Smart Battery Combiner from MAGNUM ENERGY INC
It is designed to monitor and charge a second battery using a portion of the current that is charging a main battery. This 25-amp combiner/isolator integrates an optional solenoid drive, remote voltage sense for more accurate sensing, High and Low battery cut-out to prevent over/undercharging, and automatic 12- or 24-volt operation. The basic design makes connecting the SBC as simple as it can get. Like other three-wire voltage followers, this is a very simple device to connect and setup.
$99.99