Victron Cyrix-ct Battery Combiner Kit:
This 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 engine�s electrical system, but isolates the starting battery when charging ceases so you never get stranded.
The Cyrix Battery Combiner is a microprocessor controlled heavy duty relay that automatically connects batteries in parallel when one of them has reached a pre-set voltage (indicating that the battery is being charged), and disconnects when the voltage decreases below float level (indicating that one or more batteries are being discharged).
Data sheet:
https://www.victronenergy.com.au/upload/documents/Datasheet-Cyrix-ct-120A-230A-EN.pdf
