looking at the possibilities here leads me to recommend at least checking the battery. Your vehicle is actually an ammeter, and with the ammeter "pegged" on the charge side with only 12.5 volts makes me think that maybe the alternator is wailing out a charge, and a shorted battery is keeping the voltage from rising. I would either try another battery or at least have that one checked. I suspect a shorted cell. A defective battery could destroy a new alternator. Food for thought.. Ray