Charging 4 Bats on 3-Bank Charger

I have a MK 3150 3-bank charger which is charging my 36 volt 3 battery system for my trolling motor. I also have a 4th battery 12 volt for lights and fish locator only, what is the best way to connect this battery into the system to charge so I would not need to purchase another charger? I don't mind if it takes longer to charge, since the charger stays plugged in until time to go and I only get out about once a week anyway. Appreciate your input, and if you can sketch out the wiring and post would be super great!

Without getting into a battery switching system and/or a battery isolater (which would cost more than a separate charger) you would be better off just getting a trickle charger for the auxiliary battery. I use one made by Schumaker which I got at Walmart for less than $30 last year. If the battery is only used for lights & electronics the 2 amp charger will bring it back to life in a day at most. And you can leave it connected (like your onboard charger) and it will maintain the battery as well.

What Clint said

I just use a small trickle charger (battery maintainer) on my 12 volt system. Its my starting battery, runs my electronics and the big motor charges it so the trickle charger is just topping it off and will do it over night. Plus you can leave it plugged in and they are not expensive.

Thanks for the input folks; I was just thinking since I had the 3-bank charger on board it would be handy to tap into it, but now on second thought a trickle charger defiantly is the best way to go and much safer and no risk of messing up the existing system. This is why I Love Crappie.com, have an idea or question you through it out there and see what comes back; and it could save your tail big time in the future!