BLUE SEA SYSTEMS , is a ADD-A-BATTERY is as easy to use as an ON-OFF switch, yet efficiently handles changing and charging two batteries from a single charging source.
The auxiliary battery includes a Dual Circuit Plus™ battery switch and an automatic charging relay. These components simplify swapping and automate charging, so all you have to do is turn the battery switch ON when you arrive and OFF when you leave.
Dual Circuit Plus™ Battery Switch
ACR automatic charging relay