Hello Jimmie,


I've used a relay to disconnect the auxilliary battery when either
the ignition is off or the auxilliary battery voltage is low ~11.5V.
There is a series diode and resistor in parallel with the relay
contacts to bring the auxilliary battery back up slowly when the
engine is running so the undervoltage lockout can reclose the relay
and not dump the discharged battery on the alternator.

The automotive "ice cube" relays are good for this I've seen some
rated as high as 80A.

Another refinement I did not try is a switch to override the undervoltage
lockout so I could jump the vehicle off the auxillary battery. Perhaps
the switch should be across the diode so the auxillary battery slowly
charges the main battery?


73,
Grumpy



JIMMIE wrote in news:b5cabbfd-f220-4c7e-9cef-
:

I want to add a couple of deep discharge type batteries to my truck
for camping but how do I intermix them with the normal auto battery
and charging system. I was thinking about getting a charger meant for
charging batteries from solar panels and connecting the solar panel
input to the generator. A 20A charger cost about $50USD .This would be
very reasonable if I knew it would work as expected.

Jimmie