Victron - Cyrix-ct 12/24V-120A Battery Combiner Kit Retail [CYR010120110R] MultiPlus 12V 230V but isolates the starting battery

but isolates the starting battery when charging ceases so you never get stranded

The remote on/off can be operated with a low power switch or by for example the engine run/stop switch (see manual)

and 48V systems

Similar to the MultiPlus

The SmartShunt is a good alternative for a BMV battery monitor

Victron - Cyrix-ct 12/24V-120A Battery Combiner Kit Retail [CYR010120110R] MultiPlus 12V 230V but isolates the starting batteryThis 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.