I totally agree and I can say this from experience.

I'm currently using the old fridge relay as an aux battery charging relay.

My aux battery is wired so that the fridge can run on DC even with the
engine off.  I made the mistake of leaving the fridge switch in the 12v
position overnight which completely wasted the aux battery.  That sucker
was DEAD and wouldn't even take a charge.  I drove home with it that way
(several hundred miles) and anything hooked to the aux battery worked
fine with the engine running ... but not once I turned off the key.
That bad aux battery didn't affect the starting battery one iota.

Mike

Sam Walters wrote:
> 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
>