Quote Originally Posted by NIMROD View Post
A guy asked about hooking up a 24 volt light on his boat. He has 3 batteries , (1 for starting , 1 trolling battery, 1 for accessories). Everything is 12 volt but why can't he put a jumper (neg to pos) between two batteries, hook only a postive wire to one and neg wire to other for 24 volt and still hook rest up normally for 12 volt ? This cause any problems ?
Yes you can, but there are some gotchas that you need to watch for.

You need to make sure that there aren't any common grounds on the batteries via unexpected things like the hull or intentional things like electronics noise reduction efforts. For example, you can't make any connections between the fish finder battery and the TM battery if you have a US2 motor.