After fully charging the battery,take it to AutoZone or Advance and have them pull a load test on it.This will check for a bad cell.If battery is OK an alternator with a bad diode is a possibility.Something else that a lot of people don't catch is a bad brake light switch.2 brake light bulbs plus a hi stop lamp burning will pull a battery down in a hurry.Check the lights AFTER you park the car.Might just be the culprit.
In addition to a bad battery it could be a bad diode in the alternator. If so the alternator can still work to charge the battery but drain when it's shut off. Odds are it's the battery though.
Could be interior light left on, or not turning off in glove box or trunk, or brake light switch stuck on. Basically something is still using power, but not a direct short. Draining a lead/acid battery all the way down is not good for it, so expect your battery to deteriorate.
Load test the battery. If that's ok....... look for a draw. You can check this easily. Take a 12 v test light,ground it and touch the negitive side battery terminal. If it lights up dim..... you got a draw. Then you need to find the problem.you can pull fuses and go back and do the test light test again. When it goes out, there's your circuit problem.
Sounds like a faulty battery. Take it to AutoZone and get the battery tested. It's free.
Either a bad battery or you have a direct short.
Bad battery.
Is the boot lid light going off.
You have a short in the wiring or alternator.
run a parasitic draw test and find what remains on. that will define what is needed.
