AGM batteries

Choose wisely when you buy boat batteries. The cheapest rarely prove to be the best value overall

This article about AGM batteries but what about the other types – gel cells and liquid lead acid batteries. Well, they certainly have their supporterd but in my view the AGMs currently stand head and shoulders above the others.

Indeed, there can be few electrical devices more suited to a life afloat than AGMs. They will operate in any position – upside down or on their side; will continue to function, even if punctured; and, perhaps more importantly, will accept large charging currents better than any other type. For the offshore sailor they are an obvious choice.

The ‘AGM’ stands for absorbed glass mat – a reference to their form of construction. The two other types of batteries commonly used on boats fall under the headings ‘liquid lead acid’ and ‘gel cell’ – equally apt descriptions of those having a liquid electrolyte and an electrolyte thickened to a form a glutinous gel. AGM batteries have a liquid electrolyte held by capillary action in – yes, you’ve guessed it! – in a mat of glass fibre.

The AGMs shown above are Lifeline batteries which were into their ninth year of service on Shindig when they were replaced. They were still functioning at that time, though no longer coming up to full voltage. Although not cheap, their cost spread over that period, represented  very good value indeed.

A quick caution before I wind up. Although AGMs will absorb a charge current very quickly, they can be damaged or even destroyed by excessive voltage. This means that all charging sources – engine alternator, wind turbine, solar panels, shore-powered chargers – must be regulated accordingly. With modern ‘smart’ regulators, this is no problem since all should offer an AGM setting.

And what about the new boys on the block – lithium ion batteries? Yes, they certainly have potential and one day will probably supplant AGMs. But not yet. They don't work well at low temperatures and the occasional one has been known to burst into flames.Both Samsung and Sony have had to withdraw entire product ranges.

A more spectacular incident involved a super-light single-handed race boat. A Li-ion battery burst into flames, luckily while alongside, and was thrown overboard where it continued to burn underwater for a couple of days! I have also heard tales of military vehicles reverting to AGMs.

If anyone knows more, I would very much like to hear from them.


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