Just a thought pattern change here.

Electrons or whatever are not pushed through a wire, they are pulled;
and they are pulled only as fast as the joining wire will allow.
        Devices suck power through a straw (wire); if it's a small
wire then the draw is small.

If the main battery was depleted, having my aux. battery on a 10ga.
wire from the alternator with an inline 50amp diode prevents the aux
battery backing up into the main battery through the alternator
connection. Thus you will not ruin the aux. battery attempting to
equalize the depleted Main.

If the aux. battery is depleted,and the main id good, having a 10ga
wire limits the current going into the aux. battery.  SO....
you will not ruin the main battery trying to equalize the now depleted aux.

Any attempt to bring up the charge on the aux. can be handled by the
alternator's surplus.
The alternator will give priority to the main (through the #4 Ga.
wire) and  piddle the balance into the aux. as it can afford to.
        My aux. battery relay is switched on by detecting 'oil
pressure' in my engine. Oil pressure 'grounds' out and closes the
circuit to the relay.
M2CW.
                                David(dsl82westy)

My two years experience with an Optima has not hurt my main,
whatsoever. I hurt my Optima by running it down to a point where the
radio quit, and a second time by leaving the light on over the stove.
8.5 volts..