If you are forced to stay with one battery, get a group 29-31 and your going to need a terrova or the motor guide in 55 with the maximizes. I run mine with 2 group 27 and can long line 6 or so hours. I don't know which you have the high front deck or the low deck crappie model. Helped a friend of mine on the crappie model with the big storage box up front. Had to remove front deck, installed 2 group 31's in that storage box and wired everything up with deck off. You can still access to check and add water by opening the lid. Just not enough room to put both in and wire with deck in place. Then he installed a 24 volt terrova.
Proud Member of Team Geezer
Charlie Weaver USN/ENC 1965-1979