I had 3 different brands of so called smart relays to charge my truckcamper house batteries from my truck only when the engine was running. They all failed in different ways The last one was a Cole-Hersey which failed closed while the truck was shut down with the camper off the truck. It drained both chassis batteries and the solenoid was smoking hot. Fortunately this happened while I was home. No more automatic (smart??) relays for me. I'll use a manual disconnect from now on. My solar will keep the house batteries charged up most of the time anyway.
