[shook1]

Originally Posted by Geoff

At the time the engine is switched off, if one battery is at a higher voltage than the other, the high voltage battery will try to charge the other battery until the voltages are the same. If you have some sort of one way device in that is supposed to prevent this, then maybe it has failed. The amperage rating or CCA of the battery does not come into it.

I charged both batteries fully with a battery charger. They were equal voltage. The battery in the trunk is bigger and has more amperage, but has the same voltage as the front one. The device seems to be working properly. The front battery died while sitting, but the rear battery stayed charged.

[johnta1]

The trunk battery is connected directly to the front battery clamp. So, the alternator charges the rear battery at the same time.

Where does the alternator charging wire connect at?

For such separation, probably should use a 12V power post somewhere.
(like a horn relay or something on firewall)

[johnta1]

The front battery died while sitting, but the rear battery stayed charged.

Probably has a drain on that circuit then.