What Battery Type Should You Buy?

Battery Shopping

This article has now been replaced by two new chapters, one on lithium and the other on lead acid batteries in our Online Book on Battery Installation and Maintenace.

What battery type should you buy? Liquid filled, Gel, or AGM?

The first thing to understand is that all three types are lead-acid batteries and use the same electrolyte and are subject to the same vulnerability to failure from sulfating. So there is no best type. Rather the best battery for you depends on your usage profile. Let’s look at the three types and their different advantages and disadvantages in cruising yacht use.

Liquid Filled

  • Lowest cost of the three types. (I’m assuming high quality deep cycle batteries for all types.)
  • Most robust and tolerant of charging mistakes, mainly because electrolyte that has been boiled off by overcharging can be topped up.
  • State of charge can be determined by measuring the specific gravity of each cell with a hydrometer.
  • Can be equalized to remove sulfation.
  • Must be easily accessible for checking and topping up with distilled water at regular intervals. Although this can be reduced with the use of recombining caps.
  • Must be secured upright so they don’t leak.
  • Must be charged regularly, even when the boat is unattended and unused, otherwise they will self-discharge and eventually sulfate and fail. In addition, a discharged battery will freeze, whereas a fully charged one will not (except at extreme temperatures not normally encountered where yachts are stored).

Common to Gel and AGM (Sealed Batteries)

  • Can generally be installed in any orientation, but check manufacturer’s recommendations.
  • Access for regular maintenance is not required; however, all batteries should be well ventilated, so don’t bury them in a sealed locker—kaboom can ruin your entire day.
  • Have a very low self discharge rate and therefore can be left fully charged for long periods (a year or more) without charging and without fear of freezing.


  • Generally cheaper than AGM.
  • While I have no solid engineering based proof of this, it does seem, based on our own experience and that of many other voyagers (received through the comments to this site), that Gel cells are less subject to sulfating from not being fully charged after each use than AGMs.
  • Generally higher cost than liquid filled, although this can vary depending on the quality of the two types being compared.
  • Are the most vulnerable to overcharging of the three types.
  • I have never seen a Gel cell battery that can be equalized, so once they sulfate, and all batteries do eventually, particularly with the usage profile of the typical cruising boat, they can’t be resuscitated and must be replaced.


  • Have a significantly higher acceptance current (amperage) than the other two types. This means that they can be charged in less time, particularly up to about 85% of full charge. And, by the way, according to bench testing at LifeLine Battery, AGMs, or at least their AGMs, actually last longer if charged fast, as long as the acceptance voltage is not exceeded.
  • Have a higher capacity for a given size than the other two types (10-25%).
  • AGM batteries from some manufacturers can be equalized.
  • By far the most expensive of the three types.
  • Are the most vulnerable of the three types to premature failure from sulfation if not fully charged after each discharge, which is impractical on most voyaging boats that don’t see shore power for months at a time. This problem can be largely overcome by following the protocols in this Online Book and/or by having substantial solar charging capacity. Again, I have no engineering proof to backup this assertion, and I suspect it will raise howls from AGM battery manufacturers, but our own experience, and that of many other voyagers, is just too compelling to ignore.
  • Many AGM battery manufacturers specifically forbid equalizing. These batteries should be avoided by pretty much all cruisers because this restriction, coupled with the point above, can result in lifecycle costs of as much as US$5.00 per discharge! This I know from bitter experience.

OK, What’s Best, Dammit?

That was fun…confusing, too. “But what battery is best for a cruiser?” I hear you ask. Well, it depends. Don’t you hate that answer? Why is nothing ever simple in cruising? Let’s have a go at simplifying it as best we can.

The Budget Option

If you have easy access to your battery area, a tight budget, and don’t need to leave your boat unattended for long periods, or can arrange for someone else to charge the batteries once a month if you do, then liquid filled is the way to go. We have even heard of cruisers that have had good experience using golf cart batteries that can be bought for much less than their marine equivalents.

The Middle Option

[I changed this section on Gels on 17 Jan as a result of Dick’s comment (see below) and a re-read of other comments from Gel users.]

I’m going to go out on a limb here, and probably get beaten all over the head and chest in the comments, but I generally don’t recommend Gel cells for most cruisers, for one simple reason: I have never seen any that can be equalized and, therefore, when they do sulfate as a result of not fully charging after each discharge, there is no way to bring them back to life.

Having said that, we used Gels for some years with great success when our usage was much lower than it is today. You may want to consider Gels to get the sealed battery advantages at a lower cost than AGMs as long as you can fully charge them to 100% regularly. (By the way, there is only one way to know that your batteries are fully charged.)  Or, if you simply don’t want the bother and aggravation of regular equalization, then gels are likely a better and longer lived option than AGMs.

The Expensive Option

If, like us, you need to be able to leave your boat unattended without worrying about getting someone to charge the batteries regularly, and particularly if you don’t have as much room as you would like, or really need, for batteries (again like us), AGMs are the way to go. But make sure that the ones you buy can be equalized, and that you follow the protocols in this Online Book to care for them, or there will be tears—trust me, this I know.


Like what you just read? Get lots more:

Meet the Author


John was born and brought up in Bermuda and started sailing as a child, racing locally and offshore before turning to cruising. He has sailed over 100,000 miles, most of it on his McCurdy & Rhodes 56, Morgan's Cloud, including eight ocean races to Bermuda, culminating in winning his class twice in the Newport Bermuda Race. He has skippered a series of voyages in the North Atlantic, the majority of which have been to the high latitudes. John has been helping others go voyaging by sharing his experience for twenty years, first in yachting magazines and, for the last 12 years, as co-editor/publisher of AAC.

68 comments… add one
  • Nick Kats Jan 15, 2014, 4:29 pm

    Where’s the sidebar?
    Hemidemisemicomputerliterate&allfrustrated.. Thanks!

    • John Jan 15, 2014, 5:19 pm

      Hi Nick,

      The sidebars are the two columns the right of where the post appears. If you are not logged in you should see a “Member Login” Box with a space for your username and password. Fill that in, and you will be good to go. Also, if you check the box that says “Remember Me” before logging in, you won’t have to go through this again, as long as you are on the same computer.

      If you see your user name displayed there instead, it means you are already logged in.

      • Nick Kats Jan 17, 2014, 8:36 am

        Got it, thanks John!

  • Erik de Jong Jan 15, 2014, 5:18 pm

    Hi John,

    Great piece.
    The only addition I’d like to make as a disadvantage of liquid filled batteries, is that they should be placed in a sealed and ventilated battery box in order to avoid poisoned gas in the living quarters during charging. This is also a recommendation for all sealed batteries, but much less critical.

    • John Jan 15, 2014, 5:32 pm

      Hi Erik,

      Good point, although to be fair to liquid filled batteries, recombining caps dramatically reduce the amount of explosive gas vented by them when charging.

      Having said that, I am amazed at how little our LifeLine AGM’s gas even when being equalized, as long as we keep them desulphated with regular equalization. Once they sulphate, they gas a lot more.

  • Jean Jutras Jan 16, 2014, 2:53 pm


    I am an “early adaptor” and like new technologies. Many new products and technologies, nowadays, tend to be reliable (but often more expensive). Next year, I am planning to replace my AGM batteries by the MASTERVOLT Lithium Ion ultra. These batteries will equip all the yachts of the Volvo Ocean Race, not only for the 2014-15 race but also for the 2017-18 race.

    Many manufacturers are now offering this type of batteries but is the quality and reliability the same? It will be interesting to read more about it in the next post and if someone has tried to use it, their comments will be more than welcome.

    (Sailing a Cabo Rico 42)

    • John Jan 16, 2014, 3:30 pm

      Hi Jean,

      We will be publishing Matt’s post on lithium batteries in a week or so. He has substantial experience with this technology as well as the engineering training to understand and explain what that experience means.

      I would appreciate if everyone would hold their comments on Lithium batteries of all types (there are several) until we publish Matt’s post. That way we will have all the information on this interesting technology in one place, where it will be of most benefit to all.

  • Dick Stevenson Jan 17, 2014, 4:29 pm

    Dear John,
    I thought I would share my experience with gel cell batteries as it differs some from the conclusions you reached and therefore affects the recommendations you make.
    I have been using gels for 20+ years: at first Sonnenshein (sp?) and for the last 15 years East Penn batteries. From the beginning I have used Dave Smead’s (Ample Power) charging protocols which I believe to be a bit more aggressive than generally recommended. For much of my 12 live-aboard years we have lived off the grid at anchor so we could and did go months charging at 50% and bring the bank up to 80-85%. Being in the trades during this time, there would be rare long powering that would bring the bank to full charge (possibly about once every 6 weeks or so). We have a big freezer so this charging regimen (DC genset) would be at least once a day and often twice in hot water and high temps. The gels tolerated this extremely well. We would get 6-7 years out of our battery bank.
    I continued with gels as they tolerated this use very well. I agree with your observation that gels are less subject to sulphating from not being fully charged than AGMs, although my research in the AGM realm is anecdotal and casual. When I looked into AGMs I felt their need to be fully charged on a regular basis was not possible in the live-aboard life I was then encountering. Now that I am in Europe, where we are so often in marinas, AGMs might make more sense.
    As to not recommending gels because they are unable to be equalized: this to me does not seem fair as they are not made to be equalized. I know by this you are implying a shorter life because of sulfation that is unable to be reversed, but this has not happened in my experience with gels. I get good longevity in my estimation. And certainly I have never experienced the sulfation happening in weeks or months that you warn of in such a foreboding manner.
    I am now moving into areas where I have some experience, but not a lot. I believe the equivalent to equalization for gels is a process called conditioning whereby the battery bank is fully depleted (down to 10.5v over a 20 hour period) in a graduated fashion and then brought back to full charge. I used this conditioning technique regularly before living aboard full time and felt it made a difference, especially when the battery bank was starting to show itself wearing out. Since living aboard full time, I have let this slip away as the whole process takes so long and is hard to manage the consistent amperage draw that conditioning requires. I have not felt the longevity of the battery bank has been compromised by not conditioning, perhaps full time use is in some ways kinder?
    You said you were going out on a limb and I would wish, in the kindest possible way, to cut it off by saying that your statement: “ therefore, just a few months, or even weeks, of not fully charging after each discharge can result in their demise from sulfation with no way to get them back” does not meet with my experience with gels I have been using.
    So, in summary, I install the battery bank and basically forget about the batteries for 6-7 years (or more). I replace before they start to die as that is how I run my boat. I do not have to worry about equalizing nor do I any longer condition the bank, although I keep that as an option if deemed necessary. Being an all DC boat with a large freezer, we have a higher than average daily amperage draw, so I consider the battery bank to be used hard. My casual research into AGMs lead me to believe them to be fussier and a bit more fragile so I have stuck with gels and continue to see them as a great option for those who demand a lot from their battery bank and live off the grid for long periods.
    My best, Dick Stevenson, s/v Alchemy

    • John Jan 17, 2014, 8:35 pm

      Hi Dick,

      Great comment full of a lot of good information. I think, actually, you and I are closer on our thinking about gels than it might first appear. Most of the apparent difference in our positions is my fault in trying to simplify the issue too much. I will re-write the post a bit to fix that.

      Having said that, I still think based on my own experience of using both types on the same boat, that AGM’s will likely (hard to be certain) have a longer life that gels in typical voyager use, as long as they are equalized regularly. Without regularly equalization, then gels are the clear winner as there is no question in my mind that AGM’s sulphate more quickly than gels. On the other hand, I’m also pretty confident that AGM’s are generally more robust than gels and can take more abuse, particularly over charging.

  • Dick Stevenson Jan 17, 2014, 9:26 pm

    Dear John,
    I will look forward to the re-write.
    AGMs may have a longer life if cared for in the ways you describe, but I feel I get about the same or better life than those I know with AGMs. That said, mine is a casual comparing of notes over the transom type of data collection and I believe that none I know comes close to your maintenance diligence nor do I know anyone who equalizes their AGMs. At this point, I know I will still opt for the maintenance simplicity of gels and take the chance that they may not last quite as long.
    I am curious about whether any of the readers know/use the conditioning technique I described with their gels or what the current thinking is in this area.
    My best, Dick

    • John Jan 17, 2014, 9:48 pm

      Hi Dick,

      Re-write is done, When I screw up I like to delete the evidence as quickly as I can!

      On the conditioning, as I understand it from Justin at LifeLine such full discharge and recharge cycles have no beneficial effects and in fact will shorten the life of the batteries. Having said that, I have no first hand experience to back that up. And of course anecdotal information from voyagers who have practiced this quite popular technique is really not going to tell us much, because there would be no control to compare against.

      But if we take it as a given, as I do, that the deeper you discharge lead acid batteries on each cycle, the more life you take out of them, then how can it be a good idea to discharge them to flat and recharge them?

      Do the engineers have any insight: Matt Marsh, Eric Klem?

      • Eric Klem Jan 19, 2014, 11:39 am

        Sorry John, this isn’t something that I have ever worked on or studied and I don’t have any insight. Hopefully Matt or someone else does.


        • John Jan 19, 2014, 12:24 pm

          Hi Eric,

          Thanks, anyway. Given that, unless someone like Matt or yourself, with real engineering knowledge, has good data that this technique does anything useful, I will stick with my position that it is most likely just another battery myth that has become “fact” because it’s been repeated a lot.

          There are a lot of those myths around. The other prevalent one is that it is better to charge batteries slowly than at their acceptance voltage. Boat yard “professionals” that should know better, love to spread this one. Testing at LifeLine has shown that the exact opposite is true for AGMs, at least. And, as I understand it, slow charging of liquid batteries causes acid stratification and premature failure. Any thoughts.

        • Matt Jan 26, 2014, 10:39 am

          The largest gel cells I’ve worked with are little 5-pound general purpose things. I don’t have experience with large gel cell banks.
          From what I do know about their chemistry, I would suspect that deep discharging of a gel cell would turn the surfaces of both plates into lead (II) sulphate, just as with any other lead-acid cell. Perhaps the idea is that, when the plates transform back to lead and lead oxide on recharging, the surface texture has changed to expose more fresh material…. but that would seem to imply that we’re intentionally damaging the plates to hide the sulphation. And we can’t overcharge (equalize) to actually remove the lead sulphate that will inevitably build up.
          Not sure if that’s entirely right, I’m open to being corrected by someone with more expertise in these things.

    • John Jan 17, 2014, 9:55 pm

      Hi again Dick.

      Further to the idea of discharging and recharging as a form of conditioning, actually “conditioning” refers to a process like, but slightly different to, equalization.

      Those who are interested in learning more can do so from LifeLine’s excellent manual, the area of interest starts around page 18.

    • Matt Jan 26, 2014, 10:52 am

      I think that if you know you’re going to abuse the batteries, nickel-iron ones might be a better choice than any of the lead-acid types. They’re very tolerant of being run flat, overcharged, ignored or otherwise abused, and will probably outlive the hull itself.
      The trouble is that they self-discharge much faster than any lead-acid battery (if you leave the boat unattended for six months, they will probably be dead) and they cost twice as much- if you can even find them.

  • Dick Stevenson Jan 19, 2014, 1:21 pm

    John, I have always noticed that one of the selling points of AGM batteries was their high acceptance rate. That sounds wonderful on paper, and then I wondered how many boats had the capacity to charge their AGM battery bank at anywhere even close to the battery’s acceptance rate. Couple this with your recent comments that AGMs like to be charged fast and I would think that many boats would have challenges in this area.
    Dick Stevenson, s/v Alchemy

    • John Jan 19, 2014, 2:08 pm

      Hi Dick,

      Yes, and no. There are two aspects to the AGM quicker charging feature. The first is that they can accept more current (amps), and that, or course, requires a big charging system to take advantage of. However, the second advantage that AGM’s have is that the accept a higher charge rate for longer as their level of charge increases, so even if you can’t, at first, provide them with enough amps to reach acceptance voltage, you will still reach a higher level of charge with AGM’s, in a shorter time.

      So, yes, to get the maximum benefit from AGMs you need a big alternator, but even those with smaller alternators will benefit to some extent.

      Lifeline claim that their AGM’s can be charged to 85% in half the time takes to charge a gel or liquid filled. I’m sure that’s true, in theory, but as you point out, it does depend on alternator size.

      In the real world of yachts, and based on our own experience of using a 180 amp alternator (derated to 80% output) to charge a bank of two 8Ds (510 amp hours) I would say that the savings in time is 25-30% over liquid filled or gel.

      If you often just use the engine to motor in and out of an anchorage, as we do, the greater acceptance can be very useful.

      Anyway, the bottom line is that, as you rightly point out, to reap the benefits of AGMs, you need powerful charging. That’s why we have not one, but three, AC chargers connected in parallel, since this allows us to charge our AGMs quickly off the generator and in addition properly load said generator.

      • Matt Boney Feb 18, 2014, 6:35 pm

        One point that needs clarification is that AGMs can be charged much faster than flooded wet lead acid batteries because their “charge efficiency” is much better. Wet cells may be only 70% efficient which means about 140Ah must be put in to actually raise the capacity of a battery by 100Ah. (70% of 140Ah is 100Ah)
        AGMs may be up to 98% efficient so only 102 Ah needs to be put in. That’s nearly 40% less Ah needed so 40% faster charging, and it doesn’t take a large alternator to do it. Even solar panel will charge faster.

        • John Feb 18, 2014, 8:15 pm

          Hi Matt,

          I’m pretty sure that’s not true. AGM’s have a much higher charge acceptance rate. I could be wrong, but I don’t think there is a lot of difference in charge efficiency between the three types. So while AGMs will charge more quickly, it will still take about the same number of watts in to get a given number of watts out for each type. Watts out divided by watts in equals charge efficiency. If I remember correctly from my Link 2000 that measures charge efficiency the number is about 91% for all lead acid battery types.

          Matt Marsh, Eric Klem?

    • Matt Jan 26, 2014, 10:47 am

      Installing a high output large-case alternator may take a fair bit of fuss, but I have yet to see a cruising boat of any kind where it isn’t worthwhile. The time and expense of periodically rebuilding a burned out small-case alternator (which is really just meant to power instruments and lights) more than cancels out the up-front cost of a good one. Add the improved lifespan of batteries that aren’t consistently under-charged, and the shorter engine running times, and a decent high-output charging system starts to make quite a lot of sense.

  • Dick Stevenson Jan 19, 2014, 1:30 pm

    John, I forgot to include the initiating reason for the above post. Given AGMs wish for quick charging and problems that occur with slower charging, what does this mean for the increasing number of vessels relying on passive charging methods which are invariably slow charging? Dick

    • John Jan 19, 2014, 1:48 pm

      Hi Dick,

      That’s a good point. But just to clarify, I’m pretty sure the desirability of charging at acceptance voltage applies to all lead acid batteries, not just AGM’s. In fact the problem of damage due to slow charging may even be worse for liquid filled. (not sure about gels).

      The bottom line being that even if you have good solar, it is probably a very good idea to make sure that you have a decent sized alternator and properly programmed regulator so that the batteries get a good hard charge at acceptance voltage whenever you do motor.

      The other point is that a moderate sized array of the new very efficient solar panels is capable of putting out enough current (amps), when hit by direct strong sun, to push the voltage at the battery up to acceptance level reasonably quickly.

  • CaperAsh May 7, 2014, 8:49 pm

    have any of you advanced types researched the nikel iron (Edison) battery for marine purposes?

    (Forgive if this is not appropriate suggestion-question. This sort of thing is beyond my pay grade, but this type of battery has always impressed me as deserving of more attention. Old technology, but solid and long-lasting, with many of them lasting for decades.)

  • 365Lusso May 11, 2014, 4:07 pm

    Hello, I just joined AAC, we are in the process of buying a Krogen 42 trawler to fullfill a long-held dream of economical long-distance traveling in comfort ;-). Being new to the website, I hope this is the right place to make this inquiry, and that I am not creating a conflict of interest for the website.

    Does anyone have any experience with TPPL (thin plate pure lead) technology?
    Here’s an article about it: http://www.passagemaker.com/channels/new-battery-technology/
    Here’s an example of one: http://www.odysseybatteries.com

    John, if this is the wrong place for my comment/inquiry, please move with my apology.

    Thanks, Wil

    • John May 12, 2014, 3:27 am

      HI Will,

      This is exactly the right place, so no worries there. And a thanks for the interesting link.

      Sorry I don’t have any information or experience. Can anyone else help?

      The one thing I would say is that although these new TPPL batteries do accept even higher charges than conventional gells or AGM’s I can’t see that this is really much of a break through for us live-aboards since we tend to have large banks that will take everything that most alternators or chargers can throw at them if they are AGM’s. As I explain in the earlier chapters of this book, generally our biggest problem is the latter part of the charge cycle when the acceptance rate of all lead acid batteries tails off, and, as far as I can see, these TPPL batteries don’t do anything to solve that.

      The other issue is that if you do decide to go with these batteries make sure that the manufacturer will allow you to equalize them, since there is nothing to indicate that they don’t share the AGM batteries tendency to sulphate.

      • 365Lusso May 12, 2014, 3:34 pm

        John, well, that was fun. I called Odyssey Battery and the tech who answered didn’t know what ‘equalization’ or ‘conditioning’ were. I explained them to him and he said ‘it will damage our batteries if they are taken above 15.2v while charging’. Seemed to think I was a nut of some kind. Come to think of it….. Anyhow, Lifeline AGMs will do nicely.
        PS: I don’t think the anti-spam math is that onerous at all if it keeps the web-crawlers out.

        • John May 14, 2014, 1:36 pm

          Hi Wil,

          Thanks for the report. That kind of settles that, as you say. Good of you to go through the exercise though, as we are always looking for better solutions.

          And thanks for the understanding on the spam protection.

  • Geir Ove Dec 1, 2014, 6:27 pm

    we did install 4 x 6V 390 amp Trojans, so thats 12V 780apm, and we got hold of Hoppecke pluggs to use one the cells, here is what they do,
    The AquaGen® premium.top recombinator is fitted to the battery as an external component. This avoids any temperature rise inside the battery. The separation of the recombination process from the active components of the battery allows maximisation of freedom from maintenance, as for sealed batteries, without reducing life expectancy and with no restrictions on operation of the battery.
    — And our 6 month sailing duering the summer Norway and Scotland, Our batterys did not need any refill of water.
    we have 3 x 190 watt solar, and did run engines a bit due to no wind for a week in Scotland. we have 2kw inverter making coffe every morning, ++ never did get our bank down more then 78%.


    These plugs works.

    • Marc Dacey Dec 1, 2014, 9:15 pm

      Thank you for this information. These devices seem similar to Hydrocaps (http://www.hydrocapcorp.com/) but seem significantly taller, which would create some issues around some potential battery locations. Does the Aquagen recombinator product do the same thing as the Hydrocap “vent”?

      Thank you.

      • Geir Ove Dec 2, 2014, 5:51 pm

        Mine are not tall type, they only build about 4-5cm above the batt. if you look or zooom in on the picture you can se my type in the backgound.

        • Marc Dacey Dec 2, 2014, 7:41 pm

          Thank you, Geir, for the clarification. I see them now. Much better for most installations, I think.

    • John Dec 2, 2014, 9:23 am

      Hi Geir Ove,

      Great great real world information.

      I have to say that if we did not need to leave our boat for long periods in freezing conditions, where liquid filled batteries would self-discharge and freeze, I think I would seriously consider the Trojans with combiner caps instead of AGMs.

  • Bill Wakefield Feb 12, 2015, 2:06 pm

    Hi John,

    We were faced with replacing our house bank last summer, and benefitted from this article and ensuing comments.

    I decided to use wet cells once we discovered a remote battery watering system specific to the batteries I selected. [Trojan Hydrolink]

    I can water the remotely located bank of eight 6 volt batteries in about 30 seconds (including getting things out and putting them away…

    I believe remote watering systems like this may negate that one con for liquid batteries.

    If it is appropriate to include here, following is a link to my blog post detailing that project including product information links: http://svdenalirosenc43.blogspot.com/2014/11/new-batteries-with-built-in-watering.html

    Best Regards,

    -Bill Wakefield

    • John Feb 13, 2015, 11:27 am

      Hi Bill,

      That looks like a really good solution. Thanks for sharing it. I did wonder how you know when to top up the batteries, but then I realized that with your system you just top them up on the same schedule you would normally check them, so all good.

  • Jeff Conrad Mar 15, 2016, 9:45 am

    Hi John,

    Great site, as necessity being the mother of invention, I have joined up while researching batteries due to failure of my house bank of 3 8D AGM (Deka), 198 AH each. Without retelling the long saga of how I arrived here in Sint Maarten now scrambling to restore power to my boat, I do have a couple of quick questions for you. The first deals with knowing you have arrived at full charge on you batteries. I have understood the criteria as you state multiple times in your online book, that being current dropping to about 1-2% of battery capacity while at an absorption voltage around 14.3V. As I am about to purchase new Lifeline AGMs I have been reading their technical manual where it states “The battery is fully charged when the current drops below 0.5% of the batteries rated capacity (0.5A for a 100AH battery).” Can you explain the difference in the full charge definition? This is kind of important as I will need to input this parameter into my LinkPro monitoring system in order to receive a proper indication of when I have reached full charge.
    My second question is have you any information, anecdotal or hard data, related to desulfators. Here in Sint Maarten I have bought a couple of these units from a local producer and plan to use them on my new batteries. The engineer here swears they work to not only prevent sulphation but, under the right circumstances, restore capacity to a sulphated battery.
    Thanks for your time and great site.

    S/V SeaSparrow
    Halifax, NS

    • John Mar 16, 2016, 10:26 am

      Hi Jeff,

      Thanks for joining.

      On the determination of full charge: either Lifeline have changed it (I think so) or I goofed. Either way, go with Lifeline’s recommendation, I have huge respect for the lab work they have done to determine these numbers.

      On desulfators: these things come up from time to time and many people swear by them and the idea does seem as if it might have some merit. Having said that, unfortunately I don’t have any hard data, even anecdotal, to back the claims up.

      I will write to my contact at LifeLine and see if they have any data. The last time I checked on this with them, they did not, but that was a while ago.

    • John Mar 16, 2016, 12:32 pm

      Hi Again Jeff,

      You are right, LifeLine tech manual definitely says 0.5%, so I have changed that in our Online Book, thanks for the heads up.

      Also got information on de-sulphators, post coming.

      • Rene Mar 17, 2016, 2:29 am

        My experience with de-sulphating is with truck batteries and many chargers automatically sense the need for this type of charge, usually in the 16V range for about 3 hours. I would not do this in the confined space of an engine room. When the battery has been left for too long a time and with a reading below 12V you maybe too late and be very careful if the battery was exposed to freezing temps. Protect your eyes!!

        • John Mar 17, 2016, 8:29 am

          Hi Rene,

          Good thought, more coming in a post.

  • Myles May 7, 2016, 9:55 am

    I just ran a cross a new/revisited technology. Interesting: salt water batteries. Check the the startup company Aquion. Not currently used in mobile application, less energy density that lead acid. See the white paper comparison. Could have advantages for electric propulsion (by design not conversion).

  • richard s (s/v lakota) Jun 28, 2016, 7:02 pm

    as a non-engineer yet decently experienced sailboat cruising buff i still say all this is much more art than science that often stumps so-called marine mechanics…richard s (tampa bay)

    • John Jun 29, 2016, 8:29 am

      Hi Richard,

      On the contrary, its all science. The problem comes when technicians treat it as art and don’t make the effort to understand the basics.

  • Daniel Jun 29, 2016, 10:29 pm


    maybe I missed it in one of the chapters on batteries, but I cannot recall you talking about TPPL batteries (Thin Plate Pure Lead). They have been around for several years (10?) and they have a much higher Charge Acceptance Rate then any other lead based battery.

    While more expensive then AGM or Gel batteries, they are much cheaper then Lithium Ion.

    Many articles have been written about then, here is one for reference:

    Any thoughts? Thanks


    • John Jul 1, 2016, 8:44 am

      Hi Daniel,

      No I have not written about the Odyssey batteries simply because I don’t have any experience with them. I did take a quick look at their site, but did not see anything that lead me to believe that they were in any way a breakthrough. Claims like “Twice the overall power and three times the service life of conventional batteries” without any real backup are more often than not driven by marketing, not engineering. The obvious question is “compared to what?” If they are comparing to a cheep automotive liquid filled battery, then just about any manufacturer of quality AGM batteries could say the same.

      Their “twice the power” does not seem to hold up to scrutiny in that their PC1800-FT has a capacity of 190 amp hours and weighs 132 pounds. That’s not any better energy density than say a LifeLine 8D that has a capacity of 255 amp hours and weighs 156 ponds.

      The other issue is that I could not find any information on whether or not they can be equalized. I suspect that means that they can’t be, and if that is so, that would disqualify them for live-aboard use.

      Does anyone else have first hand experience with Odyssey batteries?

  • SNOWQUEEN Jul 21, 2016, 11:13 am






    • John Jul 22, 2016, 7:35 am

      Hi Snowqueen,

      As long as the panels will recharge the batteries in a day or so after use, pretty much any good quality lead acid batteries will be fine. We have a post here that explains the three types and their relative advantages: https://www.morganscloud.com/2014/01/15/what-battery-type-should-you-buy/

      On the other hand, if your usage pattern will result in the battery being left for longer periods in a partial state of charge, then the Firefly might be the way to go.

  • evan Dec 10, 2017, 4:11 pm


    I hope you don’t mind my jumping in here with a detailed real life example.

    I have read this thread and the entire series of battery articles with great interest.

    I have recently gone through the process of trying to optimize my house battery bank on a Nordic Tug 37 (I know it’s a coastal cruiser not offshore vessel but we frequently spend weeks off & away from shore power as well as frequent trips in the dead of winter – perhaps not unlike an offshore cruiser might.)

    My system consisted of:
    Main House Bank: 4 x 260 Ah 6V Discover AGM “traction” batteries giving 520Ah of 12V power. (This would give me 260 Ah of usable capacity allowing for 50% discharge rates.)

    Secondary House Bank: 2 x 220 Ah 6V Full River AGM’s in the stern. This remote bank connects in parallel to the main bank with 2 runs of 2/0 cable (~25 feet). These function as accessory power to the House Bank and the primary supply for the Stern Thruster. (In theory this should add another 110 Ah of usable capacity.)

    Total Usable power = 370 Ah

    I have a 11 KW Onan Diesel Generator able to charge a Xantrex Freedom 25 Inverter charger as well as a Xantrex TrueCharge 40+ Amp Multi-stage Charger. The Freedom 25 can charge up to 130A quite happily if the battery bank can take it.

    I have also installed 2 x 280 Watts of solar panels on the Pilothouse roof charging via a Tristar MPPT controller. On bright sunny days, I get 25-35 Amps of charging from these panels but rarely completely charge the battery bank with solar alone. In the winter months we see far less help from the Solar system.

    I am able to monitor usage and charging via a Victron BMV-700 Shunt.

    One would think that this would be a more than adequate setup but…when out in the winter here on the BC Coast, I have a Wallas diesel heater running continuously along with a Fridge and Freezer + LED lighting & other draws etc. This gives me average ongoing power usage of 12-16 Amps or approximately 300-360 Ah per 24 hour day.

    I was finding that even starting off at a full charge at 1030 in the AM we would be down to 12.1V by morning the next day. These AGM’s are not supposed to be partially recharged so generator runs have necessarily been a good 3 hours to try and get that last 10% back into the battery bank. My batteries have instructions that strictly forbid equalization making me concerned that I should not scrimp on recharging them to close to capacity.

    It seems to me that if the battery bank could tolerate recharging to ~80% SOC, then one could reduce generator runs to 2 hours and on brighter days even forgo any generator usage.

    I understand that the Firefly AGM’s with their Carbon Foam Structure will tolerate sustained partial states of charge without damage or sulfation. In addition, unlike the standard AGM’s, they are supposed to also be able to tolerate discharges down to 10% SOC without damage. They are also supposed to be able to accept higher charging currents than standard AGM’s maximizing the efficiency of charging .

    These tempting benefits prompted me to recently swap out my main house bank AGM’s for (6) Firefly 12V 110 Ah batteries giving me ~ 660 Ah of capacity of which ~ 600 Ah should be usable; not counting the ~110 Ah from the Thruster Bank.


    I have only recently completed the installation.

    My first overnight usages (anchoring at the end of a few daily motor runs) have been in the range of 230 Ah & have not yet really stressed the system. (Morning voltages are running in the 12.43V range. ) My initial impression is that the batteries operate at slightly lower voltages for their given state of charge but that the discharge voltage curve is considerably flatter than that the old AGM’s.

    This is all a rather round about way of saying that one might consider Carbon Foam AGM’s as an alternative to standard AGM’s. If people are interested at all in this alternative, I would be happy to report back once I have more experience using the Fireflies…

    • John Dec 11, 2017, 7:42 am

      Hi Evan,

      Sounds to me like you have done a very good job of analyzing your system. The take away for me is that your old system did not have enough capacity to match your needs, mostly the heater, coupled with batteries that could not be equalized.

      Given that, I think that you made the right call on Carbon Foam, since the other alternative would have been at least 800 amp hour of conventional batteries that could be equalized, and I suspect you did not have the room.

      The problem with Carbon Foam has been that they only one size (I see they now have two) and credible reports of an unacceptable fail rate.

      That said, I’m pretty excited about Carbon Foam and think the technology is the most promising option to replace conventional lead-acid batteries on cruising boats—one I prefer over lithium.

      Cruising BC in winter on a snug motorboat sounds like fun, I’m envious.

      • Evan Dec 11, 2017, 12:12 pm

        Thanks John,

        I explored Lithium as an option but balked at the prospect of the need for a very different charging profile. I have several other AGM’s in the boat including an 8D AGM servicing the bow thruster & windlass as well as an 8D AGM start battery & smaller G31 AGM generator start battery. If I were to go to Lithium I would have had to swap these out as well. To adopt a new charging system and replace all the AGM’s would have been an enormous expense & hassle.

        The Carbon Foam make sense. I hope they live up to the promise.

        We are pretty lucky to be able cruise all year round. It’s cold at times but very beautiful. We often have anchorages to ourselves that would be jammed with boats in July.


  • Martin Wagstaff Apr 4, 2018, 2:06 pm

    Hi John, first of all what an amazing source of information. I have been a member for a while but this is my first post. I have just read everything about batteries and charging and I’m glad I did before spending a great deal of money in the wrong thing. To quickly summerise we are currently refitting our Oyster 47 to go around the world. The current batteries are alive but will need replacing before we go. Currently we have 4 x 6v batteries in series parallel for a 12v system. I will be doing a power survey shortly to work out my AmpHrs but what are your thoughts on 6v and 12v batteries? I can’t seem to find any logical reasoning to decide what is best. Martin

    • John Apr 5, 2018, 7:20 am

      Hi Martin,

      That remind me that I need to write a chapter on our new battery bank and why we configured it the way we did. But the spoiler is that it’s 4 x 6volt in two banks of two.

      All the reasons are more than I can do in a comment, but look for the chapter soon.

      • Martin Wagstaff Apr 5, 2018, 9:32 am

        Hi John,
        I look forward to reading it. I have a year before I need to take the plunge and purchase them,

  • Murray Fitzgerald May 6, 2018, 12:58 am

    G’day John.
    We are on our second set of AGM batteries. First set lasted 8 years & were changed out because we were sailing around Australia. As you said the trick is to keep them topped up. We have solar & an engine generator with a battery management system that swaps the charge between house & engine battery depending on need. It will take a good argument to get me away from AGM.
    P.S. plenty of sun in Australia!!

    • John May 6, 2018, 7:51 am

      Hi Murray,

      I hear you on AGM, although I do think that there are now some good alternatives. More in the next chapter.

  • Markus May 9, 2018, 2:24 pm

    Hello John. You wrote: “I have never seen a Gel cell battery that can be equalized…”. Please see https://www.hoppecke.com/en/product/grid-power-vr-l/ download “Operating Instructions”, page 40. Based on certain conditions, those OPzV (GEL) batteries have a procedure for equalizing at up to 2.4 V/cell for up to 48 hours. Best regards, Markus.

    • John May 10, 2018, 8:10 am

      Hi Markus,

      That’s certainly interesting, thanks for the link.

      That said, there are a couple of issues to be aware of. First off the spec says “tubular plates in combination with woven gauntlets”. That sound a lot like AGM to me, so this may just be a language issue, particularly given that the translation into English was clearly not done by a native speaker.

      The other issue is that they call for an equalization voltage of just 2.4 per cell or 14.4 for a 12 volt battery. That’s not really equalization, but rather holding the battery at acceptance level for a longer time than normal. Real equalization generally requires 15.5-16 volts.

      Given those two point, and that I’m not sure how readily available they are, I think I’m still justified in leaving them out of the next chapter.

  • John Crossen Sep 6, 2019, 12:51 pm

    Not sure this is the best page for this question but this where I was directed. What is the best way to power a 100amp windlass? Currently, we’re using the 30-yr old factory-installed cables (approx 30′ long each way), but every time we use it we get a low voltage alarm (set at 10.8v) on our chartplotter. The batts are 6 T-105s, about 3 years old, the Maxwell windlass (VW2500) is about 2 yrs old, and we’ve observed no other obvious signs of low batteries. When I installed the windlass I noticed the cables were stiff, but not solid; they reminded me of welding cables, but there was no visible signs of corrosion when I attached the new ends. We’re looking at 2 options….(1)replacing the old cables with new, or (2)installing a new, separate, dedicated battery in the chain well. But if so, what size batt, and how best to recharge it. Both are about equal in cost; which would be recommended? Also, we are running the engine when the “low-voltage alarm” comes on, but typically only at idle. The alternator is a factory standard 80 amp, with internal regulation, that feeds the house bank directly. We do have a separate dedicated start battery, that is recharged via a Balmar Duo-Charge.

    • John Sep 7, 2019, 7:13 am

      Hi John,

      First off, the low voltage alarm is not to do with the cables running to the windlass, but rather indicate that the main bank is not capable of supplying the 100 amps without dropping voltage. Given the size of your bank that either indicates a bad connection somewhere in the main bank wiring, or that the main bank is coming to the end of its life.

      So the first thing to do is test the main bank with a battery load tester and trace out why the voltage is dropping so much.

      After that’s sorted out, you may find that your windlass problems have gone away. The way to test that is measure the voltage at the windlass when it’s under load and the main bank is fully charged. I would guess that anything over about 11.7 Volts would be good.

      Bottom line, I fear that your batteries are at the end of their lives.

      (Problems in the cables leading to the windlass would manifest as low voltage and slow running at the windlass, not low voltage alarms on other gear.)

    • John Sep 7, 2019, 7:20 am

      Hi again John,

      Just noticed that your batteries are only three years old, so, assuming that they have not been abused, maybe they are OK. In that case you want to be looking for a bad connection between the batteries and the main supply buss bars (don’t forget the negative side).

      The other thing to do is equalize the batteries since, if you have not done so regularly, they may easily, after three years, be sulphated.

    • Rob Gill Sep 7, 2019, 6:23 pm

      We too replaced our original windlass about four years ago with a Maxwell (different model but same 1200 wattage). Had exactly the same low voltage alarm on our chart plotter, also even with the engine going. Our Raymarine electronics would shut themselves down if the windlass was really working (not great timing as we would lose depth, speed, rudder angle and suddenly have no chart display). If the windlass was really working, it was most likely in the poorest conditions, when we needed situational information at the helm.
      Having no power issues to the windlass, we suspected the batteries as they were 6 years old and needed replacing anyway. New LiPo batteries were installed in due course, but the problem continued. We replaced the supply and return cables with heavier duty cable to the main bus bars (factory originals probably spec’d to a budget) and the issue resolved – no recurrence since. I suspect John is probably spot on (as usual).
      Br. Rob

  • Rob Ramsey Mar 1, 2020, 12:59 pm

    Hi John & all. Great stuff on batteries! What do you think of the new lead-carbon batteries? Seeing that Victron now sells them (https://www.victronenergy.com/upload/documents/Datasheet-Lead-carbon-battery-EN.pdf) I would think they are mainstream now. The battery can be discharged to 100% DoD (Depth of Discharge), has a longer life than car batteries (500-1300 charge cycles according to Victron, not as long as LiFePO4) and are ideal for PSoC (partial state of charge) which is the way we use batteries as cruisers – never full, never empty, always somewhere in between. They are also called ultra batteries (invented by the Aussies) and are basically traditional lead-acid batteries with carbon on the negative plates which then act as super capacitors. For reasons not yet fully understood this also mitigates sulphating which is good for us. They are affordable, should be charged as lead-acid, can be fast charged, do not require charging to 100% regularly, require a lot less equalising and are safe. Reading about them I got quite enthusiastic – I can get 160Ah from a 160Ah battery. Worse, If I use only 60% I could have 1300+ charge cycles. And if they quit while I’m in Nowhereland I can replace them with car batteries because chargers, inverters et cetera in the boat are all standard.

    Seems like they have taken care of a lot of the disadvantages of lead-acid batteries.

    So, what do you think? Interesting development for cruisers?

    • John Mar 1, 2020, 8:18 pm

      Hi Rob,

      This technolighy has been around for quite a while and I have written about it at some length: https://www.morganscloud.com/2018/05/11/battery-options-part-2-lead-acid/

      Good to see that Victron are going this way. Trojan too. That said, the performance they are claiming is not that wonderful:

      ≥ 1000 cycles @ 60% DoD (discharge during three hours with I = 0,2C20, immediately followed by recharge at I = 0,2C20)

      Note that to get that quite modest cycle number they want an immediate recharge to full, something that won’t happen on a cruising boat. I think the key point is that they are only using carbon foam for one side. Firefly started that way but now use carbon foam for both sides and are getting better numbers.

      Also, I have never heard of a carbon foam battery being in any way a super-capacitor, but maybe I missed something. If so, please link me to it.

  • Rob Ramsey Mar 2, 2020, 5:38 am

    Hi John. Victron seems to be modest with their numbers and they cite claims by other manufacturers as being better but they haven’t been able to confirm.

    On the capacitor thing: adding a layer of carbon to the negative plate makes it a super capacitor. Here in Wikipedia https://en.wikipedia.org/wiki/UltraBattery and here on how it is made here (he’s upgrading a dead car battery to an ultrabattery, not what I would want to do, but it explains a lot): https://www.youtube.com/watch?v=beIwzjfkbyM. By the way … the company behind this does research in carbon batteries and capacitors: https://workingink.co.uk

    Mind you … I don’t claim to have the knowledge or expertise to say that the Victron and Firefly are hybrid supercapacitors / batteries.

    • John Mar 2, 2020, 10:57 am

      Hi Rob,

      Thanks for the link. The way I understand it only the ultra battery incorporates a super capacitor, not all carbon foam batteries. and they are not yet available. That said, it certanly looks interesting.

  • Rob Ramsey Mar 2, 2020, 7:21 am

    Also … just to add … the fact that the Pb-C (lead-carbon) batteries will operatie at PSoC (partial state of charge) means they don’t require regular charging to 100% SoC (state of charge), which is great for cruising boats that have trouble recharging to 100% SoC unless on shore power. So you can use the Pb-C battery anywhere between 0% and 80% SoC without damaging it. To me this seems a huge advantage.

    They require far less equalising, if any. They don’t require a BMS and other LiFePO4 regimes. As I understand you charge them like a lead-acid battery.

    And … as I understand it … they have lower internal resistance so can be charged faster. So if you have solar then the few hours of high electricity production are better utilised.

    Also, I must apologise … I seem to have commented on an old post.

    • John Mar 2, 2020, 11:00 am

      Hi Rob,

      Yes, carbon foam batteries are better at withstanding partial state of charge, but not all are created equal in this regard. Also note the partial state of charge is still not good for them, just not as bad as for other lead acid technologies. See the up to date post I linked to for more. Also, any future discussion on this subject should be on the new post since we will probably delete this one.

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