Originally Posted by
driveon
Hi finga, 13.8 is actually the lower limit at which a battery can be fully charged but as Splash posted he was only getting 12.97 with the motor running, so he has a serious system fault.
Goodo, thanks matey
Whether this low voltage is caused by a faulty battery and the alternator is being loaded beyond what it can produce, thus the low voltage reading or if it’s because alternator itself is the problem can’t be easily determined while he uses the existing battery ( whether it’s good or not ).
A clip on ammeter would be handy here
Splash, if your so convinced an auto elec will rip you off, try removing your boat’s battery and substitute it with the one out of your vehicle, if you still have the same voltage readings then it’s safe to say the alternator is in need of repairs but if the voltage is 13.8 or above, then the problem is most likely your battery.
A clip on ammeter would be handy here
A little warning, do not swap batteries while the motor is running, that is a sure fire way to really stuff the alternator.