I just recently bought a used boat and I was told to use a deep cycle battery for my trolling motor and a starter battery for my motor.  I am also going to hook up a new fishfinder, I was wondering should hook up the fishfinder to the same battery as the trolling motor or the starter battery.  If I run it to the starter battery will I take a chance of wearing the battery down then I would be unable to start my motor.  Any info would be great.

Thanks,

Jenga

Try it on the TM battery first.  If you get interference when the TM is running then try it on the starting battery.

Unless your outboard requires massive CCA I would use a deep cycle for the starting battery too.  Deep cycle batteries have ample CCA for starting most outboards and you will get the added advantage of the extra deep cycle capacity for running instruments, etc.