Let's take this a step at a time, first, remove the negative battery terminal and charge the battery, reconnect the battery and start the car and observe the voltage at the battery. It should be about 14 volts. Then check for a key off draw by installing an ammeter in line with the battery and checking the draw at the battery with all the doors shut and the key off. It should not exceed 50 MA. Let the battery sit for a few hours and watch the voltage, it should not go below 12.5 volts. if it does replace the battery. repost with your findings, I will be here for you.