The Trollbridge 12x24 for Lithium batteries allows you to charge your 24-Volt trolling motor batteries from the 12-Volt alternator on your main engine, from your trailer hookup, or from any single or multiple bank 12-Volt charger without risk of running down your starting battery.
It works automatically by connecting the two (2) 12-Volt trolling motor batteries in series for trolling motor mode and connecting them in parallel for charging mode. It only charges the trolling batteries when the main engine is running, and the starting battery has reached a full charge level of 13-Volts.
For installations where there's already a combiner, consider instead the Hypertech Trollbridge 24.