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. 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. Do You Need A Generator?
  22. Efficient Generator-Based Electrical Systems For Yachts
  23. Battery Bank Size and Generator Run Time, A Case Study
  24. Battery Options, Part 1—Lithium
  25. Battery Options, Part 2—Lead Acid
  26. A Simple Way to Decide Between Lithium or Lead-Acid Batteries for a Cruising Boat
  27. Why Lithium Battery Load Dumps Matter
  28. 8 Tips To Prevent Lithium Battery Load Dumps
  29. Building a Seamanlike Lithium Battery System
  30. Lithium Ion Batteries Explained
  31. 11 Steps To Better Lead Acid Battery Life
  32. How Hard Can We Charge Our Lead-Acid Batteries?
  33. How Lead Acid Batteries Get Wrecked and What To Do About It
  34. Equalizing Batteries, The Reality
  35. Renewable Power
  36. Wind Generators
  37. Solar Power
  38. Hydro Power
  39. Watt & Sea Hydro Generator Review
  40. Battery Monitors, Part 1—Which Type Is Right For You?
  41. Battery Monitors, Part 2—Recommended Unit
  42. Battery Monitors, Part 3—Calibration and Use
  43. Battery Containment—Part 1
  44. Q&A—Are Battery Desulphators a Good Idea?
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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…

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 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 https://www.victronenergy.com/chargers/phoenix-smart-ip43-charger 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.

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.

Mal L

Drew I think its worse than not charging below 0ºC as that’s where Victrons (for example) limit used to be, they changed that a while ago to +5ºC. Yes you can get those heated batteries but that’s adding more components to the system and i feel it’s like robing peter to pay paul to charge your batteries! I’d often thought if i were sailing in cold climates for long periods i would not choose Lithium.

I have just inherited a lithium bank in my new yacht, 8 x 100Ah 12V they’re a budget priced ‘drop in’ VoltX, never heard of them before so i will find out more about them.

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.