Charging house batts while driving

fulltimin wrote:
The relay that ties the 2 sets of batteries together, is only supposed to be used for starting. If the coach battery voltage goes low enough, that it won't start the engine, assuming the house batteries are in good shape, there is a button on the dash that will temporarily tie the two together, to help it start.

It is a momentary switch, and as soon as you let go of the button, the relay drops out and the 2 are separated again.

There is however, a black box near the batteries, (or should be) that the charging wire from the alternator goes into, and then it is split, so that both sets of batteries get charged at the same time, but will not backfeed into each other.

There is nothing to do, it is automatic.

There are various setups.

Sometimes the same relay is used both for the emergency start feature and to connect the batteries together when the engine is running, in which case it is activated by pressing the button or (typically) by a run circuit from the ignition switch. This is what my motorhome has, and it's pretty common but by no means universal.

Sometimes there's a separate or partly separate bidirectional relay thingy (aka BIRD) that connects the two together whenever one of them is above a certain voltage threshhold, which also allows the converter to charge the chassis battery.

Sometimes the charge from the chassis to the house battery is via a solid state isolator (a power diode), which has the disadvantage of introducing a voltage drop of about a half a volt or so in the charge circuit for the house batteries.