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.
- 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.