Question, What is the big differences between a 12 volt battery and a 24 volt battery for a trolling motor??? After my first day on my boat I came to the conclusion that the first thing I want to do is upgrade the trolling motor. It seem to be a little slow with no motor speed control for it.

You can get motors with a lot more thrust using 24v.   Only draw back is when you have to buy new ones so keep care of them during the winter months when you dont use the boat so they are not dead come spring

I had a 55 lb thrust 12 volt on my 16 ft alumacraft and best case on a week vacation I would have to charge it every other night and it struggled when the wind kicked up. I replaced it with an 80 lb 24 volt terrova and when the batteries were new I didn't have to charge it all week, now I charge it once or twice, but the batteries are close to being switched out. The trolling motor upgrade was one of my best for the boat 

The way you asked the question, let me clear something up. There is no 24v battery. The 24 volts is created by running (2) 12v batteries in series. So, the first difference would be you need room for 2 trolling motor batteries. Second would be the speed since the 24v motors come in higher thrust ratings. 3rd would be cost.

There are different types of controls on these motors. You can purchase them as a 5 speed or a variable speed. A 5 speed is exactly that, speeds 1-5. A variable speed allows you to change speeds in fractional increments.

When you buy batteries, there are different sizes of these as well defined by a group # Most common would be group 24, 27, 29, and 31. The higher the number the larger and heavier the battery and the higher the RC rating. which means longer run time.