Well. Sometimes corrosion forms between the battery post, and the conector...You can loosen the connectors and pull then off, and look.. Clean them with a wire brush while you have them off..
What does the car do when you try to start it?? If it tries to turn over, but simply is too weak.. Then it's likely that the battery is discharged.. The question is why???
Without some voltage readings I'm just guessing..
Check the resting voltage of the battery.. If its at least 12V or better, try to start the car while testing the voltage.. If the voltage stays the same or approximately you have a bad connection.. If the voltage drops way down, and the car won't start the connection is fine..
Start the car via jumper cables.. Check the voltage after it has run for 5 minutes.. If it isn't above 13V it's likely that the charging system isn't working..
To check for something draining the battery, you'll need to disconnect the battery, and place an amp meter in line.. If you have more than a few miliamps of current then there is some device that is draining you battery...
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