(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.

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.

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....

Larry