If your charging system is operating properly, you should see between 13 - 13.5 volts at the battery, but no more than 14.4V.
Also run the engine at 1000 rpm with assessories on (A/C blower, headlights, radio, wipers, etc.). Your voltage should still hover in the 13's.
12.5V is borderline. Below 12.5V, and you have a charging problem...
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