This minimum charging voltage is 13.8 volts across the battery terminals, or at the output of the alternator. A single lead-acid cell starts to charge at anything over 2.25 volts. Since a 12 volt battery has six cells, any 12 volt lead-acid battery needs at least 13.8 volts to start to charge. Check this web site: Testing battery and charging system


I think you have a bad alternator - pull it out and take it to your local parts store and they can test it on the spot.