The Add-A-Battery System comes with a Dual Circuit Plus Battery Switch and 120 Amp Automatic Charging Relay. The Dual Circuit Plus Battery Switch switches two battery banks simultaneously with one simple on/off switch, while still maintaining battery bank isolation and minimizing the risk of a dead start battery. The "Combine Batteries" function offers the ability to combine two battery banks in the event of a low start battery. Case can be surface or rear panel mount. Features 3/8" 16 tin-plated copper studs for maximum conductivity and corrosion resistance. The "Starting Isolation" Automatic Relay allows temporary isolation of house loads from engine circuit during engine cranking to protect sensitive electronics. Supports high-output alternators up to 120 amps. Built-in LED light indicates when batteries are combined. Waterproof rated IP67.