Also - I've had batteries read 12.6 volts despite being dead. It's not the voltage that measures starting power, it's cold cranking amps (CCA). When in doubt about your battery, take it to a car parts shop and have them test it or get a new battery. Cheap warehouse batteries don't last long unless kept on a trickle charger. My last $50 battery only lasted a year and a half.
Just sayin'.