David Beierl wrote:

> The relay puts the two batteries in parallel, and if the alternator were
> not charging then you're right, the battery with more charge would
> discharge into the other.  However (this is the point of the relay), the
> alternator *is* charging b/c the relay would not have closed
> otherwise.  Now the alternator brings the system voltage up higher than
> either battery can supply by itself, and both batteries charge
> simultaneously.  Whichever one is more discharged takes the most charge,
> which is exactly what you want.

This configuration fails if one of your batteries is bad (shorted internally,
the usual death of old batteries). In this case the dead (bad) battery takes
most of the charge current to the detriment of the other (good) battery.

BTDT
Larry