Originally Posted by Catbones
However, I had to take the single aux battery out since it was dead and seemed to be putting a low on the main battery. The story of that was that the previous owner has setup so that the aux charges with the main battery as the vehicle was on. He did a really bad job.
Does anyone have better ideas and diagrams of how I can put two or three aux batteries in, interconnect them and have them charge while in motion?
The most simple, reliable way to do it is with a "continuous duty" relay rated for at least the max. amperage your alternator can produce. They are available for not much $$ at any auto parts place or online. They have two large posts or lugs and one small one, the small one is the trigger circuit. You connect the charge line (heavy wire) to one large post and another heavy line between the second post and the house (aux) battery, then connect a lighter wire that is switched on with the ignition in the' run' position.
Basically the relay then acts like a switch, when the engine is running the alternator and chassis (main) battery are then all connected together. When you turn off the key the power to the relay drops and the 'switch' opens separating the batteries.