charging battery for trolling motor

Hi guys, I am a pretty new to boats guy. We bought a older 14' Fiberform boat last summer with a 50 hp. Mercury motor. I also mounted an electric trolling motor on it. My question is; I am trying to figure out how to hook up the electric system from the Merc to charge the trolling battery when the Merc is running but not discharge the Merc's starting battery when the Merc is off. Can this be done with some kind of relay that is energized by the ignition circuit of the Merc so it is automatic? I'm looking for a way to help extend the charge if we happen to be out for more than one day. I am concerned that if I just use jumper wires that I will forget to unhook and will run down both batteries.
Thanks,
kootne

A Perko switch will do what you are after as far as isolating batteries. However, I don't believe that you will gain any appreciable run time with short charges from your main motor. An onboard charger and overnight plug in is really the only way to get your trolling motor batteries charged enough to extend trolling time.

Look up " Blue Seas " for an ACR. They are spendy. I support Super Dave's position on this. I have an ACR on my tow rig. This charges my " house battery on the camper without over charging. If you wanted to. You could wire it so that your boat battery could be charged by your tow vehicle.

Thanks for the thoughts, I wondered how much boost I would get from the outboard charging system. Normally we go for a day, come home, charge the battery up for the next day trip. I was exploring options if we ever got the chance to be away several days. Maybe one of these days somebody will come up with a top material that incorporates solar charging panels. But then I might never come home.
kootne

Ok, have been lurking on this thread but will throw my 2 cents in. I have a 15 foot alumacraft fisherman. I have enough of a bow compartment for two group 31 AGM batteries. They can be laid on their side, as mine are. I have a dual on board 15a battery charger and a four way perko switch. Running a V2 bow trolling motor with I pilot. I also run two 1101 Scotty's' off the batteries. I find I can troll and run the downriggers for 5+ hours per battery. I get a good charge overnight. In the back is a 27 group battery for my electric start/ tilt 25Yammy. As a backup I added a Happy Troller in case there is no dock plug in. I modified the Happy Troller by adding a 45 degree notch as the Happy Troller can almost stop a small boat if in the factory 90 degree setting. So far I love this setup. I will be at Odell in June and was able to call ahead to reserve a boat slip near a plug in. My normal fishing day is to charge the battery at home before my fishing day, like you. In my opinion, you have set up your boat in a fashion that works for you. I got good advice from two long time KPOO members and then went for it.