I bought one of the smallest batteries interstate sells and it works for hours and never been a problem. It still was about $100 and about the size of a landmower battery in demonsions...I think 30 something amp hours.
I use a size U1 AGM battery (lawnmower siize) it's a deep cycle 35 amp hour battery. It will be fairly run down at the end of the day, but it works pretty well. I put it in a soft sided cooler bag with a shoulder strap and it is simple to take in and out to charge. Also installed a normal MinnKota plug on the battery and motor. Picked the battery up at Northern Tool for $69.99 on sale.