Plenty of room in my Classic for the 4th battery . — You could always get one of those small jump-start boxes and carry it . — Not very expensive , or heavy . — Should fit under the seat , no problem . — I run an M/K 1O1 , and after TWO days of fishing , it still has plenty of go left in it . — Youd have to run on HIGH a whole bunch in a day to pull the batteries down far enough to become a problem with starting , providing the batteries were strong to begin with . — Just my humble opinion . — Hoss