We have a ’99 Bounder 35J. A while back (years) we had a problem with the chassis battery disconnect relay. I don’t have any idea how much, if any, of this may apply to your case, but here goes…

On our unit all of the battery management functions are handled by a single black box located in the very front of the unit, i.e. where the chassis battery, radiator reservoir, window-washer fluid reservoir, etc. are located. There are three large relays located in the box. All three have four electrical connection posts: two large posts that carry the battery current; two small posts that are used to operate the relay(s). Two of these relays are "latching"; once energized to move to a particular position, they will remain in that position when the actuating signal (voltage) is removed. These two relays are used for the battery disconnect function; one for the chassis battery and the other for the coach battery. They are switched by reversing the polarity of the voltage applied to the coils: one polarity closes the relay; the other polarity opens the relay. The actuation voltage is only applied briefly, perhaps for a second. They are operated this way so that there will not be a continuous drain on the batteries from the relay coils when the relays are closed. The third large relay is the cross-connect (aux start, etc.) relay. It is a non-latching relay and is only in it’s closed position when voltage is applied to the relay coil.

This black box also manages the charging of both of the batteries from either the inverter (when connected to shore power) or the engine alternator (when the engine is running).

Our particular problem was that the chassis battery disconnect relay had gone bad; the contacts had become "carbonized". I speculate that enough gunk had built up between the contacts to form a resistance path. In fact the unit had become so hot at some point that some of the internal plastic parts had melted and deformed.

I did a "field service" to the relay by connecting some "plumbers tape" (metal strap that plumbers use to hang pipes under a house) across the two large posts of the relay (thus defeating the battery disconnect function) until I could get a replacement. BE SURE TO PHYSICALLY DISCONNECT THE CABLES TO THE BATTERIES, AT THE BATTERIES, WHEN MESSING AROUND WITH THIS STUFF!

I managed to get the schematic of my black box, as well as a replacement relay, from the nice folks at RV Custom Products, which I have attached

EdC