The Offshore Voyaging Reference Site

A Simple Way to Decide Between Lithium or Lead-Acid Batteries for a Cruising Boat

Most of us cruisers have probably either made the transition to lithium batteries, or are wondering whether we should. And, at first glance, what with all the claims and counterclaims made by each side of the debate, this seems like a horribly complex decision.

The lithium fan-boys, often in the grips of confirmation bias, scream “no brainer”, while the reactionary lead-acid stick-in-the-muds yell “unreliable and will probably burn your boat”.

Both positions are wrong.

But, not to worry, the decision becomes easy as soon as we cut through the claims to an understanding of the fundamental difference between the two chemistries.

And, further, once we have that fundamental locked into our heads, it becomes easy to select the right system for us and our boat.

What’s that fundamental?

Lithium batteries store about four times as much usable energy for a given weight and volume as lead-acid batteries do.

Yes, I know, four times seems high given that a good-quality lithium battery weighs about 40% of a lead-acid battery, but the difference is that lithiums can be consistently cycled through about 80 percent of their range and lead-acid about half that.

Lead-acid salespeople claim 50%, but 40% is more realistic in typical yacht use due to tail off in charge current, making full charge every cycle unlikely, although solar does help with that.

And, yes, the savings in weight of lithium tends to be more than the savings in volume.

So let’s just stipulate (as the lawyers say) that four times is a rough approximation. Don’t worry about it…for our purposes it will do. Remember, this article is about a simple way to decide which batteries to buy, not an challenge to get into a mind-numbing debate about every detail.

If you are a lithium-lover and want to use five, or a lead-luster who wants to use three for the factor, that’s just fine with me; besides, it will make little difference to the final logical outcome.

OK, so now we know this basic difference between the two battery types, the decision comes down to two questions each of us need to answer for ourselves:

  1. Is having either the same capacity at one-quarter the weight and volume, or four times the capacity at the same weight and volume (or a mix of the two variables in between), worth the cost of changing to lithium?
  2. If the answer to question one is yes, then the next question is, are we willing to take on the complications and hassles of installing a seamanlike lithium-battery system?

Wasn’t that nice and simple?

But wait. What about defining costs (question 1) and complications (question 2) in a world of fan-boys and salespeople from both sides screaming at the top of their lungs stuff like:

  • Lithium will last twice, five, ten times longer—the number claimed will vary with the speaker’s ability to tell a whopper without blushing.
  • Lithium will burn your boat.
  • Lithium is safer than lead.
  • Lithium is far less reliable than lead.
  • Lead batteries die in less than a year with typical cruiser use.
  • Lithium is so expensive that it never makes any sense.
  • Lithium is actually cheaper than lead because it lasts twice, five, ten times longer.

As so often in these kinds of arguments, all of the above claims contain some truth, but most are more wrong than right.

Getting To Truth

The good news is that, with a bit of logic, getting to the truth is easy. Let’s do that.

Claim: Lithium lasts longer than lead

Let’s check with Victron who make several kinds of batteries of both chemistries, and then add in our own experience:

  • Lithium 80% discharge: 3000 cycles
  • AGM Super Cycle battery 40% discharge: 1000 cycles
  • Gel long life 50% discharge: 2500 cycles¹
  • LifeLine AGM manual 50% discharge: 1000 cycles
  • LifeLine AGM real-world experience on McCurdy and Rhodes 56 (no solar and brutal usage) 50% discharge: ~1500 cycles (see Further Reading for details)

So where does the truth lie? Assuming that both types of batteries are well taken care of, let’s call it three times as long for lithium, although based on our own experience, that is being kind to lithium, not that it matters for our purposes.

Note that I’m assuming high-quality lithium cells from a reputable manufacturer and that they are treated absolutely perfectly for their entire lifetime—one BMS failure or wiring mistake can total a whole battery, or even the whole bank.

Also be aware that, as I understand it, even reputable manufacturers sell cells that don’t meet the standards of mainstream companies (like Victron, Mastervolt, and Lithionics) into the secondary market, so the claims you often hear that “the cells I got from no-name-lithium are just the same as those used by Victron” is likely not true.

To mangle a quote from George Orwell’s Animal Farm, “all lithium cells from a given manufacturer are equal, but some are more equal than others”.

¹The claimed cycle life for gels is interesting.

Claim: Lithium will burn your boat

This one’s easy. Assuming we are using Lithium-Ion-Phosphate batteries from a reputable source, properly installed, this is crap. Heck, ABYC just went through the exercise of actually trying to get a lithium battery to thermal runaway without success.

As far as I’m concerned, this one’s a deader.

That said, if we buy no-name cells off eBay, without safe containment, and then cobble together a bank using the instructions from some wan….err…presenter on YouTube, without a full understanding of what we are doing, all bets are off.

Claim: Lithium is safer than lead

The argument is that lithium cells are individually controlled by a BMS and therefore will be isolated if something goes wrong.

But the problem is that said argument assumes that BMSs never fail, clearly rubbish, particularly given how complicated they are.

And all that complex wiring in a lithium system, or even inside a “drop-in” lithium battery, can be a cause of a fire in and of itself. This is particularly true of the sketchy cheap internal BMS batteries available directly from the far east—see Further Reading.

Also, lithium batteries enter thermal runaway at a lower temperature than lead acid, and if they do catch fire are harder to put out and produce more toxic fumes.

Anyway, the bottom line on both this claim and the one just above, is that assuming good-quality batteries of either chemistry, fires are generally the result of poor system design or installation, not battery chemistry, so let’s move on.

Claim: Lithium is far less reliable than lead

This one is fundamentally true in that lithium systems are far, far more complicated than lead, and more complication pretty much always equals less reliability, particularly at sea.

Also, lithiums can be totalled by just one charge or discharge mistake, whereas good-quality lead-acid batteries are fantastically tolerant of screw ups.

That said, if we use equipment from reputable vendors, and install the right backup and monitoring systems, this is now a manageable issue that should not deter anyone from installing a lithium system that makes sense for them and their boat when measured against the above fundamental: much higher energy density.

Claim: Lead acid batteries die in less than a year on a cruising boat

True, if we don’t charge them properly and leave them in a partial state of charge for weeks or months.

But that problem was fixed 15 years ago (by this Online Book) and now that we have efficient and cost-effective solar panels, even with a small array (properly managed) it’s a very easily solved problem.

Bottom line, if we get less than 1000 cycles out of good-quality lead-acid batteries, we need to go look at our installation and charging practices, not blame the chemistry.

Or, to put it another way, changing to lithium to fix a faulty lead-acid system that would have, when fixed, met our needs, is a giant logic fail.

Claim: Lithium is so much more expensive than lead that it never makes sense

Rubbish. There are a whole bunch of scenarios where lithium makes perfect sense, even if it costs much more than lead. Saving weight and volume is a huge benefit, particularly on smaller and/or lighter boats.

And every year, as lithium systems get better, more reliable, and less expensive (in inflation-adjusted terms) there are more and more scenarios where lithium makes sense.

Claim: Lithium is cheaper than lead because it lasts so much longer

Not true, at least at the moment.

This is an assertion that lithium fan-boys love to support by comparing above-mentioned no-name cells to quality AGMs and/or conveniently forgetting all the expensive surrounding equipment required to build a seamanlike lithium battery system.

And lead-acid lovers can be just as biased, comparing a primitive system based on no-name golf-cart batteries from Costco², charged by the shitty OEM alternator that came with their engine, with an all-singing, all-dancing seamanlike lithium system.

What’s the reality? At the time of writing, a good, although very rough rule of thumb would be that a seamanlike lithium system will cost four times what an equivalent usable-capacity lead-acid system will.

Three-quarters of that added capital cost is the price difference in the batteries, including BMS(s), and the remaining quarter is the cost of the added equipment to build a reliable and fault-tolerant lithium system that is not required for lead.

However, the per-cycle cost over the life of the lithium batteries will be only about twice as expensive as an equivalent lead-acid system. The difference between this factor and the one in the paragraph above being that the lead-acid batteries will be replaced two to three times as often as the lithium.

²I’m not suggesting that golf-cart batteries from Costco are always a bad idea, just that for the purposes of this article we need to compare apples to apples. In fact, golf-cart batteries can be a great option for the right usage profile.


We have come full circle.

To make the choice between lead-acid and lithium, all each of us needs to do is shut out all the screaming and yelling and answer the two questions at the top of the article for ourselves.

One more time:

The decision comes down to cost as against energy density, and willingness to take on the complications of a seamanlike lithium installation.

It really is that simple. Everything else is just noise.

Our Technical Comfort Level

That said, there is one other issue that we should all think about before pulling the trigger on lithium:

Designing, building, troubleshooting, and maintaining a reliable, seamanlike, regulatory compliant, and insurable lithium system is a complex project that only boat owners with a good technical understanding will be able to DIY, although reading through this Online Book will help.

Others should make sure they have a really good technician lined up to do the installation, not Bubba at the boatyard, and also think deeply about what they will do if the system fails far from said technician—our suggested backup system can help with that, see Further Reading.

I also highly recommend sourcing the entire system from a company like Ocean Planet Energy who specialize in systems for offshore voyagers.

When installing and maintaining a lithium system, we want one competent organization to call when stuff goes wrong, staffed by people who understand boats and give a shit. Not a bunch of different companies staffed by honey badgers who will all point the finger at each other.

Lead/Lithium Hybrid Systems

One other point before I end this:

There are a bunch of YouTube presenters suggesting adding lithium in parallel with an existing lead-acid system. And there is even one product (at least) purpose-built for this usage.

But once we understand that the sole reason for taking on the complications and expense of lithium on a boat is saving weight and volume, it becomes obvious that adding lithium in parallel to an existing lead-acid system is not only bad practice and potentially dangerous, but also makes very little sense in most scenarios.

If we are going to do lithium, we should go the whole hog to get all the space and weight benefits we paid the additional money, and took on the added complications over lead-acid, for.

Further Reading

³Rod starts this otherwise excellent article with a rant claiming that lead-acid batteries typically last just 150 deep cycles. And he is kinda-right given that he is quoting a survey of cruisers who had clearly not followed the recommendations in this Online Book. And, as I write above, with the advent of reasonably priced solar this is an even easier problem to solve. So ignore that first part, it does not apply to smart AAC members.


If you have any questions, please leave a comment.

If you want to tell me why I’m wrong about the above factors, that’s fine, I always have more to learn.

That said, make sure you remember that I’m assuming a seamanlike load-dump (blackout) proof lithium system with proper backup, designed for full-time offshore voyaging, that won’t make your insurance company cancel your policy (see Further Reading)—let’s compare apples to apples in the discussion.

More Articles From Online Book: Electrical Systems For Cruising Boats:

  1. Why Most New-To-Us Boat Electrical Systems Must Be Rebuilt
  2. One Simple Law That Makes Electrical Systems Easy to Understand
  3. How Batteries Charge (Multiple Charging Sources Too)
  4. 5 Safety Tips For Working on Boat DC Electrical Systems
  5. 7 Checks To Stop Our DC Electrical System From Burning Our Boat
  6. Cruising Boat Electrical System Design, Part 1—Loads and Conservation
  7. Cruising Boat Electrical System Design, Part 2—Thinking About Systems
  8. Cruising Boat Electrical System Design, Part 3—Specifying Optimal Battery Bank Size
  9. Balancing Battery Bank and Solar Array Size
  10. The Danger of Voltage Drops From High Current (Amp) Loads
  11. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 1
  12. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 2
  13. Battery Bank Separation and Cross-Charging Best Practices
  14. Choosing & Installing Battery Switches
  15. Cross-Bank Battery Charging—Splitters and Relays
  16. Cross-Bank Battery Charging—DC/DC Chargers
  17. 10 Tips To Install An Alternator
  18. Stupid Alternator Regulators Get Smarter…Finally
  19. WakeSpeed WS500—Best Alternator Regulator for Lead Acid¹ and Lithium Batteries
  20. Smart Chargers Are Not That Smart
  21. Replacing Diesel-Generated Electricity With Renewables, Part 1—Loads and Options
  22. Replacing Diesel-Generated Electricity With Renewables, Part 2—Case Studies
  23. Efficient Generator-Based Electrical Systems For Yachts
  24. Battery Bank Size and Generator Run Time, A Case Study
  25. A Simple Way to Decide Between Lithium or Lead-Acid Batteries for a Cruising Boat
  26. Eight Steps to Get Ready For Lithium Batteries
  27. Why Lithium Battery Load Dumps Matter
  28. 8 Tips To Prevent Lithium Battery Black Outs
  29. Building a Seamanlike Lithium Battery System
  30. Lithium Batteries Buyer’s Guide—Part 1, BMS Requirements
  31. Lithium Batteries Buyer’s Guide—Part 2, Balancing and Monitoring
  32. Lithium Batteries Buyer’s Guide—Part 3, Current (Amps) Requirements and Optimal Voltage
  33. Lithium Battery Buyer’s Guide—Part 4, Fusing
  34. Lithium Buyer’s Guide—Budget: High End System
  35. 11 Steps To Better Lead Acid Battery Life
  36. How Hard Can We Charge Our Lead-Acid Batteries?
  37. How Lead Acid Batteries Get Wrecked and What To Do About It
  38. Equalizing Batteries, The Reality
  39. Renewable Power
  40. Wind Generators
  41. Solar Power
  42. Watt & Sea Hydrogenerator Buyer’s Guide—Cost Performance
  43. Battery Monitors, Part 1—Which Type Is Right For You?
  44. Battery Monitors, Part 2—Recommended Unit
  45. Battery Monitors, Part 3—Calibration and Use
  46. Battery Containment—Part 1
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Alastair Currie

I am not in a position to criticise what you suggest as my electrical energy upgrade is only just starting. I would like to thank you for bringing a bit of clarity to the decision process. I am very much a late adopter of technology, not because I am a luddite, but rather I want to benefit from technology maturing and associated price drops. So, in the market for changing out old dual alternators, to a single alternator, split charging via diodes to something else, regulators, AtoB, BtoB and nearing end of life T105s et cetera, I am beginning to consider lithium. Two primary reasons: my current set up will be removed entirely, so blank canvas and I would like the space that the current bank takes up to be freed up for something else. However, if I answer your questions as noted: 1) Yes, 2) No. Hence for me, I will be upgrading system for lead acid this time round. I would of course want to make sure that the new equipment is at least lithium charging capable at some future point.

Alastair Currie

John, I don’t know where you find the time! However, an article on “build or modify a lead-acid system” so that it is lithium ready, would be appreciated by many I am sure.

Andre Langevin

I have been the slave of Lead-Acid battery for 40 years. Started my life as a mechanic responsible for truck maintenance, burnt alternator and dead batteries. On my current exploration boat, i had 8 Crown CR-235 from 2008 to 2015 then i bought another set in 2015 that i just discarded (from July 2015 to December 2022 = about 2000 cycles. The secret for golf cart battery longevity is 1) watering, watering watering and 2) daily charge after night discharge. Water consumption has been around 1 liter per battery per month under daily 50 % discharge in a liveaboard situation in the Caribbean.

I switched the 8 CR-235 to 4 Victron 200 AH Smart Lithium in December 2022 and i think the most important point to the analysis is that Lithium stop the dependency to the daily charge. Which mean that it open the door to transient charge/discharge scenario like solar (a cloud passes and no charge while there is still a DC load that discharge), induction oven and other transient use.

this has completely changed my life, much more than the weight savings or the added capacity.

The Lithium system has been made ABYC TE13 compliant by careful engineering of the BMS interaction with all charging and load component has well has the monitoring system.

(on the picture, hard to believe that these are equivalent)

Andre Langevin

Example of a  seamanlike load-dump (blackout) proof lithium system with proper backup, designed for full-time offshore voyaging,

There is a starting set of 2 Victron AGM battery (2) and 4 Smart Lithium Victron battery in two separate bank each with its own BMS. The Balmar 250 Amp alternator feed the AGM with a charge profile set at 14.2 volts. At that voltage the AGM are happy and its the recommended absorbtion voltage for the Victron Lithiums. The Alternator is also connectd to the ALTERNATOR STUD on the Victron Smart BMS 12 200. The Smart BMS 12 200 has integrated switches and mosfet transistor to separate and interconnect based on voltage rules, the Lithiums, the alternator and the DC loads.

The system on our boat is completely redundant, all DC loads including engine start can either be run from the AGM, from Lithium bank 1 or Lithium bank 2.

The system has been designed to meet ABYC TE13 and be highly redundant.

Impermanence Lithium jpg.jpg
Andre Langevin

John this is the feature of the Victron Smart BMS 12-200. It receive the alternator through a dedicated stud, which get triggered at 13V and disconnect at 14.4. The DC system is on another stud which is not deactivated unless an internal problem in a lithium battery. Since the alternator is also connected to the AGM there is no dump load. The alternator stud also serve as a programmable shunt so to limit the amperage output of the alternator. I have two separate system so having both disconnecting at the same time would be very low probability and in any case there is still 200 Ah of AGM.

1611304722_upload_documents_775_500-BMS210055000_Smart BMS 12-200 (top).png
Andre Langevin

Hello John i don’t want to argue but the system is fault tolerant with two separate set of lithium batteries each governed by its own BMS. Since the BMS take care of all external events, what remains are internal events (cell out of balance, temperature, low SOC). Both set are connected to the same DC wire that provide all current to the main panel including the autopilot. They are thus governed by Ohm law. If one set of battery goes down by an internal event, then the other set is still there and no DC shutdown occurs.

I agree with you that a backup is needed. I’ve put an excerpt of the TE13 that say so. But the backup can be another Lithium set…

Capture d’écran 2023-01-13 082934.jpg
Star Tracker

Diffferent strokes and all that, but I’m going with something similar, as much as I like Lifepo4, and firmly believe my next round the lead acid will be replaced with an LTO battery, but I need to do a bunch of testing first. I don’t trust the lithium on it’s own exactly, doubly so since I’ll be rolling my own for cost reasons. Lead acid (AGM, MagnaCharge, has higher reserve AH, and same CCA as the victron equivalent, better warranty and 1/2 the price at retail vs my cost with victron) engine start, one isolated bilge pump mounted higher in the bilge. In use, each battery bank will be charged by any charging source, but the lead acid will see me through surprises. It worked out to be no real difference in price to put it all together using a Lynx set from Victron vs buying fuses individually. Similar to the above BMS, but I’m doing it using a ML-ACR with remote switch for over-ride. Alt will always be direct to the lead acid, but the manual switch can be used to start off the lithium. To solve my back and forth on nav lights, VHF/plotter etc, I am reversing a set of 3 way toggle switches; 2 in one out. So by default my start sequence will look like this: over-ride ACR with remote switch, start. Nav light etc switches in Pos 1 means everything is off lithium. If it goes down for whatever reason, toggling the switches to Pos 2 means my AGM will keep critical systems going. It does require a bit more manual interaction, but to me gives the best combination of reliability and minimizing extra thinking bits. I have a funny story about chasing a GFI in a Nordhaven recently. Ended up they rail mounted them behind the panels requiring tools to access. All was well until the thinking bits reached a different conclusion than anticipated.

Evan Effa

Weight savings & the number of cycles on the life of the battery are only part of the appeal of a Lithium house bank.

Much better charging efficiency means the generator runs can be significantly shorter (huge benefit IMO) and the solar charging is more efficient. Maintaining a constant voltage well above 12V (even when under higher loads like running a microwave to warm up leftovers) is also appealing as some instruments and devices (eg Wallas heater) are more sensitive to low voltage states.

My 660 Amp-hour Firefly House bank will drift pretty close to 12.1V by the time I’m at a ~ 300 A-h deficit & there is no way we are warming up leftovers when we are anything but close to a full charge.

The Fireflies are still working well enough but my next house bank will almost certainly be LiPO4.


Evan Effa

Hi John,

I should have been clearer in my description. My inverter keeps going but the house bank voltage drops off from say 12.4V to 11.9V after a minute or som of microwave use. It’s winter and the heater is running and I don’t want the voltage to drop less than 12.0V

As I understand it, a LiPO4 house bank would not do this.

Also, AGM’s like to be brought up to a full state of charge frequently, so that last 10% of charging is important for AGM battery health whereas the Lithium bank, or the Fireflies too for that matter, are happy to be in a partial SOC and do not need that extra hour or so of generator run.

Evan Effa

Not to put too fine a point on it, but my experience this past weekend underscores an issue I would think a LiPO4 house bank could solve: Rapid charging.

My wife & I just did an overnight trip to one of our favorite anchorages ~ 2 hours away from where we keep the boat. ( I have switched to a bulk charging / no float regimen for our 660A/h Firefly house bank when on shore power so can get to the boat with it less than fully charged.) On arrival we were still down ~120 A/h from a full SOC despite a 2 hour run on a 120A alternator. I ran the generator for an hour around supper. After our overnight stay with the heater running & a few extravagances (think Starlink & streaming a movie) we were down to -335 A/h in the morning

After raising anchor & heading home, I had the generator running, feeding 2 battery chargers & with the engine alternator working well (it was a foggy morning so no help from the solar panels), we were pushing 210 Amps into the bank. An hour later this was down to 95 Amps at 13.74V.

By the time we arrived home, we were still only at 90% SOC and pushing ~70 amps into the bank.

This doesn’t matter much in this scenario as we returned to port to plug into shore power; but I would like the house bank to accept everything we can throw at it in as short a time as is practical. If we had been moving to a new anchorage we would still be starting the winter afternoon/evening in a deficit.

My understanding is that a LiPO4 bank would have maintained this ~ 200A+ acceptance and be nicely full in an hour and a half. (Other sources & Andre’s comment seems to support this expectation.) This would be a very compelling reason to consider LiPO4 over lead acid.

– evan

Evan Effa

Thanks John,

I don’t wish to hijack this discussion any more than I have already; but, as a case study, it may be useful for this question of why go LiPO4 when good old lead acid may be good enough…

This past weekend was an anomaly in that we left the dock with a significant SOC deficit but if we had been at anchor for a couple of days, the daily charge requirement in winter would still be in the 300+ range. (Your comments will prompt me to do an another inventory on my loads again but I do not see much that I can cut out of the load side of the equation. We have all LED lighting and I have, in the past looked hard in vain for any parasitic loads.)

Summertime is not so much of a problem as we have 550W of solar and long days to maximize power delivery.

In wintertime though, at anchor with a NMEA 2000 network with redundant GPS’, wind instruments, a fridge and a freezer, & a diesel heater running 24/7 along with the inverter supplying power to a laptop (with requisite anchor alarm) and a few other assorted devices we average 13-15 amps draw per hour. We just added a new Starlink which looks to be drawing another 2 – 4 amps when it is on, potentially bringing us even closer to a rather horrifying 18-20A per hour. In winter, the solar panels don’t add much to the charging input so, yes, with potentially 360 A/h per day at anchor, my house bank is probably too small.

Nevertheless, adding more lead acid storage to get to 1200 A/h (even if I could find more Fireflies and find the space for them) would not address the problem of the prolonged daily charging cycle to replace ~350 A/h with the tapering charge profile of the lead acid batteries limiting the efficiency. With a 9.5 KW generator and a Magnum 2800W charger, and a 40Amp Xantrex auxiliary charger,(+ 120A alternator) I don’t really think that I need more charging capacity, I just need the bank to be more efficient at accepting the offered charging current. I really do not want 4 hours of generator time per day to get back to 100% SOC.

Thus; LiPO4 makes the most sense for the next house bank.

Stein Varjord

Hi Evan,

You seem to be tech savvy enough to make good choices already. You’re probably right that lithium is the best solution for you. Since you wish to use the fast charge properties, you need to be careful with which lithium setup you chose. Most drop-in batteries (12V battery box with BMS inside) are limited on current capacity. They’re usually better than similar size AGM, but not by as much as many think. To exploit the insane current capacity of the raw cells, you need either a very serious integrated battery, no drop-in, or better; separate 3,2V cells with external good BMS etc.

I’d also suspect that you can find ways of reducing consumption. I notice that you run a laptop via an inverter. Perhaps also phone chargers etc? That will, according to Rod Collins and several others, make your items consume at least twice as much as it would if you turned off the inverter and ran them via a DC converter straight from 12V. Your laptop is probably running on 19V DC. All that back and forth transformation, DC to AC, low voltage to high voltage, high voltage back to low voltage, AC back to DC, makes more trouble than the stated efficiency numbers from the inverter maker indicate. An inverter on a boat should close to always be off.

Evan Effa

Thank you Stein.

Your suggestion to turn off the inverter is a very good one, as the obligatory basal draw from the Inverter unit is quite significant. The primary reason to keep it going was to feed the laptop but as you suggest, a simple 12V to 19V step-up converter for the laptop and converting the phone chargers to DC USB outlets should decrease the need for the inverter to be on for the majority of the time.


Evan Effa

Thank you Stein.

Your comments are very helpful and have prompted me to make some changes.

I’ve done a little more inventory as to my power usage. My Magnum Inverter is drawing close to 3 amps at idle. (~70 A/h per day).

I have been using the inverter to power the navigational laptop and various AC chargers to phones, iPad, flashlights etc. thinking I need these on at all times. It makes much more sense to get these off AC and onto DC power sources in order to allow me to turn off the inverter (except when needed for brief usage of a few AC-only appliances).

I bought a DC-DC converter to power the laptop. That works well.

All smaller devices can be charged through DC USB outlets

The Starlink does not run 24/7 but when it is on, it has a significant impact on my power usage. I have the parts on order to convert the Starlink to DC power which should reduce its power requirements from ~ 55-60W to 30-35W. (With no need for the inverter and this reduction in power draw, the Starlink related loads should go from 6-7A down to ~ 2-3A.)

Anyways, I appreciate your thoughtful comments (and John’s comments) to nudge me to look at the house loads a little more closely.


Evan Effa

Hi again John,

Your advice and comments are appreciated.

Although leaving an anchorage in the morning with a charge deficit of 180 A/h is not uncommon, it might seem a little odd when the boat was docked and on shore power.

A little background explanation is in order:

I was noticing that when using the boat after being on shore power for a while, the house bank would drop its voltage rather quickly with only a modest draw; but this would improve after even one or two discharge /recharge cycles. I had been using the typical float charge when on shore power at our home dock.

This has begged me for quite a while but I was unaware that Firefly had modified their manual to be more specific. As per the revised Firefly manual, it is recommended against having the Fireflies left in a float 100% SOC and using a different charging scheme for their “storage state”. I recently switched to their recommended Constant Current / Constant Voltage charging regimen for the FF house bank. The Magnum charger will start charging when the house bank voltage hits a programmed number (12.1V) and charge for a specified duration (10 hrs) or until the charging current hits < a set current (5A). (I have the Lifeline start battery isolated on a separate charger circuit with a typical Lead acid float profile.). The fridge, freezer, heat pump etc. are all on AC. Virtually all DC loads are off except for my aging Raymarine autopilot which does not take kindly to being turned off and on and the NMEA 2000 network. (The total DC load is trivial when at the dock.)

This new regimen seems to be working in that the FF bank maintains its voltage better under load when first out but it also means that if I get to the boat at a point when it is at a nadir in the cycle, it may have a SOC deficit when we leave the dock. This is why we started our overnight trip at a deficit.

I may need to change the parameters to a higher trigger point for starting a recharge cycle but will not likely go back to the typical float regimen.

(I do need to do a deep discharge to ~10.5V with an aggressive recharge cycle as per the manual (equivalent to an equalization routine) but I need a chance to do that over a few days with all instruments and sensitive electronics turned off.

Anyways, I hope that fills in the blanks a bit.


Evan Effa

Hi again John.

When at anchor, I use my laptop to run Coastal Explorer and to maintain a continuous anchor alarm. I also appreciate knowing the outside temperature, wind speed data, AIS info etc. so feel pretty committed to keeping my NMEA 2000 instruments up and running.

As I’ve posted above, I think I may have found some other power savings and with your suggestion of using DC-DC converters to mitigate some of the negative effects of lower voltages on voltage sensitive devices, I think I may have a workable solution to my situation.

I will look to adding another 40-50 Amps of charging.

I hope you don’t mind all my comments on this thread; but, perhaps others might find it useful in considering similar issues with their own systems?


Evan Effa

I have been doing a lot of thinking about the many issues discussed here. A number of good suggestions have been made and I am seeing a number of ways that I might carry on with my FireFly house bank for a few more years.

Your suggestion of using a DC-DC converter to power the Wallas heater with a controlled, more favourable, voltage makes very good sense. I think I will try that, as on paper, it is probably the most voltage sensitive device on the DC side of the system. (I have yet to have it cut out or quit at even 12.0V but in the written specs, it wants a minimum of 12.6V for startup and operation.)

It might also make sense to have a DC-DC converter to provide more consistent voltages for the more sensitive instruments and NMEA 2000 network?

With these devices protected against lower voltages, I could be less concerned when the house bank voltage drops a little below 12.0V…

Evan Effa

This discussion has been quite useful to me. Thank you.

I’ve been wrestling with this question of replacing my Lead acid (Carbon Foam) house bank vs LiPO4 for my boat for a while now.

One major factor that prompted me to consider LiPO4 was that I was under the impression that I should never allow the house bank voltage to drop below 12.0V.

In hindsight, I am not sure where I got that idea. I was under the impression that my Wallas Heater and the NMEA 2000 network and its associated devices would not tolerate voltages lower than 12.0V

Like so many things, it doesn’t hurt to review the documentation.

The official documentation for the Wallas heater is rather sparse but states that it does not like to be started at a voltage below 12.6V; but it has never stalled in its startup cycle even at voltages as low as 12.3V. As it turns out, it will apparently tolerate operating voltages as low as 11.0V…

Your comment that NMEA 2000 networks and and instruments should be tolerant of voltages well below 12.0V prompted me to look into this and indeed 9.0V is listed as the minimum for the NMEA 2000 network. I would not a want to be anywhere near that but it suggests that dropping into the 11.8 – 11.9V range would not cause any grief.

So, if we can allow the system to drop into the 11.9V range I would have at least another 1/2 to full day of usage in dark winter days on the hook and the need to consider a LiPO4 upgrade decreases significantly.

The slower charging is a nuisance in that the generator runs in winter stretch into 2 – 3 hours a day but we have a very quiet and fuel efficient generator that really deserves to be run regularly anyway.

I have also been prompted by this discussion to look to reducing the demand side of the equation. I have have made changes that allow me to turn off the inverter. I’ve converted the navigation laptop, all phone chargers, Starlink and assorted other devices to DC power. Doing this, it looks like I can reduce my average power requirements by around 5A or so or ~ 120 Ah / day: Not insignificant.

Anyways, I appreciate the useful information and guidance here.

Thank you.


Evan Effa

Hi John,

Yes. I totally understand why voltage is a poor indicator of a battery bank’s SOC.

(I have a Victron Shunt and monitor the SOC pretty religiously through Bluetooth. )

My point above was, that despite the state of charge, I was loath to let my system drop below 12.0V fearing damage to the more sensitive DC instruments and devices in the system.

Having looked at this arbitrary threshold more closely though, it looks like there is no reason to not make something like 11.7V or even11.5V the new limit to how low I can go. It does not appear to be a risk to the instruments or even the supposedly voltage sensitive heater.

(I appreciate that deeper discharges shorten the overall lifespan of carbon foam batteries but these Fireflies seem to perform better with occasional deeper discharges.)

This might be a good question for another post:

How low is too low for house bank voltage?

Andre Langevin

Indeed Victron recommend to put charge efficiency at 99% for their Smart Lithium. I notice since my installation that the battery charge faster because of this increase efficiency. Its notable.

Mark Wilson

Dear John et al

My current batteries are failing, all together and all at once. They need replacing. So I read all these articles with urgent interest.

The seductive allure of lithium is obvious but the need to apply backup systems and three types of batteries combined with a complicated array of control systems ? Aargh! I just want some new batteries so I can go sailing.

All these discussions must be catnip to those who imbibed Ohms law with their mother’s milk and who delight in these matters but to those of us who were too thick to pass their advanced mathematics exams they are excoriatingly and opaquely irritating. I hesitate to say tedious for they do matter. But my eyes cloud over and I am filled with a deep sense of ennui.

Warren Buffet advises investors, and here I paraphrase, that if you don’t understand what the company does don’t invest in it.

So I deduce from all this that if I invest in a unitary bank of Victron super cycle AGM batteries and the requisite Victron gizmos to control it I will get at minimum 3 years non stop usage and probably more. Realistically, including void periods when life and fate prevents me from going sailing, I will get five years. (As long as I don’t do something deeply stupid but then the same caveat would apply to any batteries.). By which time I will be 73. If I’m still fit then I will thank my lucky stars and then look again at lithium. Maybe by then it will be cheaper and simpler to operate. Planning ten years ahead at my time of life might be seen by the gods as presumptuous.

As for the weight and space saving that comes with lithium its a nice idea but I’m a single hander in a 40 footer and the the battery compartments are already conveniently located near the engine and the middle of the boat and there is adequate storage space elsewhere. (I’m a bit of a weight watching fascist as regards the storing of the boat.) I have an adequate solar array so charging isn’t a major issue. And from what I read the new AGM batteries offer the option of deeper discharge than the previous generation so I may well already be getting more bang for my buck.

Andre Langevin

Mark its not that complicated, manufacturers like Victron have scientist and a lot of knowlege more than us sailor so they have product that support the ABYC TE13 requirement and enable mixing AGM and Lithium safely BECAUSE they are clever and they designed the AGM to have the same charge profile as the Lithium. I wish i could help you.

Ignat Fialkovskiy

Will Adventure 40 have lithium or Acid?
or better to ask, which one you think it should have.


Ignat Fialkovskiy

thank you and I think that’s the shortest answer to the question posed in this article 🙂

Mike Pitzer

I have two Lifeline 8D, 255 amp AGMs on my coastal used boat. They lasted 6 years before being killed by negligence of Marina staff. Given my usage they could have lasted another 3 years. I have been reimbursed for replacement costs. Lithium then became lustium. I thought why not with such a head start on the cost? Well, once you find out that you need to replace your 9 year old magnum Charger/inverter and it’s controller and a new regulater to replace the old Balmar 612 and maybe some type of isolator and a new Victron shunt and other miscellany and pay a true professional to install it and set it up you realize that the gain isn’t worth the cost. Not for 5-7 day cruises. If I were going on a long term cruise then I would switch. So, for me it’s about how one uses his boat and is there a break even point when your a 75 yr old sailor. I am now recovered from Lustium.

Stein Varjord

Hi John,
As you know, I can be counted on the side of the lithium fanboys, and I’m definitely on the nerd side of it. I’m happy to report that I agree with every word in this article. Of course one can dig into loads of detail with specific systems, but the point here was to make it simple so that people can see the main points. Applause for that!

I also like to say that if your lead acid batteries do the job well now, why pay 4 times the price for the same job? Why buy a Ferrari if you will never drive faster than 100 km/h / 60 mph? If we regularly spend extended periods with solar etc as our main charging source, lithium is a life changer. Same if we regularly use a lot of power. If we either plug into a dock most nights or use our engine most days when we’re cruising, lead acid is totally able to do the job. We won’t notice any reason for paying a lot more.

One detail, that doesn’t diminish anything of the above evaluation: The point mentioned by Evan about improved solar charging is something I’ve also noticed on several boats that have gotten lithium and kept the solar setup unchanged. According to the battery monitor, the Watt hours gathered per day goes to very roughly double what it was. Changing from lead to lithium sort of makes the solar panels “twice as big”. I can’t give a complete explanation for it. It’s just an approximate observation. My guess is that several factors are relevant.

Probably the most important factor is that lithium will always receive all the charge current it is given and store it in the cells with about 98% efficiency, even when close to fully charged. Lead acid can also be quite efficient at low to mid charge levels, above 90% efficient, but as the charge level rises and the resistance increases, charge efficiency drops fast. Closer to full, the current acceptance also wastes much charge power, especially from a big array. This happens well before 90% full, where most boats tend to hover a lot in the day time.

But again: This solar thing is a detail that is nice to know, while the simple main issues relevant for making a choice are the ones discussed in the article. Letting our love for the lates tech take us for a ride can often (but not always) be expensive and give little benefit.

Eric Klem

Hi Stein,

Your comment on the picture of Rob’s lithium install reminded me of this comment. I didn’t respond when you made it as I didn’t want to front-run the article on battery and solar sizing but I think a major contributor to people reporting more solar output with lithium is likely that they installed more battery capacity and are more willing to be at lower SOC. It could also be that people increase their usage when they increase their capacity. With a small battery bank, on the cloudy days you get forced to run another charge source like an engine or generator to not drop to too low of an SOC and that energy is now not solar energy. On the sunny days, you are more likely to get into absorption or float so no useful solar then. For max solar efficiency, we would have a giant battery that would never drop below minimum SOC and never get into absorption.

There is a straight up charging efficiency too but it isn’t nearly as big as the difference claims some are making.


Eric Klem

Hi John,

I think this is a very good way to look at it. I started to think about the advantage of faster charge acceptance rates as I was reading but your earlier comment is right, this is actually a bank sizing question. There is some truth to charge efficiency differences but it isn’t a large difference until you get to high states of charge with big charging sources on lead acid which is poor system design anyways.

In my case, I would love to save the weight but I don’t think it would make huge performance differences for us and I am sure I could get bigger gains by spending the money on other things like fancier sails. Ignoring the money issue, I want to wait until lithium becomes more widely adopted and things like insurance requirements, availability of components, etc. becomes a lot more stable. I certainly think it is possible to build a good lithium system these days if you have the right technical knowledge but going back to your questions, I am not willing to put in the effort yet.

Future proofing systems is a very interesting topic. I can say that professionally I have learned to completely ignore it unless someone really can give you hard interface requirements for something that is close to completion of development. Everything else just ends up being a huge distraction and you end up compromising your design for all sorts of hypotheticals that usually end up not actually being compatible when that thing ends up eventually being developed. Unfortunately, future proofing is one of those things that sounds good in a white board discussion and it is a buzzword in marketing so it eats a lot of time for almost no value. That being said, if I were replacing something like our alternator external regulator right now, I would try to get one that has the most controllability but I wouldn’t spend significantly more money or effort to do it. I expect to replace our GC2’s in probably 2 years with another set and when they die it will be 10-12 years from now probably. Whether LFP is the prevailing chemistry at that point (solid state?), whether a company like Wakespeed is still in business to keep updating their interface with BMS’s, etc. are all questions that have answers that are significantly lower than 100%. This doesn’t make something like the Wakespeed a bad product but I would buy it because it was the right product for now as opposed to a hypothetical future. Maybe I am pessimistic but I usually find that I end up having to replace entire systems as a unit and upgrading to a fundamentally different thing like lithium is not possible to do piecemeal.


Stein Varjord

Hi Eric,
Good points, but I think there’s still room for making it easier for those who will install lithium in the near future, as several A40 owners might do in the first year of ownership. It’s far easier to predict what can make their life easier the first year or two after purchase, while also fitting lead acid.

Choosing the right chargers is one example. This model from Victron is completely programmable via an app, gives 50 Amp at 12V, (25 Amp with the 24V version) can be parallelled with another of the same for more power, is water resistant, has no fan. Most modern boats go for inverter chargers, but most smart cruisers have no need for AC when unplugged. Owners can then add the inverter they wish. A pure charger seems like the right standard solution. Great for lead acid and perfect for any other battery chemistry I know of now.

Another thing that could be future proofed is to go for fuse systems that have large gaps when in the open positions, to prevent the arcing lithium is capable of. There are probably many more issues, but I’d assume one would stick to modifications that don’t add noticeably to cost and complexity, and that might also enhance the system quality for a lead acid system.

Stein Varjord

Hi John,

I think the no charger and no shore power installation might be a good call. I’ve never bought a new cruiser, so maybe this doesn’t count, but with any new to me boat I’ve invariably replaced everything related to shore power, including the charger. The consequences of faults are too big to accept anything but perfect. I think I’d do it on any new boat too. My definitions are probably not universal, to say the least, so avoiding discussions with people like me is smart. 😀

By the way, the mentioned charger does take any shore power voltage and frequency you feed it and makes out of it exactly what you told it to feed the batteries, and how you told it to do it. Pretty neat stuff, but since it’s such an easy thing to install, I se no reason why the builder should do it.

Frederick Gleason

A40 Comment: Possible Owner upgrades and improvements.

Is there is a large protected surface for mounting necessary electrical equipment and cables? Preferably behind some large cabinet doors.
Is there a large well ventilated battery box or cabinet inside and not part of the engine area?

Is there relatively easy access to run cables for solar and to and from the engine area? Where is it?
Having a small boat without these features I am very aware of the need.
Thanks for the good discussion.

Eric Klem

Hi Stein,

I wasn’t thinking specifically about the A40 when writing my comment but if I were the one making the decisions, I would still generally apply it. In line with John’s comment, I would try to set things up per best practices with the right fuses, bus separation, etc. but I don’t see that as chemistry specific. What I wouldn’t do is try to select fancy components such as chargers, alternator regulators, etc. with too much emphasis on lithium. I do think a significant portion of buyers will go out and immediately switch to lithium so in many ways this isn’t even future proofing, it is now proofing. But I would certainly not drive up the base cost measurably or create a situation where you could drop in a lithium bank with doing nothing else because invariably the consumers won’t even agree with what has been done as is shown by endless debates on the right way to set it up, this is not a mature implementation yet. Back to future proofing, I think the best you can count on is to make the system upgradable now but if someone switches to the latest in battery tech in 15 years, they will likely require more than new batteries. Heck, many of the electronics don’t last that long anyways.


Pierre Boutet

My comment will seem off topic but its not. Read on. I just read the story of that french crew who was attacked by Orcas west of Portugal. After multiple attacks by a groupe of whales, the rudder post was torn out from the bottom of the hull and the boat finally sank. Why I’m bringing that story here is because when reading it, (“Voiles et voiliers” february 2023), the skipper explained he was using his main VHF instead of the handheld, because the transmission was much better. Also, in addition to the manual pump, he turned on all electrical pumps (bilge, shower). The most interesting words he said was that he was surprised that the batteries were still working under 3 feet of sea water.

Which is the point of this comment. Would Lithium batteries, with their sophisticated electronics, would still work under 3 or 6 feet of sea water? For how long? As we can imagine, it could be crucial to keep working batteries under any kind of emergency where the batteries might be submerged.

Andre Langevin

Battery can work under water for some time and its documented in many boating forums. But i wouldn’t expect this to work for hours…mainly a few minutes. Any battery under sea water is shorted and start producing oxygen and hydrogen through electrolysis. Lithium battery electronic will go crazy with sea water and i would much more confident that AGM or Lead Acid would operate underwater than Lithiums.

Stein Varjord

Hi Pierre,

The Lithium cells themselves are totally sealed, unlike any type of lead acid battery. That means the cells, where the chemical action happens, will not be influenced by immersion. Most of the lithium batteries on cruisers are some type of drop-in battery. They normally use a non sealed box with open electronics. That will go just as you expect. The battery will certainly fail pretty immediately when the box is flooded.

However, there are versions that are fully sealed. They will work at least as long as lead acid batteries. Either way, I assume it won’t be too long. When both battery poles are immersed in salt water, quite a lot of current should flow between the poles, which should empty the battery. This is just my assumption. Perhaps somebody here has some numbers?

It should be theoretically possible to make a completely protected system, where also the poles and all electronics are sealed, but I doubt if it will be worth the effort.

Pierre Boutet

To sum it up, if the lithium battery’s BMS is not water proof, it would fail very soon after the battery is submerged.

I’ve seen a guy who has setup his electrical system such as the critical instruments (GPS, VHF, etc) are powered by a small 12 volts lead acid battery (a motorcycle battery I tink) which is placed very high in the boat. So in the case of a serious and prolonged water ingres, these critical devices would still work even if the main house batteries are submerged and discharged. This would be an interesting backup plan, both for a lithium bank and a lead acid bank.

Taras Kalapun

One point on installing separate charger and inverter versus combined.
I’ve installed Victron 12v 50A smart charger because it can work anywhere- it accepts 100V and 220V, while MultiPlus inverter/charger you need to buy for specific voltage.

Andy Schell

Great distillation John. We have the MCA surveyor coming Monday to do the full commercial inspection on FALKEN, and a big part of it is the electrical system. The MCA don’t yet have a commercial standard for house lithium batteries, so we, along with Bruce at OPE, have had to provide a compelling case as to why they are safe. It’s been an interesting process.

The new system is fully operational now!

Drew Frye

What about the restriction on charging below freezing? My understanding is that most (all?) BMSs block charging below 0C. That has been the non-starter for me on both my last and current boat. Because they are multihulls and the batteries are in the bridge deck, they absolutely go well below freezing for days or weeks at a time, even if I am on the boat. Heating (like Tesla) makes no sense, I do need a sump pump, and I don’t completely trust shore power not to fail for either local or ice storm reasons. I use solar for winter battery maintenance. I could figure out a lead work around, but that’s one more thing. And even if I did, they would not be useable for winter cruising. Perhaps mostly a multihull thing, though a shame, since we have the greatest motivations to save weight. We’re seeing a lot of lithium on new multihulls, but if you ask the builders about sub-freezing operation, you get blank stares.

Drew Frye

I should have mentioned that storage below freezing on the hard is not a problem, as long as the charging sources are disconnected. Battery bank locations in the bilge generally do not freeze if the boat is in the water, although this depends on the specifics (how deep in the bilge, remembering that getting batteries wet is bad). Obviously, not that many people expereince conditions where both the bank is below freezing and they have need to charge it.

Stein Varjord

Hi Drew,

The “do not charge below freezing” is a real LFP chemistry issue, but it’s also partly a lead acid issue. The reason the BMS cuts charging is that the cells actually do get damaged by it. They can be charged at far below freezing, but then at very low current. Winston use a variation of chemistry with Yttrium to get LiFeYPo4, and they claim normal charging down to minus 5 C and at reduced rates down to minus 20 C (minus 4 F). They can probably go slightly further than the other makers, and I can’t dispute those numbers with any facts, but I’d be very careful with pushing those limits. Crossing the line apparently doesn’t cause sudden death, as some other actions can, but does cause much accelerated ageing.

In remote locations in Norway most lit sea lead markers have solar cells and lithium batteries. They’re expected to tolerate a bit of charging even at below 20 C minus (- 4 F) and still last many years, so it can be done.

We also have a cat and the batteries are also in the bridge deck, but in the salon sofa, where it does get chilly on really cold days, but never close to freezing. Our home harbour is Amsterdam and we live on our boat so we heat it, of course. If you need your batteries to give power during the winter, thus need charge, and also don’t keep the boat heated, I’m assuming that the amount of power needed is very limited? A seriously low charge Amperage might do the job? I don’t know. If you do want lithium for the house bank, you could disconnect it in the winter and let the backup lead acid backup battery do the job as house bank?

So LFP cells are not happy with proper charging below freezing. Are lead acid batteries better? Yes…, and no. They do not charge well below plus 15 degrees C (60 F) and the further below the harder it gets. At below freezing, they give minimal power out, and take even less. Also they must absolutely not be discharged significantly, as the previous mens it’s hard to reverse and the battery fluid in a low charged LA battery is closer to water than acid. Water will freeze and destroy the battery. A decent charge current will heat lead cells and (lithium cells too, if you really push power), but neither are straightforward tasks.

I have no better solution than, if we use our boar in really cold places, we should keep the batteries in or close to the heated areas. If we store our boat, when not in use, in proper cold, we probably need to have either:
– No power consumption at all, and batteries disconnected, or
– Very low consumption and very slow charging, or
– Somehow keep the batteries slightly warmer than the ambient temp.

All three strategies can be done with any battery chemistry, even though:
– Lithium drop-in batteries often will self drain from the BMS and electronics inside the box, that can’t be disconnected, so they need a charge now and then.
– Lead Acid prefer to be kept charging at a low maintenance voltage, like 13,4V. Charge at least once a month, preferably when the weather is non freezing.
– Separate lithium cells can be stored for years without charging, especially in a cold or freezing environment, as long as they are completely disconnected from everything, also free from the battery monitor, BMS etc. The cells alone have extremely low self discharge.

Andre Langevin

Hi Stein, i live in Quebec and have stored my boat in winter with Lead Acid battery inside the boat, connected for 31 years. As you know we have temperature in the -20C for long period and winter is officially from December to March. No load and all battery disconnected i got 7 years of service on my first set of flooded lead acid battery and 8 years of service on the previous set. Now i have switched to Lithium and i’m quite concerned about what will happen next winter. I may bring the battery back home for the winter season…

Stein Varjord

Hi Andre,

I share your question mark about the new lithiums. The answer depends on what type and configuration they are. I’m certain that you can find a good routine for how to handle them in the winter.

I just wanted to hang on to your mention of flooded lead acid traction batteries, which are very competitive to any other battery type! They typically live 2-4 times as long as any AGM and can take far more abuse without dramatic issues. On raw data, they’re easily the strongest contender to lithium. The weaknesses (compared to consumeer adapted AGM batteries) is that they regularly need to be checked and refilled with distilled water, and that they emit a bit of explosive hydrogen gas when charging. That’s no exaggeration.

I was nearby when one big 48V bank exploded. An open boat, but a huge bang and much damage. Nobody hurt, except for acid burns and beeping ears for some days. (Not me). Caused by a tiny spark in a relay (we think). All those were removed.

Just to illustrate what this type of battery is capable of, I’ll mention (as I’ve done before in other contexts) that I work with electric tourist boats in Amsterdam. Skipper + tech supervision. Almost all of the boats have 2V FLA cells in 48V to 96V banks. The type you find in fork lifts and golf carts. The company I mostly work for now (Flagship) has 43 boats (at the moment), up to 20 meters (65 feet), 120 passengers and 40 metric tonnes. They run 8 to 15 hours nonstop every day the whole year and charge in the night only. The bank lasts typically 5 years of this heavy use. Flooded Lead Acid traction batteries are impressive stuff, but no set and forget.

A small number of the smaller boats here (other companies) have moved to lithium, but that’s not mainly to save weight and space but to get much faster charging. The night often gets a bit too short for the lead charge cycle, even at high current. The bigger boats charge at 96Volt 140 Amps, which is the same as 12Volt 1120 Amp… When all the boats come in and plug the chargers late in the evening, that harbour has a serious power draw! Try that at any normal marina… 😀
(We use several industrial 3-phase 240V = 360V supply lines). The current tapers off after some hours, so the batteries take too long to fill completely.

My main point is, there are very good alternatives to AGM and lithium. AGM might have the weakest performance of them all, especially their durability, to make the vendors happy, but it is indeed very easy to use…

Drew Frye

One parting thought, slightly off topic. I like to post a voltage/temperature chart next to the panel as a reminder of where they are on the curve, both charging and discharging. This means little to the typical summer-only cruiser, who sees only a small temperature range, but it helps me. The other thing I have to remember is that solar becomes much less with shorter days and low sun angle, which is kind of why it’s winter. Small wonder solar is less talked about in the UK and northern climes.

Michael Van Eeden

Wow learned a lot here, I”m not that clued up yet, on all things battery and charging.

But I took simple laymen commonsensical
approach, I thought.

I Figure out what my usage would be and tripped it.worst case.

Then figured out what solar I need to put back in 5hrs a doubled it. +120amp alt, small but good in gray days to top up..

Then figured what storage I would need and tripled it. So I did not have to go below 30 40% draw down in 24hrs for my daily use.Have cut offs to shutdown unused system banks at anchor.

I cycled those AGM battery’s for 7 years, never plug-in to shore

Well I did, plugin ever 3 months to desulfate with a good q 8 stage charger.

So for me, AGM are very affordable. and easy install and management, cheaper components Plus AGM are recyclable apparently Lithium are not, so feels better to use them.and simple ….

Plus, I dont have microwave, (still dont trust them) I just have fridge freezer, phone laptops,cameras,nav,radios,pumps Cordless tools the basics stuff,

So keep the system lean as pos… seems to work…
And I understand it, and also I can troubleshoot.fix at sea, well most of the time…For me its one of those cases I dont want to fix what ant broke.

Stuart Jackson

Nicely well balanced article. One thing I think we will see is internal BMS batteries becoming the standard (even Victron have introduced them). Agree there are some low quality products, but if done properly, I think this is a better solution since if one battery does fail, it switches itself off and one or more others wired in parallel can keep going uninterrupted. You still need to be careful with alternator connections but there are good products (DC-DC chargers) to help with that.

I am in process of binning my 3 Victron 130A Gels in favor of 3x 280A Lithiums that are the same size. Expensive, but then little in sailing makes sense financially.

Douglas Bruce

Congratulations, J&P, on an incredibly thorough analysis. I may have to read it a few more times before I truly understand. What a treasure you two are to the cruising community.

Gordon Hinds

I am on the low side of storage vs demand. I have two lead acid 130ah house batteries that power a fridge, nav array (when on), led lights and a autohelm.
I leave my inverter in which I will change now as I suspected it draws power.
The change over cost and hassle to lithium is not inconsiderable. I want to be able to cruise for weeks maybe months and not be concerned about if the battery has enough charge.
I have a wind gen that pumps out about 15amps, and a single solar panel that does about the same. My concern is the autohelm (octopus/raymarine) and fridge. I’m guessing I am a bit short. So I am thinking of just adding a third battery rather than going li which will give me the storage I need but also cost a bomb – north of $10,000… as opposed to maybe $600.

Bruce Schwab

Hi Folks,

I thought put in a short note here regarding two things that have come up in the comments; fast recharging, and supporting critical loads.

On many of the system upgrades we do, the house bank (if 12V) is upgraded to 24V, or even 48V (actually 51V if Li). The reason for this is much faster alternator recharging.

The critical and/or general loads can remain 12V (or 24V), and run from a backup battery that is kept fully charged from the house bank, using a B2B charger (or sometimes we use MPPT solar controllers). The size of the backup battery is determined by the loads, and how long you want to run off it in the case of a main battery bank cutoff.

Smooth seas

Michał Palczyński

This is my first comment here. So on the beginning I would like to say thank you John for that page with tons of interesting articles! English is not my native language so I’m sorry if sometimes I use wrong word. 

I use to live aboard for 15 years sailing to remote places. For me LFP was a game changer. I changed from 800Ah (12V) AGM to 700Ah (12v) Winston cells with REC BMS. After two years of using it I can say that I’m lithium fun boy 😉

When I wake up and the batteries are around 50-70% I can boil water on my induction stove. Under 300amps of load the voltage is around 12.6V! Impossible performance with LA batteries. Even if I had a ridiculous 1500Ah. Even @50% SOC I can run my watermaker (160 amps) for more than one hour. 

The system now is much more powerful weight 88kg (LA was 260) and taking 3 times less space. 

Another benefit is the charging efficiency of LFP is close to 100%. For LA that is around 85%. So with LFP we need to return less energy with the same usage….and so on….Others mentioned other benefits like more efficient solar charging, long time with partial state of charge, faster charging, low voltage drop, flat voltage curve. Big advantage is also no need for equalization and building an electric system around LA weakness.  

And what is the price? 

Let’s compare Lifeline GPL-4DA AGM Sealed Battery 12V 210Ah with a couple of lithium options. 

Let’s assume 40% standard DOD (depth of discharge) for AGM and 80% for LFP. 

For example we need around 300Ah usable energy. So we need 4x210Ah=840Ah of AGM, or around 400Ah of LFP. For lifeline the cycle life is 1000, for Winston cells 5000 (manufacturer data), for the rest of LFP 3000.  

We should not compare a number of cycles but number of usable Ah during battery life. 

So for Lifeline it will be 840Ah x 40% x 1000 cycles = 336 000 Ah

For Winston DIY: 400Ah x 80% x 5000 cycles = 1 600 000 Ah

The price for Lifelite is 4×700$ = 2800$

For Winston is 1600$ – price from reputable Sky Power. To make the battery work you need BMS, contactors and other gadgets – so add 1000$ (REC MBS with 2 contactors). When you change batteries in the future BMS and contactors remains the same. But ok. 1600$+1000$ = 2600$

So what is the price for one usable Ah?

Lifeline – 2800$/336000Ah=0,0083$

Winston – 2600$/1600000Ah=0,0016$

That means that ONE usable Ah with Lifeline AGM is more than FIVE times more expensive than a DIY pack built from raw Winston cells. And Winston cells are one of the best on the market. 

But ok. Not everyone is able to assemble and use DIY pack. So what about Victron?

The price for 2x Victron LFP Smart 200ah is 4000$. Plus 1000$ for BMS(one time In the lifetime) and we have 5000$

400Ah x 80% x 3000 cycles = 960 000 Ah

5000$/960 000 Ah = 0,0052$, 1,6 times less expensive than AGM

The route with Battleborn is 2,2 times less expensive. Ok. Drop-ins are not very good for yachts but for the same money and for redundancy you can have two Battleborn banks. 

What about the state of art., and one of the most expensive LFP – Lithionics @ Ocean Planet Energy?

They don’t have 400Ah. 

Price for 320Ah is 4600$+2200$(BMS) = 6800$

320Ah x 80% x 3000 = 768 000 Ah; 6800$/768 000 Ah = 0,0089$ for one Ah, 6% more than AGM

Price for 555Ah is 7200$ + 2200$(BMS) = 9400$

550Ah x 80% x 3000 = 1 332 000Ah; 9400$/1 332 000Ah = 0,0070$ – 16% cheaper than AGM!

If someone sees a mistake in my calculations, please correct me. Summary on the picture below. 

LiFePO4 batteries are not only better but also cheaper (much cheaper with a DIY pack). I think that for people that live onboard, that is a complete game changer. I have never heard that someone installed LFP and went back to AGM. For me, there is no way back. 

Let the (electric) force be with you


Battery comp.jpg
Eric Klem

Hi Michal,

This is a general comment that has been on my mind for some time but your comment got me thinking about it. One thing that I wonder about in a lot of cost comparisons is how people count cycles. At one extreme, you have the liveaboard cruisers who are on their boat something like 300+ days a year. If they are sized so that they have to use a generator once a day and have no other significant charge sources, then they may get 300 cycles a year. But I think this is pretty rare, most of these people will have solar, wind or hydro and will only use the entire available SOC range relatively occasionally and most of the time will be cycling over a much more limited portion of the range. Even people who are planning to use a generator a lot likely are only getting something like the equivalent of 100-200 full cycles a year. With 5000 rated cycles, that would be 25-50 years for the heaviest users.

Going to the other end of the spectrum, there are plenty of people working full time using their boats for a 2 week cruise a year and then some daysailing. Many of these people seem to be also going lithium from what I see. These people might realistically have more like 10 cycles a year. Even if these people go for max sailing performance with minimal battery and charge twice daily with an engine, they are still in the range of 30 cycles a year which would be 167 years.

I think we need to include an age check in any cycle life claims. Batteries all degrade with time even with no cycling and the amount depends on the specific chemistry and the storage condition. Electrical requirements also change, in 20 years we may all be charging with a different source, using significantly more energy or running a different bus voltage or something else. This all also assumes you don’t have a fault or make a mistake that destroys your whole pack at some point. These days, electronics are generally quite good but still a failure of part of your BMS can make a pack useless for all but the very technically savvy. Finally, how many people actually keep a boat for multiple decades, I think the number is actually quite low. My guess is that very few people will make lead acids last more than 10 years and lithium more than 20 even in ideal conditions so a lot of people may not be cycle limited but age limited. For truly heavy users, cycles is probably a decent measure but even many full time liveaboards and certainly most seasonal cruisers may find that the math is driven differently.


Rob Gill

Hi Eric,

What an interesting comment – I have been pondering the excellent reporting available in our MasterVolt BMS, by interrogating our bank of 3 x 180 A/hr LiFePO batteries, which were installed in February 2016.

For us, that economic decision was easy, our AGMs and old generator were end-of-life, and not replacing the generator paid for our new system, even without considering the maintenance and fuel savings we have since enjoyed.

Since then, the BMS tells me that we have only used ~330 cycles on our battery bank, and I was questioning this being so little after 7 years. We average something like 80 nights per year on board, plus one full-time six month cruise to the SW Pacific.

I was thinking at this rate of cycling, not only will I be dead before replacing the bank, but our kids might be too…!

From your comment, I think what we are experiencing is the benefit of our solar directly supplying the bulk of our load during the day. At night, our current draw reduces dramatically as we aren’t opening the fridge or freezer and the nav systems are off, unless we are on passage.

So the batteries are only experiencing the difference, plus our reduced winter season usage (but increased battery draw) when we have our bimini and flexible PV panels stowed away .

Thank you for demystifying this for me…!

Eric Klem

Hi Rob,

That is a great data point. I wonder how your BMS calculates a cycle. With batteries that give a relatively constant amount of energy over their life regardless of DOD, it is pretty easy to divide the total energy used by the usable capacity to get something realistic.

Wouldn’t it be great if you never had to do anything with batteries again.


Rob Gill

Hi Eric,

Sorry, can’t help there – I even had to Google what DOD stood for..! I just checked the manuals for the MV shunt and MasterView display, but no insight there.

As you say the BMS knows the capacity of the bank and our usage. Not sure if it is more sophisticated than that, in monitoring and adjusting the capacity in the calculations as the bank ages. To be fair, the conservative minimum rating of 2000 cycles quoted by MV probably allows for degradation of the battery over time, so it may well do.

Our cycle count usage data point, is flattened and flattered by around 50% on account of me interrogating the MV EasyView BMS monitoring system soon after we bought two MV batteries originally. I noticed our new batteries were 28 months old (elapsed time since first charge is recorded in typical thoroughness) and the dealer hadn’t told me they had been in MV stock for a while.

On questioning, we were sheepishly given a free third 180A battery, to act as redundancy for going offshore, which I had planned for space-wise in the electrical locker, but hadn’t ordered as we didn’t need the capacity. So now we can have two out of our three batteries in fault mode and automatically isolated by the BMS, and still have a perfectly useable house bank.

And our experience to date is the batteries have been very much set and forget (and monitor).

Like Michal I’m a fan, but I suspect for different reasons.

Rob Gill

Great point John, we too are “seeing the glow!”

And it’s bad for their displays – Raymarine certainly advises in their operator manuals to turn off and cover up all screens from sunlight when not in use. Our Raymarine cockpit chart plotter mounted in the elements by the helm station is coming up 10 years, and still works and looks like new.

Never given this any thought before, but now you make me wonder if many boaties unused to navigating after dark without instruments, feel if they don’t have their plotter going, they are somehow lost?

I have even been told we are UNSAFE at night for not running our AIS, which is a fair point if we ever had to anchor in a fairway or shipping channel, but that’s certainly not by choice. Personally, I find it counter-productive for small craft to leave the AIS on when anchored – it just creates screen clutter and makes it hard to spot boats that are underway, as we approach an anchorage in the dark – we have a few that we know well and happy to enter.

It might be a helpful post John in your anchoring book, giving some guidance for people in how to create their own situational awareness at dusk, so they feel secure without a chart plotter and instrument bank telling them what they can see with their eyes (and ears) if they know how and have a little confidence to try it.

Perhaps some hints like which bearings will be of no use after dark (or even create confusion). And in remote anchorages on a back night, with no lights, how to pick unlit features ashore that you can find after dark with a powerful torch or searchlight, or will see against a clear skyline. And perhaps when to turn on the instruments and have them running..?

Personally, I enjoy the dusk ritual of getting my situational awareness, choosing my bearings, noting the magnetic compass heading, checking time of moonset/rise. I find it helpful to visualise what things will look like after dark when the wind shifts, or tide turns. And then what is the relative position of other boats around us, and how are they lit so I will be able to figure out if they are moving relative to us.

And I always pop up again before bed, when it’s fully dark, just to lock the situation in my head, if we ever have to get up in a hurry.

Writing this up in the log, then staying out in the cockpit as darkness falls, observing the fading skyline, seeing more lights appearing, listening to the Kawau (NZ shags) arguing over the best perches, these are some of the most magical moments on the water…!

Evan Effa

Situational awareness is good but if you want to sleep at anchor, I would argue that an anchor alarm is pretty essential. We always turn on our anchor alarm which uses GPS units serviced by our NMEA 2000 network.
So, we may not have our chart plotter on but my navigation laptop is always on at night with an anchor alarm set. It has saved our bacon more than a few times & quite frankly, when the wind is howling at ‘oh-dark 40’ I want to quickly know how hard & what direction it is blowing and where we are. If we’ve dragged, I want to know without any doubt.

My NMEA 2000 network with an Actisensense Ethernet adapter, Airmar wind instrument, Vesper XB-8000, Furuno 330 GPS & Airmar heading sensor and a thermometer does not draw very much power at all (< 2A ) but provides excellent / invaluable situational awareness.

Around here we get to listen to the cormorants squabbling over their roosting trees. For that we use our old fashioned senses 😉

Dick Stevenson

Hi all,
I am interested in Evans declaring an anchor alarm being essential for sound sleeping and what others do.
In many thousands of nights at anchor, I would guess, I have set an anchor alarm just in the past couple of years and then, just a few times when things were predicted to get boisterous and hard stuff was close-at-hand. I generally sleep very well, but also seem to awaken to an increase in wind or change in the noise and check things out. Situational awareness is from a telltale compass right over my head. If I am facing the same direction I was when I set the anchor, I figure I am good for a gale.I also have several (if possible) shore lights or the like to orient myself if I happen to go on deck.
I am of two minds about becoming better acquainted with anchor alarms. One, will it interfere with the sense I have that I will awaken if something untoward is occurring: too much reliance on the alarm? Or am I in for an uncomfortable surprise when I sleep through an event that the alarm might have warned me about?
I am not a Luddite, but I am wary of electronic stuff creeping insidiously into my life and tend to eschew the seduction of relying on electronics/tech etc. in all its seductive manifestations.
Random thoughts, Dick Stevenson, s/v Alchemy

Dick Stevenson

Hi John and all,
Yes, having a bearing to the anchorage entrance and orienting lights in memory good practice, but I would probably not choose to bail out of an anchorage at night in a fire drill of some sort with just a bearing.
For those who are as slow in new tech as I am: my chart plotter in the cockpit is great, but takes forever to get going which always concerned me if needed quickly. I am not generally an iPad user so it took awhile to realize that our iPad with its nav system is almost instantly on and with a quick fix.
It impresses me what a significant increase in awareness and confidence going from a few bearings in the dark to seeing the boat icon situated just where I thought in the anchorage. Another way of saying this is that, for me, night-time fire drills where the boat needs to be moved can be impressively disorienting: and instruments help.
My best, Dick Stevenson, s/v Alchemy

Dick Stevenson

Hi John,
I also have never had to bail on an anchorage, but knowing the way out has become something like carrying an umbrella: if I prepare to bail, I will not have to do so.
Yes, agreed about night vision. Our SOP for situations like this is for Ginger to be looking at the iPad while I am at the helm with what situational awareness I can have in the dark and Ginger calling out information from the iPad. This would take a few minutes until the cockpit chart plotter fired up and could be used in its night mode. Even then Ginger would be monitoring the instruments while I drive.
Yours like a good solution, especially for single-handers.
What small plotter is your choice?
My best, Dick

Dick Stevenson

Hi John, Agree about Furuno. I will look. Thanks, Dick

Evan Effa

It’s an Interesting suggestion to set things up to allow the the Vesper to run separately from the NMEA 2000 network. (I do have the external GPS unit for the Vesper & it is probably the most reliable GPS of the lot. ) I will look at a more power friendly setup using the Vesper.

As to the necessity or desirability of an anchor alarm? We dragged at least twice with a Rocna 25 kg (roll bar) anchor. One of those occasions was at 0200 after a wind & tide shift that had us coming within 20 feet of a rocky shoreline that would have destroyed the boat. I was very glad to the have set the anchor alarm.

We switched to a SPADE anchor & an all chain rode in 2016. We have never dragged since but I never want to assume that it couldn’t happen…. We are often anchored in lonely places up the BC coast where there are no other boats, no shore lights & unpredictable wind shifts. The anchor alarm is non-negotiable.

Eric Klem

Hi All,

I too can’t imagine having the electrical draw of all the electronics going all the time. Our NMEA 2000 network draws around 0.6A (12V) and the chartplotter draws a little under 2A so if we left it running all the time and didn’t have stuff like the VHF going, it would not quite double our electrical usage. We do sometimes run an anchor alarm and for this we keep a small standalone chartplotter at the nav station that draws 0.4A when the screen is relatively dim. This also doubles as our backup to our main unit so we have it wired to a waterproof plug with receptacle at the nav station and on deck and mounting points in both places too.

We have tried other methods that I didn’t like as much. We used to have a small black and white plotter above my head in the bunk but I found it too tempting to look and it couldn’t be seen from anywhere else so if the wind suddenly picked up and I decided to get up, I now was electronics blind for a little while getting something else running. I have also tried a smartphone which I didn’t particularly like and at least when I measured the one I had several years ago, the power draw was no less than the small chartplotter. I had not considered using the AIS unit so I will have to think about whether that makes sense in our case. One thing that I do think is important in many instances is not having to spend a bunch of time booting stuff up if you do have to get moving. Unfortunately, modern MFD’s boot a lot slower than ones from 10 years ago but I still find them much faster than computers although computers are a lot better than they used to be. In our case, we can see the plotter down below reasonably well from on deck so can use that while the on-deck one boots. At least the new radar doesn’t take 3 minutes, that used to feel like forever when the visibility went to nothing and there were likely to be other boats dragging.

Dick’s comment about when to use an anchor alarm is certainly an interesting one. The majority of the time we do not but most New England nights are pretty calm, if there is much wind or any other reason to worry like a bad bottom or reversing current, I set one. With any light at all, I can easily see our jib halyard and how it is deflected in the breeze from in the bunk and it is great at telling me our angle to the wind. Our boat sails at anchor a little so I expect to see some movement but if we were to ever be closer to beam on and staying there, we would know that there was an issue. If there is any actual change in anything, then I get up and check ranges, other boats, etc.


Dick Stevenson

Hi Eric, You remind me: my “alarm” is my line to raise the inboard end of my pole which, at anchor, slaps when the wind gets much above 25kn or so. All efforts to quiet it have so far failed and I do not mind the wake up call. My best, Dick

Marc Dacey

This is what we’ve found by “going big” with lead-acid batteries. We can stay at high states of charge because our usage patterns rarely dig down deeply before we are back to pumping in amps. “Cycles” is so ill-defined as to be meaningless: if you draw down by boiling water or vacuuming at noon when solar is peaking, is that a cycle? Or pretty much a wash? I draw more deeply on passage, but that is also when we tend to motorsail more in coastal mode, so the big radar and related nav devices don’t really hit the battery bank the way they would if we were just sailing at dark o’clock. It’s too ambiguous a measure.

Stein Varjord

Hi Michal, John, Eric and Rob,

Good points, and seen from different sides.
There’s no doubt that a good lithium bank treated right will live way longer than any lead acid bank, and that this will influence the lifetime cost a lot. At the same time, we must accept that a quality LiFoPo4/LFP bank on a boat will most likely not die because it has reached its maximum cycle number. Rather, it’s way more probable that it dies from:
1. Over charge or over discharge, resulting in sudden death. (Slow over charge is reportedly the most frequent reason for premature death of LFP.)
2. Age, accelerated by hot environment, constant high charge level and more.

Since lead acid banks are also often mistreated and die far earlier than they should, the difference in expected cycle numbers is probably still fairly realistic, albeit with lower numbers for both. It seems like a realistic guess could be that a good LFP bank lives about three times as many years of similar use as a good AGM bank. (Flooded lead acid banks live at least twice as long as AGM, tolerate far more abuse and are cheaper… Gel also live at least twice as long as AGM, and cost roughly the same, but are not tolerant to abuse.)

In the data sheet above we can notice that Winston has by far the highest number of lifetime Ah. 16 million, while Victron is at about 9 mill and Lithionics in between. This is especially interesting, as the three all use the same Winston cells. The numbers should have been the same. Reading the white papers from Winston will reveal that they tend to name the true achieved limits, while not necessarily emphasising the conditions they are using. Victron on the other hand, tend to report levels that are quite conservative, to avoid complaints. Still, they’re all relating to lab conditions, which are not the same as multi year boat use, especially in that aging doesn’t interfere and that the cycles are quite high draw and done in short order.

Comparing between traditional battery banks and lithium when applied on an existing boat will also give a pretty different cost calculation than mentioned, if the system is done right. Just exchanging the lead acid bank means that’s it. There are no system rebuild costs. Changing over to LFP will imply some significant costs in replacing much of the electric system. The battery bank itself may be far less than half the cost, if the system is done right. (By going drop-in, we can fool ourselves into believing we got it easy…)

As Eric mentions, from cycle life we can calculate perhaps 50 years of service life from our LFP bank. However, that’s not realistic, unless we look at LTO (lithium titanate) cells, which for several reasons is a poor choice. LFP cells can indeed live long, but probably not much more than 20 years of actual use, no matter how they are treated. They do age. “Dendrites” is a key problem. Since the tech hasn’t existed long enough yet, we don’t really know enough, but one bank belonging to Rod Collins of has some actual numbers. That’s Winston cells and it’s been run extremely hard, for testing purposes. That’s his business. It’s capacity is still higher than its rated capacity and it’s about 14 years old now, past 3000 cycles if I remember correctly. It seems likely that 20 years will not be a problem, but it can also suddenly fail.

Either way, to benefit from the long life expectancy of LFP, we need to push the envelope some. A boat that is normally plugged in at night or runs the engine most days anyway, will probably not notice a benefit from getting LFP, unless the boat regularly has an extremely high power consumption. A boat that regularly has extended periods at anchor or under sail, mostly charging from solar, will certainly notice! We can all crave for the new cool stuff. In this case it’s the wrong choice for the majority of normal boaters. For the group on AAC, it’s perhaps a good choice.

Stein Varjord

Hi John,

I forgot to specify that I was thinking about liquid filled traction cells. They’re normally in 2 V single cells or semi traction in 6 V batteries. They are far more durable (hand heavier) than standard liquid filled batteries available in normal boat shops.

Michał Palczyński

Hi Eric, Rob and Syein
I was also wondering how BMS counts cycles. I think that for my REC one cycle is when the sum of partial discharges reaches 100%. When I’m on a boat using battery heavy my average on BMS is 11-12 cycles during one month. I have a 700Ah battery bank and my average DOD when I wake up is in range 200-300Ah. Most of the days solars take care of loads and they charge the batteries. So three 30% partial cycles make one cycle for BMS.
12 cycles in one month are less than 150 cycles during one year. So even if that 5000 cycle life is too optimistic and my batteries will last only for 2000 cycles it would be 15 years of full time cruising!

So Stein is right. most probably they will be dead not because of the number of cycles but because of their age, or because I make something stupid 😉

For me switching from AGM was not very expensive. I had already Victron programable solar chargers and Balmar external regulator with temp sensor. I know that wakespeed is better but I didn’t wannt to spend more money and with balmar is still possible to tune all variables right. So except the cells 90% of the cost of switching was BMS, two conntactors and couple of meters of thick cables. The cost of the system can go crazy if you hire someone. Especially in US where labor is extremely expensive.

And of course, I agree that the benefits of lithium are for peole who spend more time off the grid than connected to shore power. More off grid, more benefits. For those who make 10 cycles a year, LFP is waste of time and money.

Marc Dacey

As I did not see the keyword I was searching for, I will give another viewpoint. 1) I think lithium is wonderful, particularly for lighter cruisers. 2) We are sticking with quality lead-acid, because our boat is steel and we’ve built our battery compartment pretty much directly above the keel tanks and directly below the mainsail’s center of effort.

That’s right, it’s about 330 kilos of “internal ballast”.

This has made our boat actually stiffer in a blow. The six L-16s we’ve had for the last seven years have been babied, rarely if ever below 75% SoC, and watered and equalized on a schedule. The addition this winter of a new wind generator charging source on top of the four solar panels should not charge the range at which the house bank stays charged via the Victron setup, the Outback MPPT and the simple alternator, which is easier to embiggen elsewhere (and to add that slick Wakespeed regulator!) if that becomes necessary or preferable in some foreign harbour.

If I had a light performance cruiser, I would absolutely go lithium, and top-end, too. But in contemplating a circ, getting lithium shipped to exotic repair locales is still problematic, as far as I know, in ways that lead-acid batteries are not (save for the weight of the freight). I would also have a “wall” of lithium for an off-grid house, or, if they exist in five years, some of the next wave of battery chemistries.

I am only switching up our existing house bank because the batteries are long in the tooth, not because they are showing any signs of failing.

So we are in the odd position of liking a big tray of lead under the saloon stairs. If we didn’t have the boat we do, I would hold quite different opinions and am ultimately agnostic on the topic.

Robert Cart

I have not read all the comments so this may have been covered but I believe you are dead wrong with your whole hog lithium advice. The hybrid approach is the only practical answer to lithium. You can make the lead acid bank small but you need it for many reasons. You still get the benefit of Lithium and the cost and complexity is well worth it. Boats going all lithium are struggling. Your opinions and advice are usually more thoughtful. This seems more like bias. Best to speak with some people who know about such matters before making bold and incorrect assertions.

Jesse Falsone

I recently designed and installed a LifePO4 system in my boat. Absolutely more expensive because of the added equipment necessary for a reliable system but, at least in my case, the economics were better than you have presented here if you consider the longevity and assume you are keeping the boat for at least the life of good lead acid batteries, properly treated. First, the lithium batteries I installed were not appreciably more expensive than the really good AGMs I had. In fact, the prices were nearly equal (about $600 for each of three Kilovault HLX+ 100ah batteries to replace the Odyssey Grp 31 AGMs, one of which I repurposed as my new start battery). And the Kilovaults are UL1973 approved unlike many other top brands (we can argue over the merits of different ULs but Calder says 1973 is what you want). Since I already had “smart” charging equipment in the AGM system, my adds were a DC-DC charger, a T-Class fuse block, separate pos/neg terminal blocks, and larger 2/0 cable. I also threw in a battery isolation switch. There are some hassles to the system, like the need to remove the lithium batteries for storage because of the cold winter temps. I got a fuzzy answer from Kilovault but it seems the best course of action is to remove them in the winter and place in storage mode. I’ll place an old AGM back in the system for lights and bilge pump when I’m aboard on the hard. So, whether I keep this boat for the long haul or replace before the economics of the switch evens out, my view is that I tackled a tough technical problem and learned a lot in the process. This will make the decision and the process of going lithium on my next boat far easier.

Jim Schulz

John, how would you define the difference between a lead-lithium hybrid system (which you say to avoid) and a lithium system with a lead-acid backup for critical systems (which you describe as seamanlike)? I’ve read your article here but I’m still not 100% sure I understand. Is it just the way the system is designed ie in series not parallel? Or is there something else I’m missing?