I think the best unit I have seen for charging off the engine is the Stealth unit. It is pricey. The following link is to a discussion forum of these units.
http://www.wmi.org/bassfish/bassboar...ors/t65753.htm
Might help in making a choice or at least where to look next. I just put in a minn Kota on board charger and a power switch on the 24V outlet side of the batteries and plug a cord into my console when I am near power. Hope this helps.