I got lucky and was able to purchase a couple of jelco batteries for $20 each that are used for electric wheelchairs. Totally maintenance free and they last for the duration of the fishing trip. I use one for the trolling motor and one for the lights if I'm night fishing.
Check with any company that provides electric wheelchairs because these batteries are routinely replaced whether they are bad or not and the two I have work like new.
Good luck!
AMERICANS: Willing to cross a frozen river to kill you, in your sleep, on Christmas, totally not kidding, we've done it.