Your trolling motor battery is starting to go bad, and it's time to replace it. If you aren't that mechanically blessed in trolling motors, then this could end up being a difficult task. The last thing that you don't want to happen is being in open water with your brand new battery, and your trolling motor just died on you.
Trolling motors have 3 different power systems: 12-volts, 24-volts, and 36-volts. This information is vital because all batteries come in 12-volts, so you may end up buying multiple batteries. Below is a few ways to determine that your motor is getting the correct battery.
The most obvious answer here is the biggest battery that you can fit in your boat. There are 3 different groups to choose from, which are the 27, 29, and 31 groups of batteries. The 27 group is going to give you your lowest AH at around 115. The higher the group the more AH you will get. This means that if you buy too big of a battery, you will have a higher longevity, but you could be burning a hole in your pocket.
If you buy too small of a battery, then you run the risk of either not being able to handle treacherous waters, or you could be paddling back to the dock. Always Consult with your owner manual to determine recommendations if they are available.
Mother Nature also has a play into what you should purchase with your battery. If you live in a place with constant choppy waters or the risk of a storm at sea, then you're going to have to buy the best battery available. Places that are located on the coast or near a major source of water will want to look at those options.
My personal opinion is to ignore that starter batteries, and focus more on the dual-purpose or deep cycle batteries.
No matter what choice you make always remember to have a smart on-board charger, or have a portable charger available just in case of emergencies. Batteries that are in the groups of 29, and 31 can be very heavy.