Trolling Motor Outlet

What kind of battery should I buy for my trolling motor?

First of all, buy a deep cycle battery. This can be a flooded lead acid battery or a deep cycle AGM battery. The first one is cheaper, but requires regular maintenance in the form of watering. The AGM battery is maintenance free, but a little more expensive.
Do not use a car battery / starter battery. These are made for short burst of energy, and not for providing power over a longer period of time. A starter battery can only be discharged to a maximum of 50%. Good deep cycle batteries can be discharged up to 70 / 75%.

Next comes driving time. How many Amps do you need. A 12 Volt trolling motor will use about 1 Amp per pound thrust. So a 40 lbs trolling motor will use a maximum of 40 Amps. Depending on your boat and the speed, you will use a certain amount of power. Multiply this by the estimated driving time to calculate the power you will use on your trip.
For example a 12 foot Jon boat with a 40 lbs motor at half speed will use 20 Amps per hour. We stay an average of 3 hours on the water. We need a battery with at least 60 A/h.

Important to know is that most batteries show the capacity based on a 20 hour discharge. So an 100 Ampere battery at C20, will deliver 5 Amperes every hour over a period of 20 hours. A deep cycle battery will provide an average of 80 tot 85 Amperes over a period of 5 hours (C5). As mentioned before the maximum discharge is about 75%, so in a period of 5 hours you can only use 75% of 80 = 60 Amps.
In our example a 100 Ampere deep cycle battery will be the minimum we need.