Ryk wrote:
On 15 May 2006 07:50:23 -0700, in message
. com
" wrote:

The basic premise for calculating alternator size is 4:1. Your
alternator output in amps should be at least 1/4 the amp hour capacity
of the battery bank it is being asked to charge.

I have read this in all sorts of places but haven't seen a good
explanation for the why of it, other than the common sense notion that
it would be nice to get from 50% to 75% in not too much more than an
hour. I agree that it seems like a sensibly balanced rule of thumb.
I'm not sure I see why charging a 400 AH bank at 50 A would be a bad
idea. Maybe something about not running the alternator full out for
hours on end?

65 A into 200 AH seems to be working for me....


The thing is, you're not throwing a constant 65a into a 200AH battery.
When the typical integral regulated alternator kicks in, the voltage
setting is usually in the 13.8-14.2 amp. range. Higher on modern
vehicles. The Alt. is forced to throw a full amp load in the Battery,
but as it charges, the voltage will remain the same on the alt. output,
however the amperage will decrease down to say 7-10 amps. The more
fully charged the battery, the less amps it takes to maintain it at the
set voltage.
Ryk