You want to get the highest RC (Reserve Capacity at 25 amp draw). The RC is measured in minutes. Basically you put a nonstop 25 amp draw on a battery and see how long it will last. Also, some companies size 29 is the same as size 31... Some 29s (like ACdelco) have higher RC than some 31s. If you look at the dimensions they are the same size just called a 29 or 31. The Trojan SCS225 is the highest RC battery available with 225 RC. The next is the ACdelco or Interstate 29s with 210 RC. The next is Exide 31s at 205 RC. All these batteries have different dimensions (slightly) so it might be a matter of what fits. Trojan also makes a SCS200 at 200 RC.

If you want a bad to the bone trolling motor battery get a Trojan T1275. It has an RC of 280 but is one hunk of a battery... I got two of them!!!!! I have trolled all day long the last couple times out and it says I had 3/4 battery power left...

DoubleG