I had 2 year old group 27 batteries that came with my boat and they did that few weeks back. I was going fine doing 1.5 on 4 and the next time I looked it was on 8 to maintain that speed. I went and bought some new Group 29 size (Everstart Maxx) and I did 1.6 from Cole's point (Lake Marker 2) to almost Toby Tubby (Lake Marker 8). It stayed between 3 1/2 and 4 1/2 the whole way.