The Offshore Voyaging Reference Site

Battery Options, Part 2—Lead Acid

Update 2024

While I still think carbon foam technology has huge potential, I have been hearing credible and disturbing accounts of quality control problems and premature failure and therefore no longer recommend Firefly carbon foam batteries.

In Part 1 of my evaluation of battery options to replace the aging house bank on Morgan’s Cloud, I took an in-depth look at lithium batteries and concluded that they were not right for us.

So that left three types of lead acid batteries in the running:

  • Carbon foam (strictly speaking these are a variant of AGM, but let’s keep it simple).
  • Liquid filled
  • AGM (the incumbent)

As in the last chapter, for the purposes of this discussion I’m assuming a 12-volt 800-amp hour house bank, the size we now have on our boat and a reasonable average capacity for many cruising boats.

Let’s dive in:

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Dick Stevenson

Hi John,
One of the appeals to me of gels is that they do not need or demand being equalized to give good to very good service. I have been using gels for 25 years now and have never suffered sulphation problems which, short of cutting open a battery, I suspect would only be shown by a shorted cell. One of the appeals to me of gels is that I install them and more or less forget about them. Not having to equalize for me is a huge bonus. I get 5-7 years on a battery bank.
My best, Dick Stevenson, s/v Alchemy

Dick Stevenson

Hi John,
By the way, I want to note how much work I suspect went into this really superb survey (the whole series of battery articles). I suspect many of your articles have emerged from deeply thinking about everyday life on a boat, but an article such as this battery article must have had you in the “library” for long hours doing research. Your efforts are appreciated.
And I agree with your comments about gels. For a while I kept data records on usage (when I got my first battery monitor), but that got old pretty quickly and once you miss a few recordings of data, all gets suspect.
In any case gels are a compromise that works for me: gels allow me to not worry about attaining a full charge at those times when it is a challenge to do so. Nor do I have equalize. That and their at-rest slow discharge rate and tolerance of occasional abuse has kept me in the gel family. The compromise: I suspect I replace them more quickly.
My best, Dick Stevenson, s/v Alchemy.

Max Shaw

Another great article John. We were starting to go through these deliberations last year with a hope to replace the house bank in a slow, deliberate manner when we arrived in New Zealand. This was cut short as the batteries rapidly started dying on the passage from the Marshalls to the Solomons. By the time we got to Vanuatu it was clear that we just needed to replace the batteries and keep it simple as there are limited options even in Port Vila.

We put in flooded 6V batteries again this time going with Trojan due to their good reputation and that ever import criteria of availability. This time though we treated ourselves to the Trojan Hydro-Link system as half of our 980 Amp-Hr (at 12 V) is under the aft cabin bunk. The Hydro-Link was easy to install and easy to use. No problems so recommended so far based our usage over the last six months.

Thanks for all the good work with the website.


SV Fluenta
Presently Tauranga, NZ

Serge Paul

Hi John, unless i ma All mix-up i thinking Thérèse is a mistake in your spread sheet for The cost. The Banksy gour for surette and Trojan should be 500 and not 1000. Pleasure tell me if i am Wrong.


must also correct paragraph re costco batteries. 800 AH will require 8 batteries not 4 so cost would be $800 not $400.

chris Groobey

Great article as always. I want to second the concept of separating batteries into redundant banks.

Our FPB64 was originally fitted with a bank of 12 two-volt batteries made by a German manufacturer that did not keep stock in the US and had an 8-week lead time for replacement. Our batteries needed to be replaced when we bought her and we learned over time that multiple FPBs had suffered failures of individual cells, thereby (as you say) impairing the entire bank. So that manufacturer went off our list pretty quickly, even though it was the drop-in solution and we were wary of moving away from the boat’s original specs given the thought that had been put into them by the designer and builder.

After doing much of the same research you describe, and due in part to your earlier recommendation of Lifeline AGMs, we contacted Lifeline and eventually met with them at a boat show. They had excellent (and patient) customer service and it was they who suggested that we change our configuration from 12×2 to three banks of 4 six-volt batteries. Their view was, look, our batteries don’t fail but, if one does, you will still be good to go, plus our six-volt batteries are easier to find than two-volters if you ever need a replacement. Lifeline’s form factor was such that their batteries had the same footprint as the ones we were replacing, but were shorter, so they were essentially drop-in as well.

So we took their advice, did a minor cabling reconfiguration and now have a battery system that we are thrilled with and that is as redundant as all of the other systems on our boat.

We too considered Firefly, even meeting with a member of Bruce Schwab’s team, but they only offer limited sizes and we would have needed 20+ of their batteries to reach the capacity we wanted. Plus we had heard the same quality control, multiple changes of ownership and limited availability concerns you have mentioned. It will be great if/when they get their act together.

We also considered liquid but between maintenance (our batteries are hard to access), the risk of spill (not good on an aluminum boat) and intolerance of vibration, we decided against them. But another FPB owner has gone that route and is very happy. They went with golf cart batteries on the thinking that they are cheap to replace and can be purchased anywhere.

It might also be good to mention the importance of updating the settings on your alternator regulator and inverter/chargers to match the requirements of whatever batteries are installed. Lifeline provides decent information on what settings to use, although not as specific as we would like given all the settings available on our Balmar and Victron equipment that, I assume, if matched to the batteries would provide even better overall performance.


My thanks as well for providing this experience Chris and wondering if you would be willing to share more details of what caused the Hoppecke 2v cells to fail in your FPB64? You may not know if they were already shot when you found and bought the boat but if you have any additional details on this I would be very interested to know as I’ve been considering using these batteries on a new boat my wife and I are building that is in the same family of boats as the FPB’s. I don’t recall if they were using OPzV (Gel) or OPzS (FLA) batteries in those first FPB64’s? And were all 12 of the 2v batteries failing or just one or two?
Thanks for any additional insights you can provide on these 2v batteries Chris and if you’d prefer I can contact you privately to discuss further.


Chris Groobey

Hi Wayne,
Thanks for your note. I’m happy to help but need a day or so to get back to you, we have a long passage tomorrow to Montenegro that means an early bed tonight and most of the day tomorrow offline. I clicked through to your webpage but (likely intentionally) did not quickly find your email. Mine is moc.yeboorg@sirhc, look forward to hearing from you. –Chris

Dick Stevenson

Hi Chris, Way off topic, but a couple of thoughts that were wonderful for us: anchor off Kotor for an amazing anchorage and rent a car to go up the mountain and inland. Just a fabulously beautiful country.
Enjoy, Dick Stevenson, s/v Alchemy


Hi Chris, thanks for the quick response especially while you are in the midst of heading out. Enjoy your passage to Montenegro and if your travels happen to take you further East to the beautiful coasts of Turkey we are building our new boat in Antalya and we’d love to meet up with you and Carolyn.

Please take time to enjoy your passage and there is no rush with your responses to my questions, would just appreciate learning more from your experiences with the original OPz batteries as well as your new setup.

Thanks too for the reminder that our Emails are not readily available on our Mobius.World blog. Quite unintentional and we will fix that shortly.

Easiest Email for us is dlrow.suibom@enitsirhc.enyaw and I’ll send you a short note from that Email so you have the connection.

Safe and fun travels, Wayne

Gino Del Guercio

I installed four Fireflies a year ago and have been cruising full time with them. So far I’ve had no problems. One thing to note though, while you can drive them down to 20 percent, most boat electronics don’t seem to like voltages below 12, so realistically you can’t really go below 50 percent. The issue is mostly irrelevant for me since after installing a big alternator and 340 watts of solar on top of my davits, my charge rarely drops below 85 percent. Bliss.


I too have the Firefly batteries: (6 x 110 Ah) in a single house bank in a Nordic Tug 37. We do a lot of winter cruising with shorter days and often little solar input from our 550 Watts on the pilothouse roof.

The Fireflies seem to be working well but like Gino, I have noted that once in discharge mode they drop their voltage fairly quickly to around 12.75V and then have a flatter, slower discharge from there. (cf with our old LifeLines or Chinese AGM Traction batteries)

IIRC, our deepest discharge to date has been ~ 340A with a Voltage of ~12.2V.

I would be not want the voltage to drop below 12.1V, for the same concerns around the health of the electronics, so the the true useful capacity is probably only down to 50% SOC. It’s reassuring that the bank will not be damaged by a deep discharge but by then I would be worried more about my far more expensive electronics than the battery bank.

On the positive side, they will accept a charge very quickly and are supposedly happy in a partial state of charge so that I do not need to be concerned when at the end of the day, solar or a short generator run only recharges to ~ 80% SOC.

These are expensive. The real cost and the ultimate advantages or disadvantages will be more apparent when we have a few more years of use and a good sense of their ability to maintain their performance.

Gino Del Guercio

An update on my Firefly batteries, now going on four years. I recently installed the new Balmar SG200 battery monitor, which includes a state of health reading. It indicates my batteries are still at 100 percent state of health and I have no reason to believe this is not correct. The Fireflies are still going strong and we’ve been cruising full time since we installed them and almost always anchor out. I wish all my systems worked as well.

Gardiner L Schneider

All this talk of esoterics is interesting; however, for price and rugged construction it is hard to beat 12v 8D lead acid batts. All countries have diesel trucks, most diesel trucks use 8Ds. Trucks have lots of vibration and motion, more than many boats. We usually find they last for about six to eight years and charge well off of our 280 watts of solar over the hard Bimini. We turn our electric fridge on in the Spring and do not turn it off again until we pull the boat in the Fall, unless we travel South. Even left for a week at a time, the batts are still up upon our return. At about $200 a piece, 8Ds work for many of us.

Marc Dacey

These sorts of considerations, and the fact that we have excellent access for watering and related maintenance right in the middle of the boat, led us to the L-16 (floorsweeper/forklift) 6V format from Crown in series-parallel. We got the type used in off-grid solar installations on racks, at 55 kilos each, they are not for the squeamish. Still, they are excellent to date and this form factor, while not as ubiquitous as the 8D diesel battery, is far more common than a more exotic choice.

We went for this option on the basis of our existing access (under the saloon companionway stairs), the utility of having that mass as internal ballast under the mast and on the centerline (less plate area uncovered on a heel), the short runs to buss bars and the alternator, and the bank’s sizing to operate within a relatively narrow and high SOC range of 70%-90% based on usage and charging sources. As you point out, one’s tolerance for batteries with a technological edge is dependent on many factors; we went with “easily fixable in foreign ports”. That said, I have no objections to AGMs on ACRs off the house bank to keep start and windlass batteries topped up, and Lifelines now have a track record to recommend them.


Last fall we replaced four 6V interstate golf cart batteries that were 7 years old with new Trojan (T-105) golf cart batteries and a remote watering system. The self watering system makes the task of topping off the water in the batteries very easy to do and only takes a few minutes. It fills each cell up to the proper level without spilling, without having to move the batteries to remove the caps, or having to remove the caps, making it safer to add water when needed. So now there is no excuse to do proper maintenance on the batteries on a regular basis. If someone goes with liquid filled lead acid batteries I would strongly recommend investing in a self watering system. We did consider the Firefly batteries but the cost was a factor, same for lithium. The golf cart batteries were chosen because it was a battery I could lift, move, or replace if necessary without help, in other words weight was also an issue.


Hi John

I think if you have found with something that works for your situation stick with it.
I replaced my 13 year old lifeline agm’s last year, not because they had failed but I felt it was a sensible time after upgrading all the wiring and adding solar panels. I have owned my Island packet for 6 of the 13 years, my demands are less than yours and I only cruise 4 – 5 months a year. I found your report on lifelines at the time very useful.
I’m still using the 2002 gpl27 starter battery. I have never equalized any of them yet!

SC Brown Bear

Marc Dacey

When we bought our current boat in 2006, it had a pair of Trojan T-105s installed in 1988 aboard as a (rather modest) house bank. While I think the boat was on shore power and therefore charging most of the time by its first two owners, that’s still an enviable value for money. I used them for about six months until I realized they weren’t really getting above 75% SOC anymore. But they never failed the crank the diesel or light the lights.


Hey John SC Brown Bear,

What climate do you store/use your boat? I ask because Lifeline AGM’s are heat sensitive, regarding overall battery cycle life.

“Battery life decreases by 50% for every 10 degrees C rise in temperature” from Lifeline Manual.

David Pyle


“My preferred option would be six of the four-volt batteries split into two banks of three, each yielding 900 amp hours.”

Unless I’m reading this wrong shouldn’t that be 2 banks of 450 amp hours each, totaling 900 aH.

Roger Neiley

Thanks for a very thorough article, as usual. On our Saga 43 I got 15 years out of two 4D Lifelines before noticing any significant decrease in capacity. Replaced them 3 years ago with two 8D Lifelines which are fed primarily by three 120 watt SunPower rigid PV panels. With the boat on a mooring here in sunny So Cal, the batteries are almost always held at float charge levels which seems to be good for them.
When you’re ready for solar, suggest you check out, run by Tom Trimmer. Fair pricing on high end products and superior service in my experience.

Keep your real world info coming please!

Roger Neiley

Jim Rabkin

Hi John
We have bog standard lorry type batteries which i put in three years ago. But the difference is that they are sealed and i cannot top up fluid levels. I have a small altenator which i suppose will not gas them up but means they will not spill acid either
Thanks Jim

Eric Klem

Hi John,

Well reasoned analysis. I like the GC2 size 6V flooded batteries for our use but can see the appeal of some of the other technologies. The first electric car I built used 24 Trojan T145’s in series and watering those was a real pain but the 4 that we have on our boat are not too bad. Keeping our boat on a mooring, we didn’t get great service life out of batteries until we got solar. Now I tell all my friends to at least get a 30W panel (most people seem to have only ~200 ah capacity) and put it somewhere not so much to do the bulk of the charging but to bring the batteries back to 100% while they are away and equalize them.

The firefly batteries are really exciting and something that I have been watching to see how they work out. Since we get ~8 years on a set of batteries, it is not clear how much benefit they would have to us but for people who do constant PSOC usage, they are really interesting. Maybe someday when you buy a used boat you won’t have to count on needing to replace the batteries.


Bill Wakefield

More good stuff, John. Thank you.

I believe Carbon Foam or Smart Carbon will likely be the primary contenders our next time around.

We went through a similar decision making process when we bought this boat in 2014- knowing the house bank [8- Trojan T105 6V comprising a 900AH 12V DC bank] was on its last legs [after ~6 years full time use by the previous owners, and ~2 years of dockage at the brokerage where they were allowed to go completely flat several times…]

The short version is we replaced with same- partly so we didn’t have to make any infrastructure changes at the time, and because they were readily available where/when we decided to take on that project.

That is when I learned about the remote watering systems from online research. Like yours, our bank is not conveniently accessible, so discovering remote watering systems tipped the decision back to FLA from AGM- saving ~40% of the cost at that time. [It is also worth noting that- at that time- the Trojan dealer was unaware of these remote watering systems- even the Trojan Hydrolink system we chose.]

It takes me about 1 minute [2 if I’m distracted…] to grab a jug of distilled water and the squeeze bulb watering assembly, connect to the quick connect fitting leading to the bank, squeeze the bulb [like priming an outboard] to water all 8 batteries, and put the water and filling assembly away. Highly recommended for those choosing FLA technology- accessible or not…

Below is a link to a write-up [with photos and relavent links] of our less than exemplary house bank replacement if anyone is interested. [We reused the utilitarian arrangement that came with the boat to make short work of replacing the house bank.]

Cheers! Bill

Carl Howe Hansen

Thanks, John, for your timely discussion on a timeless subject. On the subject of outfitting a cruising sailboat, I think batteries have dominated my limited research time. I am on the tail end of restoring Destiny, my classic Phillip Rhodes designed sloop. I am far from wealthy, so cost has always entered the equation. Being a extensive refit, space was not a concern. Then, Ah capacity and charging ability deserved consideration as I have 400 watts of solar panels (and that’s it, forever…) .
Full disclosure is necessary. During my research I was contacted by Firefly to see if I would sell their batteries to my clients in the residential solar field. Although information on the technology is somewhat limited, the source of that information ie. Nigel Calder, Practical Sailor and others, produced a unanimous opinion that the technology is sound, but unanimous opinion that the company, Firefly, was experiencing growing pains. New ownership, financial strains, relocating manufacturing to India, and delays and more delays, have contributed to a less-than-confident feeling within the battery community.
I met with Bruce Schwab at the Maine Boatbuilders Show this spring, and he echoed some of the frustration with the delays, but was quite enthusiastic about the future of Firefly.
So is Nigel Calder and many others. And that is good enough for me.
With that in mind (the technology is proven), I decided to order some batteries. It took three months, but my batteries arrived this week along with assurances that the company is catching up with orders.
I’ll add my two cents worth to your ongoing discussion as I put Firefly to the test. Quality control will be determined over time, but, unless another technology surfaces before QC is proven, Firefly seems to be the way to go.

Carl Howe Hansen

Rex O'Regan

With the liquid filled batteries is there much concern of leaks while underway? I thought that the caps have a hole drilled in them which would run the risk of electrolyte getting sloshed out in a heavy sea, or even worse in a knockdown.
Or is my imagination worse than reality and in the real world it is such an insignificant issue that this isn’t really a concern?


Hey Rex,
We have about 10 years of Interstate 6v Golf Cart battery experience in cruising boats (which has been 3 battery banks). Generally, our experience is their leak-y-ness isnt a problem, but they always do leak a little. Everytime I water them or check them or replace them, they have either a little residue on top from leaking (like a little dried-white circle around the caps) or the sides of the battery are a little wet, or rarely a few drops around a cap, or a crusty-ness or moisture underneath the batteries. I am not sure what has caused this, as I am not sure of any over charging situations, but I would 2nd the recommendation to secure the batteries in a drip-tray or battery box or something to catch a little drips that I have always noticed.


Bill Koppe

My previous yachts have all had Lifeline .and lasted around 3-4 years.
This time in build I bought 15 group 31 from Bruce Schwab.
After sitting for almost 2 years all batteries showed 11.75 V except 1 at 10.6.
I now have Enerdrive smart chargers permanently on.
Bruce was very helpful in checking to see the house bank of 12 could cope with the large loads the Andersen 110 electric winches would give.


This year I’ve been on a delivery of an old Melody 34 which had three liquid filled Lead Acid batteries as its powerpack. To my astonishment these batteries were 10 (ten!!) years old and still had measurable 100% of their capacity. The owner had unplugged the landbased charger completely and relied solely on the solar panels and intermittent charging from the engine generator.
I did no extensive tests on this system but what I found was that the consistent top-up from the solar panels was usually sufficient to keep the batteries charged. The owner uses the inverter to power his notebook, and even the radar was on from time to time, so the batteries were at roughly 60% in the morning, after two or three hours of daylight they had recovered to 75%, being fully loaded in the afternoon, mostly without engine as we were lucky to be able to sail most of the time.
I did not believe this story but found it to be true in the course of the delivery. Unfortunately I forgot to check the make of the solars, but it seems clear that a permanent and vivid top up is key to battery longevity.


Actually this is a liveaboard boat, the owner is on the boat at least 10 months/year. There is no shore power charger in use, everything is done using solar and engine gen.
Looking at his usage I doubt the batteries are ever discharged below 50%, but then again seldomly at true 100%. I asked him about his equalizing strategy – he had no idea what I was talking about 😉 He is “just using” the batteries and simply keeping them topped up and recharged, meticuously.


Hi John,
Very nice series on batteries.
Was wondering if you considered TPPL (Thin Plate Pure Lead) offered by Odyssey and others as an option for lead-acid? Nigel Calder was a fan before the Fireflys came out. We’ve been using them for 5 years of cruising with good results. We have a 700AH bank and plenty of solar, so our usage profile is fairly gentle. They tolerate full 14.3V charging at any charge state, and supposedly will take 80% discharges without damage. The advertised ability to take as much charge as you can give them we’ve found to be untrue, as internal resistance limits charge acceptance rate similar to other AGMs. The company does not encourage equalizing, which may rule them out for you. After reading about Nigel’s fairly successful efforts to revive an older bank of dead Odysseys, we decided to charge them more aggressively when power is available, as they seem to tolerate that well. So far so good. They do a good job retaining their capacity even when abused. The story we are told is that army tanks use them, and when the generator is running they have a target-able heat signature, which they’d like to avoid, so when stationary they they run on the Odyssey batteries until below 12V and the electronics start shutting down, then start the motor to recharge. Apparently they tolerate that abuse pretty well. The DOD’s battery budget is way higher than ours so we treat ours as gently as we can so they’ll last longer. After 6 years of cruising they are going strong. We would buy them again.

Paul Koenig

Another Odyssey user, but not a cruiser. Installed 4 x G31 batteries for my house bank. My boat, a ’05 Jeanneau SO40.3, has limited space and has molded spaces for 4 G27 or G31 batteries. The boat is also in a sailing club in the SF Bay Area and is used for daily as well as weekend charters and spends most of the time on shore power. The Odysseys were installed in Feb 2016 for the 2016 Pac Cup race from SF to Hawaii. The stock charging system was swapped out for a Grasser small case alt, ext rectifiers, ext regulator and a serpentine belt system. During the race, our daily usage was about 165-170Ah or 45% of rated capacity. We ran 14 days getting to Hawaii and 16 days coming back going from 80% SOC down to 35-40% SOC, charging back to 80% using the alternator. In Hawaii we were able to charge to 100% on shorepower.
One of the reasons I went with the Odyssey TPPL was the acceptance rate, which for my batteries had a recommended charge rate of 40A / battery or 160A for the bank. So far the bank is holding up fine, with the Balmar SmartGauge still showing 100% SOC for the batteries. I have not done a 20hr discharge test to truly measure the health of the batteries. We are doing the race again this year, but have added a 600W Watt&Sea to charging under way.

Philip Wilkie


This series has inspired me to go off and do a lot more reading around the whole topic of batteries and energy storage in general; in that respect thanks for generating that spark of interest. The very old Nickel Iron batteries I mentioned earlier are one point of interest I’ve encountered along the way.

At the other extreme it’s clear that new battery and supercapacitor technologies are being researched heavily in Universities around the world. Australia has a real lead in super-capacitor research and application, we are already seeing several local companies successfully commercialising some exciting new ideas. The most promising area seems to be a hybridising of Lithium chemical storage with graphene capacitive storage. I have to admit that some of the documents I’ve downloaded make my head hurt on first reading; but there does seem to be some genuine reason to think that within five years or so we will see some commercialised products that successfully combine the lifecycles and robustness of super-capacitors and the energy density of lithium.

Here is one product that has been recently released that packs in 3.6kWhr of storage in a compact unit for around U$3500:
It’s generated a fair bit of controversy locally because it may well be based on Lithium Titaniate and not Supercaps as advertised; but the real and proven performance is pretty impressive all the same. It’s indicative of what’s possible and what we can expect to see in the next decade or sooner.


Stan Creighton

I am putting Lifeline AGM batteries in a new boat now being built in New Zealand. 1800 AH, and it’s going under the main berth mattress. The boat is all aluminum as is the bed frame and battery enclosure. Looking at this a few days ago it occurred to me that there may be a health hazard sleeping right on top of so many batteries. I’ve looked pretty extensively to see if this is the case, but the literature varies from “nothing to worry about” to “it will certainly hurt me”. I have the option of adding an aluminum top to the battery enclosure which would create (I think) a type of Faraday box to act as a shield if needed. It seems like there is a difference with electromagnetic forces from AC vs DC sources as well but this, too, is confusing to me. Do you or your readers have any real world knowledge about this and shielding?
Stan Creighton MV Buffalo Nickel

Stan Creighton

John, I found an electrical engineer/ consultant on electromagnetic fields (EMF’s), testing and health risks and recently had a good telephone consultation with him about my question of locating batteries under the bed on our boat, see post above and
The short answer is that DC power sources or users produce little or no EMF, so batteries under the bed are fine. AC is the source of most EMF with inverters and the genset being the largest producers, but any AC user can produce EMF. There is a magnetic component and an electrical component to EMF and any testing should test for both using the appropriate gaussmeter. He says he is familiar with my question because many RV’s put batteries AND inverters under the bed, and that could be a health risk. Additionally, it seems that by 10 feet, most EMF dissipates so that is his rule about locating AC equipment near a bed or area where one might spend a lot of time. Of course on a boat that could be problematic, but I don’t worry about this stuff in general as we live with constant exposure to all kinds of bad things. That said, if I have a choice about locating AC components in the future, the material I learned from this exercise is stuff that will at least cause me to stop and think where to locate it. My inverters are fortunately more than 10 feet from the bed. FWIW.
Stan Creighton
MV Buffalo Nickel

ANDREa Jasson

Hi John,

my battery/charging configuration makes me wonder what to do, now that I must replace my very old wet cell batteries (for either Gel or AGM).

I currently have a 400ah house battery bank and one engine battery. The way the alternator charges both banks, is via a “smart” voltage-sensing rele’: if the alternator is running, it will first charge the engine battery. Once a certain voltage is reached, the rele’ will click and put the engine battery in parallel with the house bank. The opposite happens when shorepower is connected: the battery charger charges the house bank first, and when a certain V is reached, the engine battery gets put in parallel with the house bank.

This gives me great trepidation when it comes to choosing batteries because when I operate the windlass (typically just after turning the engine on), the house and engine batteries are in parallel –> the windlass draws from engine and house batteries as well (to my horror, I just noticed this! ).

I also have a dog, hence hoover running on a 1200W inverter, which draws about 90-100 Ah… quite a bit.

Do you think this charging setup is normal?
A local electrician keeps telling me AGM’s a not good, Gels are way better. I thought AGM is better for rapid discharge (as is the case with the windlass running on the house bank, when house and starter are in parallel). What do you think?

Many thanks

Bob Buck

Last fall at boat show time I began looking for replacement batteries for my house bank, which consisted of six Lifeline Group 31s wired for 24 volts. I have used Lifelines for many years and have been happy with them. I estimate that I got about 1200 cycles out of the old bank, although I admit I try to be careful about how I treat my batteries. Still, I was hoping to find something better. I briefly considered Lithium Ion, but the price tag for both the batteries and the necessary system upgrades put me off that idea. Safety, was also a significant consideration.
I was intrigued by the Firefly carbon foam batteries after reading an article by Nigel Calder, but two Firefly dealers at the Newport Boat Show said they couldn’t recommend them due to quality control issues. My boat yard also said they’d had issues with Firefly batteries, although they also said the company promptly replaced any batteries that had problems.
In the spring I was about ready to order another set of Lifelines when one of the Firefly dealers called be to say they thought the quality control issues had been resolved and that they could get me six Group 31s before my launch date.
I was hesitant, but ultimately decided to give the Firefly batteries a try. After one season of use, the Fireflies have exceeded my expectations, which is to say they have performed as advertised. The quality control problems I’d been warned about have not materialized (leaking at the posts and vents, and failure within the first month). For me the true test of any deep cycle battery is their longevity, and only time will tell, but so far so good.

Matt Marsh

I’m becoming increasingly comfortable with recommending the Firefly carbon foam batteries, as time goes on. Their quality reputation now seems to be that of an established manufacturer addressing a lot of tough markets, rather than that of a startup with a cool new thing as they were for the first 6-8 years.

The capital cost is a hurdle, but the significantly improved cycle life (versus conventional AGM) on the 50%-80% cycle routine makes me think that total lifetime cost, after factoring in the time and labour of installation & eventual replacement, probably now favours the Firefly over 6 to 10 years.

Also, for boats that rely on an electric autopilot and/or carry deployable thin solar arrays – and therefore tend to be much harder on the batteries while sailing than while anchored – you’re normally supposed to size a flooded or AGM pack to handle the on-passage loads with a 50%-80% daily cycle between engine runs. This makes it heavier and bulkier than is necessary to do the job the other 25 days of the month. The ability to *occasionally* extend the cycling to 40%-80% or even 30%-80% without a dramatic compromise in longevity, while still using 50%-80% as the basis for routine use, means that you can be less conservative in the sizing calculations, getting the same performance and similar longevity with a lower nameplate rating and less total weight.

Hans Boebs

Hi John,
I came across a new (to me) concept of a battery system and I wonder if that could be a viable solution for many cruisers. It seems to combine advantages of both worlds: lithium and lead acid.
But as you know, I’m not an expert in the field of electrics, so maybe you could have a look at their product, the LE300 ?
Thank you and Merry Christmas to Phyllis and you !

Hans Boebs

Hi John,
they recommend 1 unit per 100Ah of lead acid capacity, for me that would mean 2 units. Price per unit is € 338,- or so.

Hans Boebs

Just looked it up: it’s 388,94 EUR

Hans Boebs

Hi John,
you are very probably right. When the time comes to do something about my battery bank I will examine all options. My goal with this post was just making you and the community aware of yet another possible configuration that I never before heard about.
On Snowball I get excellent service from my Sonnenschein Exide Gel battery 12V/230Ah which has worked for 8 seasons now and is still going strong. I rarely discharge below 25% DOD and I very rarely use the engine expressively for charging, that only happens on passage after a few days without sun, so that the solar panel has little chance of keeping pace with consumption. (Which is low: LED lights, few electronics, a small but big enough fridge)
Given my usage profile I briefly thought that this “care and feeding” lithium supplement could improve the already good longevity of my gel bank. 52 amp hours is more than I use on most days so these could be taken out of the LE300 and be recharged very quickly when I’m using the engine anyway. And there is this space in the battery compartment… But you are right, I could buy a new gel battery and have money left, compared to two LE300s.
Another thing I noticed when browsing through the datasheets of my gel battery: The Sonnenschein dryfit solar series can and apparently should be equalized ! Although I’ve never done that. Look at this link, scroll down to page 5ff and look at 2.6 and 2.11

Kindest regards

Terence Thatcher

John: For years I have run 4 Trojan flooded 6 volts, combined to make 12 volts. I get a theoretical 450 amp hoiurs (20 hr rate) and fairly long life. I have a Group 27 starting battery, which is probably a waste of space and couhld be more house bank, as long as I monitor battery SOC. (My battery locker limits additional batteries.) Maintaining charge in winter is easy in Oregon’s mild climate. Solar keeps them topped up But I now need to replace the 6 volts. Trojan makes equivalent 6 volt AGM batteries and I would be happy to get rid of the watering obligation. In return for going AGM, I would lose about 20 amp hours total and gain 6 pounds in weight per battery. (Don’t understand why; just quoting spec sheets.) I also will be upgrading to a Wakespeed regulator. I would appreciate your advice on my choice. I am leaning toward the liquid old style, but remain uncertain. As usual, I appreciate your willingness to share your wisdom. I find it amazing Morgan’s Cloud did not fly off the market, but my circumstances do not allow me to make an offer.

Marc Dacey

Every time I have trudged past where you’ve got MC, I have wondered about that aspect. On the other hand, having left Ontario, we’ve gone quite evangelical on the topic of Nova Scotians acting like adults during this whole pandemic. Good luck with the coming of spring.

Back on topic, I find the “chore” of watering is directly proportional to the ease of access, which is usually not the owner’s choice on a production boat where battery servicing is often apparently an afterthought. We remove a couple of steps and have full access, so it’s form factor, not battery type, that takes a larger branch in our decision tree. But again, I like to examine the terminals, clean what needs cleaning and look at the cabling every few months anyway, so watering and equalization and checking the SG is not a big deal for us.

Steven Bibula

We were about to upgrade our house bank with eight Firefly Oasis 116 Ah carbon foam batteries, when the distributor and system designer, Ocean Planet Energy, informed us -just a few days ago- that Firefly suddenly no longer recommends more than four of these batteries in a single bank, even if there are multiple series groups of four ganged together in parallel. It seems that Firefly will not honor warrantees on their batteries used in banks where this guidance is violated. Well, this restriction would seem to rule out Firefly carbon foam, with respect to most of the cruising community. I thought others would like to know about this development.

As for our new house bank, we have altered course, and our new heading is Lithionics LFP with internal BMS; much lighter and less space for roughly the same usable Ah, 70% DoD, easier charging and -sigh- approximately 2.2 x price of the FFCF.

Paul Borondy

Hi John, The batteries I use on my yacht are Lead Crystal which have liquid electrolite when the batteries are flat and then the electrolite turns to crystals when the batteries are charged. Performance is 6000 cycles to 20% DOD and 600 cycles to 100% DOD They do require periodic resetting if continuously shallow cycled and I do this once per year. It involves discharging the batteries to 10.5 volts then charging them up at the rate of 30% of the amp hour capacity. In my case I have 3 x 90AH batteries in the house bank and reset them with 3 x 12 volt 30A chargers charging each battery separately with the generator. For normal charging I use a DC to DC charger which is fully programmable to set the charge voltages correctly for the batteries. It accepts 50Amps of solar charge and has alternator input which means you can control the alternator charge voltages.

Paul Borondy

The Lead Crystal batteries are from Betta Batteries. They have been on the yacht for 6 years and I haven’t had a problem. The battery model is 6-CNFJ-90

Kimball Atwood

Hi John,
I’m looking at an add on Yachtworld for a boat that has “New Victron Super Cycle AGM batteries – 3 x 125ah (2022) – Note: These batteries are a new type of AGM which approach lithium in some respects, and matches the Carbon Foam Firefly batteries performance – capable of up to 100% depth of discharge occasionally, and 60-80% frequently without damage.” Have you heard of this, and does it sound legit?

David Hayward

The link
None of this is particularly easy, to the point that I wrote much of this Online Book to deal with just these problems.

Goes to the Anchoring Book

Iain Dell

Hi, John and Happy New Year!

While certainly not wanting to start digging new rabbit holes for you to go down I wondered if you’d come across this before LE300 Smart Battery System | Lithium Battery – BOS AG (

I understand the principle to be that the Li battery, connected directly to the LA house bank, functions as a reservoir for any excess charge that the LA bank cannot accept, before then acting as a charger itself when required.

The system seems very simple. However, while I’m an ‘electrical dyslexic’ I’m cynical enough to question sales bumph which doesn’t mention the inevitable compromises. Grateful for your views…..


Iain Dell

Much appreciated; thanks John. I was probably reading the blurb with too much ‘Hogmanay spirit’ remaining in the bloodstream….

Christin Miller

A positive for lithium house bank. And a Odyssey battery for engine.
On my Allied 36 there was 3 lead acid batteries when I bought it 4 yrs ago. 2 weeks later I smelled sulfur and realized I had a hot runaway battery ready to explode. I took precautions and removed from the boat.
My replacements;

For the engine battery I installed a Odyssey battery that I had bought 4 yrs earlier. So now it has been in use for 8 years and still going.

Building my House bank. I went with 2 200 amp hr grade as lifpo4 lithium battery’s. I believe they are as safe or safer then lead battery’s. Their construction is four metal encased waterproof batteries then those are incased in a sealed plastic battery case. They each have a controller board that shuts down the battery if I goes under or over charge condition. Also shuts off in under and overheated conditions. To connect to my system nothing else needed. My 55 amp alternator changes it at 14.4 with no problem for 2 yrs. I had bought a Dc to Dc charger to charge the lithiums but never installed it.
Cost batteries of battery bank. Batteries including shipping 800 x 2 = 1600. Dc to Dc charger was 280. $1880 total. This cost less then most lead acid battery systems. RJ energy where I the battery’s expects 6000 to 8000 deep recharges and battery’s would still have 80% capacity.
In three years that thy have been in service I can count the deep discharges on one hand. My solar keeps them topped off.