> The voltage regulator does just that, maintain a fixed voltage. Typical
> is ~13.8. The batteries current draw is based on its charge acceptance
> which will be a function of state of charge, (SOC), and temperature.
> The other limit will be the wiring resistance of the charging path.
> Typically, a group 41 will draw about 20 amp/hours from 20% to 80% state
> of charge. Then it will taper down to as little as 2 amps.
>
> Dennis
>
>
> Just finished installing aux battery in my '90 Westy, using regular
> group 41 and the
> existing relay system. Everything is up and running fine.
> Recent discussions about relays, isolators, separators, over or
> under-charging, etc.,
> makes me ask this question.:
> Does the voltage regulator sense when the main battery is fully
> charged and slow
> down the rate or something, and if so, what happens at the aux battery
> if it was still at a lower state of charge?
I guess what I did not realize and what Mark D. kindly pointed out:)
is that with dual batteries and a relay the batteries are connected in
parallel,
so what the alternators voltage regulator senses
is the demand in the system ( ie the demand from both batteries) NOT just
the  main battery
so if the system demand drops off because BOTH batteries are charged then
the regulator kicks in.
So in this case if the main battery is fully charged but if the Aux is still
in a discharge state
then the voltage regulator will not kick in because there is still demand in
the system by the aux battery.
And the main battery will not be over charged because its 'charge
acceptance' is low so no current will flow
to that main battery (albeit current will flow via its + terminal because it
is connected eletrically to the system)

now if i still have this wrong hopefully Mark/Dennis will pipe up

now i have a 2 question i would like confirmed
Is an over charge condition to a battery only when the voltage supplied by
the alternator is HIGHER than
the voltaged required by the battery during re-charge

Is an under charge condition to a battery only when the voltage supplied by
the alternator is LOWER than
the voltaged required by the battery during re-charge