For a trolling motor you want to use a Deep Cycle battery. I bought a BIG Delco Voyager deep cycle at Murray's for about $60 I think. You're not going to get much cheaper than $60 for a decent deep cycle battery.
Just replaced my seven year old Delco Voyager deep cycles with another pair. The old ones weren't bad, I just felt it was time. With success like that, I wasn't going to switch brands, I beat the heck out of my trolling motor batteries 9 months a year. I got another pair of M27MF's they will run my 70# 24V Minn Kota for a weekend of fishing if the wind isn't blowing so hard I have to run the motor on 10 all day.
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