Yesterday, while fishing offshore on our Seafarer, we ran with the battery selector switch set on #1. After drifting for about three hours with the GPS and VHF on we headed for shore. The engine cranked over fine. We stopped again about a mile from the inlet and fished again. The engine barely cranked over this time and I had to switch to battery#2. I noticed that my volt meter was showing a charge of almost 16 volts. I charged the dead battery back at the slip and it came right back and cranked the engine over. The engine is a 1996 Evinrude 200. My question is ,is it overcharging and killing the battery or is it not charging. I also noticed that the GPS shut down and the VHF radio went from channel 68 to 88 and started beeping. I look forward to the help.