OK, just one more thing. Well, maybe three.

1) You charge your aux battery with an external charger to (say) 13.5 volts,
but your vehicle battery is at 12.9. Start the engine, the relay closes, and
the higher charged battery drains into the lower, as they're now in
parallel. Make sense? Or would the alternator output compensate for this as
it charges the vehicle battery?

2) From what I've understood, the only type of battery that will charge to
100% in the aux position is a standard lead/acid starter battery, not an
AGM, not a deep cycle. The AGM's and deep cycles need special external
chargers (which I have, but just for clarification.) True?

3) Has anyone done a mod that allows for convenient external charging of the
aux battery? I'm thinking some kind of a receptacle you could just put a
plug into and thence to the charger? Too much trouble to be removing the
driver's seat every time you want to charge the aux battery, for my taste at
least. A receptacle of some sort would make it easier.

Thanks all--informative posts here.

Geza