T&T: Batteries

[Robin Brueckner]

Batteries are somewhat like engines: they have their power ratings but making
comparisons is never easy. You just can't tell what's inside and how long it
will really last.

Generally, AGM's offer the lowest overall cost per amp hour delivered, but
their initial cost is often much higher than traditional lead acid wet cells.
In my research I have never found gels to make economic sense aboard boats.

The key measure, not always easily available, is NOT the number of discharge
cycles, (which IS a contributing factor)  but rather the total life amp hours
delivered. In other words, how much power can you get from the battery over
it's useful life.

Wet cell batteries can generally be bulk charged at about 25% of the capacity(
so a 220 amp hour 8D,for example, can be charged at about .25 x 220 or 55
amps) while an AGM will accept four times that rate (that is it bulk charges
at its stated amp hour capacity) or nearly four times as fast.

I don't recall if AGM's can be more deeply discharged (I seem to recall
different claims by different manufacturers)  but I have not researched in
detail in about six or eight years. But AGM's do deliver more life cycle amp
hours than wet cells and should therefore last longer.

I don't believe AGM's for just starting  engines offer any real advantage;
however, mixing types may not be possible as they require different charge
settings than wet cell. AGM's should be considered for deep cycle
applications.

If you currently have wet cell batteries, you'll need to be able to adjust
your charger and alternator regulators to "AGM"..if not you'll need some new
charging equipment as well.

Alas, the formerely great Glacier Bay website technology discussion on  marine
refrigeration and battery power is no longer available.
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