id stick with any 31 class battery, as long as its a deep cycle battery, no real need spending big money for a optima, you may want to try and get a bigger trolling motor so it doesnt have to work so hard, i had a 30 with samme deal, bought a 36 not much difference but i bought a bass pro brand its a 45 lb transom trolling motor and it pushes the boat all day long on one battery, i have it set up on a little 12 foot boat , just for a small restricted reservoir close by.