Help?

I have a alumacraft 1648ncs that was utilized only as my duck boat. Im now turning into a fishing boat for the summers (I can't stand seeing it sit). I'm running 1 exide deep cycle marine battery. Connected to it I have a minn Kota trolling motor and my Baja designs light bar (which is off currently along with the blind). So.... I bought a small fish finder, wired it in, and the damn thing won't turn on. It seems like it's trying, a white screen flashes but goes off. I have 12.20 vdc at the fish finder plug. Manufacturer says operating voltage is 10-17vdc. Any thoughts? I really don't have room to add another battery.

 

I think I figured it out. I'm powering off of the light bar lead which has an inline 25 amp fuse on the positive leg. I'm thinking if I switch it out to a 2 or 3 amp I'll be fine. Just gotta make sure I remember to switch it back before duck season...