The method I used to connect my two batteries was to put relay switches on
the ground wires coming off of my two batteries. The positive wires of the
batteries are permanently connected together so the relays on the negative
wires connect and disconnect them.

I made it so that the both of the two relays would connect the grounds of the
batteries only when 1) the alternator was on or 2) when the key was in the
starter position. This way, I would know that when the alternator wasn't
putting out current, the relays would disconnect the batteries and prevent
one bad one from draining the other one--except when I was trying to start
the vehicle, in which case both batteries would contribute force.

I added a parallel path for the ground wires using manual switches so I could
decide which battery would be "always connected" and which battery would be
"connected only when vehicle running."

Thus, I had two devices on each battery ground wire, for a total of four, all
interrupting wires leading to ground. With the vehicle engine off, only one
of these four wires would be connected. With the vehicle running, three of
these wires would be connected.

Confused?

I don't blame you. It took me nine months of drawings and theorizing to
figure out why I needed to do it this way.

derekdrew@aol.com