Jim said:
Running off the same battery should not be a problem. A fsihfinder does not use allot of battery.
I respectfully disagree. The problem with this has nothing to due with the fishfinder affecting the battery, but the other way around. I have had this problem firsthand. First of all, you supposedly will see trolling motor interference on the sonar. I never
seemed to have this problem. However, on a deep cycle battery with your trolling motor, if you were to run it at full speed from full charge down to maximum safe discharge (12.2 or so), you will notice start to get a little slower as the day wears on. This is normal, as the voltage drops, and doesn't affect the trolling motor. However, your sonar will not be happy with that voltage drop, and will start to turn off every time you hit the trolling motor pedal. Royal PITA.
Way back when, before I had an electric start outboard, and only ran one battery in my 12 footer for the trolling motor, I used a small go kart battery to power my sonar. It was a little bitty thing, and would run my Fishmark 320 all day. At Wallyworld or Home Depot, you can get small lawn tractor batteries, for something like 20 bucks. If you are going to be running only a trolling battery, I would recommend getting one of those for your sonar. It has a bit more amperage than my little go kart battery, so you should have absolutely no problem with it.