Now I have an old evinrude 45lb thrust 12 volt. Thinking of going with a new 55lb thrust. Will I see much difference in power and will I see much difference in battery life? I have a lowe 170 bass boat and the 45 has been ok but 4th speed is out of it. It works but runs the same as 3rd. While I'm asking does anyone know how to fix this problem?im not good at trouble shooting electronics.. Thanks any advice is appreciated.

Old trolling motors lose power over time.  Fact of life.  If you are replacing and going with the same wiring, go for the biggest 12 volt motor you can find - probably a 55 lb thrust.  If you have room in the battery compartment, check out how much it would cost to to to a 24 volt system.   You won't believe how much easier your boat positioning chores will become if you upgrade.  The variable speed feature alone will save you hours of fishing time.

A 55lb thrust will be much different than the 45lb motor and it will be a little easier on the battery. Think of it this way - if you were having to be on "full throttle" for your current setup, you will most likely be able to drop down a gear, let's say to 4th, because you have more thrust. You'll end up not using as much battery for your 12 volt system.

Of course, the good thing is that you'll gain more control in the wind just in case you need to go full throttle. If it's in your budget, I say go for it.

Then, maybe you can get the other motor fixed by a local repair shop and sell it to recoup some of the costs of the new motor

Thanks guys I think I'm gonna go with a 55lb.

yup the variable speed saves tons of battery life. you can go .5 or .75 instead of clicking the 1st setting.  i have Minn Kota Endura Max 55lb and love it

If your in a bass club talk to your federation I got a 1400$ 109 mg for $900 brand new