Well I bought a Interstate 29 series marine deep cycle for about $90 and still haven't hooked up two in parallel yet. I haven't needed to really, although I probably will soon. The one battery has been holding up to a full day of trolling just fine with plenty left over at the end of the day. Will keep ya'll posted if any problems come up. This battery actually has better ratings than the 31 series. Good folks to deal with down here in Perry at the Interstate Store.