Yandine Trollbridge 12X24 Volt Battery Combiner (TD 1224) allows charging of the 24V (dual12V batteries wired in series) trolling motor battery from the 12V alternator off the main engine or trailer hook-up without any risk of discharging the starting battery.
The TD 1224 works by automatically connecting two 12V batteries in series when running the trolling motor and in parallel for charging. Charging is automatic when the main engine is running and the starting battery has at least 13V charge. When not trolling, the second trolling battery backs up the first trolling battery which can be used as a house battery.
Trollbridge TD 1224 Diagram: