Larry wrote:
(Mic) wrote in :


The advantage of a larger alternator is that it recharges the
battery faster (or a bigger one in the same time).


Simply, unfortunately, NOT true. The chemical reaction of converting lead
sulphate back into lead metal plated on the plates is, unfortunately, a
slow reaction. The slower you charge it, the "better" it does the
conversion. Charging a 330AH golf cart battery at 120A is only surface
charging the plates so the voltage rises. It is also dangerous as 120A
shoved against a 2V cell produces something a little less than 240 watts of
heat and will boil the electrolyte in a few minutes.

This may be true, but a slightly different scenario is reality. I use
4 Trojan T105's, for a total of 425 to 450 Amp-hours. My alternator
is a Balmar 110 Amp controlled by a Xantrex 2000R. If the bank is
discharged more than a third, the charge rate is about 100 Amps, and
it will stay over 80 Amps until its at about 85% full. If the bank is
down to 50%, its not uncommon to charge at the 100+ Amps rate for a
considerable time.



Luckily, batteries and alternators are smarter than captains and wannabees.
The initial charge current from low cell voltage drops rapidly off the
alternator's current limit to something more reasonable, then tapers off
safely because the voltage regulator won't allow the voltage across the 6
cells to go over 14.2V, if it's setup properly. The 120A alternator is now
charging the batteries at 20A like it should....plus the load current the
boat is drawing at regulator voltage.

Yes, it would be silly to have a large alternator on a 200 Amp-hour
bank, since it would be unlikely to ever charge at over 60 Amps.
However, it you have a 400+ Amp-hour bank, it works and it makes sense.

BTW, my first set of batteries gave 6 years of hard use - they were
probably done in more by being neglected last winter then by being
overcharged in the summer. But since the replacement cost for the
bank was $280, that's only about $46 a year.



What current should the batteries be charged at, initially?

http://www.batteryuniversity.com/partone-13.htm

"It takes about 5 times as long to recharge a lead-acid battery to the same
level as it does to discharge."

"The charge current for small lead-acid batteries should be set between 10%
and 30% of the rated capacity (30% of a 2Ah battery would be 600mA). Larger
batteries, such as those used in the automotive industry, are generally
charged at lower current ratings."

Good advise.

Why boaters buy huge alternators to charge golf cart batteries remains one
of life's mysteries....

It depends on what you call "huge" and how many golf cart batteries
you have. A 110 Amp alternator is a very good match for 4 T105's.