Our 12V backup battery controller provides backup power for small devices consuming up to 3 amps. The controller has two inputs and one output.
Input 1: Power input from main battery.
Input 2: Power input from an auxiliary backup battery (Lead acid, VRLA, SLA, etc.).
Output: Power output to device.
When the main battery has sufficient voltage, it powers the output and charges the auxiliary battery. As soon as the main battery goes down, the auxiliary battery takes over to continue powering the output. When the main battery comes back, the controller switches back to recharging the auxiliary battery.