Since April, I have added a voltage-activated low-voltage battery disconnect relay that I sometimes refer to as a “protective relay” which protects my truck’s accessory battery from overdischarge. While the accessory battery when discharged doesn’t impact the starting battery (much), it does ruin the accessory battery chemistry by causing the battery plates to sulfurize and warp. Only one or two extreme discharges can greatly reduce the maximum charge and discharge of the battery – even with deep cycle battery that I use for accessories.

While wiring the optional reset switch was a bit confusing, the relay itself was quick and easy to wire. It monitors voltage on the wire going from the battery to all loads inside the truck and in the cap, and when the voltage dips below 12.1 volts for more then 30 seconds, it drops all loads. While this leaves the truck cap and lights running off the inverter dark, 30 seconds after I start the truck and start charging the battery, the relay resets and connects the load. The relay resets when the battery voltage rises above 13.5 volts for more then 30 seconds. This gives the battery a chance to start charging before a load is connected to the system, and minimizes stress on the truck’s electrical system.

It seems to work well at protecting the battery and avoids the annoying chirp of the inverter at low voltage. I like how it works without further action on my part, and I don’t have to worry about monitoring battery voltage to avoid overdischarging the battery while camping. Moreover, it automatically resets about 30 seconds after I start the truck, so I don’t have to go digging around to find a reset switch.

The model I got was $50 from Cabella’s. There are other models out there, but this model has an integrated relay and has an easy to use voltage cut off adjustment and reset button, and automatically resets. So that’s what I use.