Troy,

I may be missing something here, but I don't understand why what you say
about the bad battery damaging the good battery in a relay system would
be correct.

The relay switch connecting the two batteries is not closed except when
the engine is running and the alternator is therefore putting out
current.  OK, maybe not for a few seconds after start until the rpm's
are up enough to put out the alternator light, but after this.

If the alternator is pumping 13.5 volts or more  (actually anything
above 12V I think) into both batteries, as will be happening when the
relay switch is closed, how will the weaker battery being drawing from
the stronger battery.  Would it not both be getting current from the
alternator instead?  Unless it is the alternator that is the defective
part of the charging system, the alternator is always going to be the
source of the current a battery draws as its voltage is higher than that
of the other battery.

And if the weak battery won't take a charge from the alternator, how
would it be drawing off the stronger battery anyway?

Sam

--
Sam Walters

Baltimore, MD