The Blue Sea Add-A-Battery Kit # 7650 includes the Blue Sea Automatic Charging Relay (ACR) # 7610 and the Blue Sea Dual Circuit Plus Battery Switch # 5511e.
The ACR #7610 automatically combines battery banks when charging and isolates when discharging. It isolates the house loads from the engine starting circuit during engine cranking. This prevents your electronics from receiving low voltage during engine cranking.
The Blue Sea # 5511e Dual Circuit Switch will allow you to manually combine the two battery banks in the event of a low start battery. It will handle up to 1000 amps for ten seconds and 350 amps continuously.