Victron - Cyrix-ct 12/24V-120A Battery Combiner Kit Retail [CYR010120110R] MultiPlus 12V 230V The MultiPlus switches to inverter

The MultiPlus switches to inverter operation when no AC source is available

The SmartShunt is a good alternative for a BMV battery monitor

and 48V systems

The alternate source typically is an inverter

Remote on/off

Victron - Cyrix-ct 12/24V-120A Battery Combiner Kit Retail [CYR010120110R] MultiPlus 12V 230V The MultiPlus switches to inverterThis Battery Combiner Kit is for adding a 2nd or 3rd battery system in your car, boat, RV or any other vehicle. It utilizes the simplest and most advanced technology to keep all your batteries charged using the engines electrical system, but isolates the starting battery when charging ceases so you never get stranded.