I suspect that my 1990 Beneteau 41S5 with a Perkins Prima 50 engine might not be working. How can I tell? I waited untill the bateries were a little low at 12.1 volts per the gage on the main switchboard. Then I started the engine and reved to 2000b rpm. gage was still at 12.1 volts. However when I run the dockside 115volt AC charger the gage quckly goes to over 13volts. Help please. Thanks from Ole George