Simple, measure the voltage on the batt after its been runnign for a while, it should be about 12.5 or higher. If its not, start the car and measure voltage, if its 14v and up, get a new battery, if its low, get a new alternator. If the battery has voltage, and the alternator is putting out power, kill the car,disconnect one side of the battery, and measure the power being drawn with a meter. If its a few hundred mA or higher, you have a serious load somewhere. Bracketracer has a decent way a figgureing it out posted, try that…