Bad Battery Causing Alternator to Show Low Voltage?

[dumo3672]

I am trying to troubleshoot if my alternator is bad or if my battery is bad. When the boat is idling and the battery was charged the night before the voltage is 13.8 to 13.9V. After fishing for a couple hours the battery will read 11.7V with the motor off and 12.6 to 12.7V with the motor at idle. Even at 3,000 rpms it doesn't go above 13V. I believe it is just a bad battery but shouldn't the alternator put out 13.8V to 14V regardless of how charged the battery is?

Boat is a 2003 Nitro with a 225 Optimax

[yam911]

The alternator can only put out X current while maintaining Y voltage. When the limits of X are met then Y will drop.

I am guessing that it is the battery.

[lpugh]

I am not guessing, battery bad or insufficient for the job or both.

[billnorman1]

Yes it should put out 13.8+ volts at 3000 RPM unless the battery is drawing so many amps (to charge) that it is pulling the voltage down. I think once you start it and it goes from 11.7 to 12.7 at idle the alternator is pretty good BUT you are probably using a FF or such to check volts. You should be checking these readings at the battery.

[FrancoCialone]

My Optimax puts out 14 volts at all times, and my starting battery is always fully charged the night before, sounds like the battery needs replacing.

[dumo3672]

Switched out the battery and there have been no issues. At least with that part of the motor