i would have to see the set up...
first a few questions...
how is the "house" battery recharged

electronic shift ? not sure what you're talking about ?

you're more than welcomed to give me a call and discuss this...it's gonna be easier to talk rather than type...609-214-7556.you will get me on the phone untill 9p tonite,than,all day tomorrow...

let me add a little more to this...

a third battery powering electronics is definatley the way to go,it however,has to be wired to remain independent of the other batteries,but,you will need a way to recharge this.i like the "sense relay" method of recharging.most new outboard engines have multi battery recharge capabilities,personally,i don't do it this way,i still like the sense relays.

the reason i explain "call me,and we can discuss this",the idea is,i can explain it better this way versus,typing it,sometimes,what's being said gets confused,plus,when you're talking to me on the phone,other questions come up,we can cover those too..make sense ?

like i said,gimme a call,i can steer you in the right direction to get yourself squared away for the coming season...