The required voltage at the batery is a minimum of 13.5 volts and a maxium of 14.5. At idle with lights, A/C and wipers on it has to mantain 13.5 volts. You need to have the battery check and clean your terminals good.
Have you checked the voltage at battery positive and negative terminals while engine running. Reading should be between 13.2-14.4 voltage. Either lower or higher reading indicates bad battery; bad connection(to ground) or shorting in wiring.