One of the biggest enemy of a battery is vibration. Optima Batteries are built to avoid this problem with their spiral wound technology.

Vibration in a boat will ruin a common battery faster than an optima. Just look at the difference between the way the Optima is made and the way regular batteries are made.

Optima's also charge up faster and hold a charge for a year at a time. You can't say that about any other battery.

Go Optima Blue Top and get the largest amp/hr rating you can get for the trolling motor.

Also check the output of that battery charger and make sure it's working ok.

I have two optimas in my boat now and have been using the trolling motor on Electric Motor Lakes only and that puts them though the mill. I have been using them for 4 years now. I think they will give me 6 years of good service.