First, get a regular taillight bulb. Remove one battery terminal with the motor NOT running and key OFF, and position the bulb to electrically complete the circuit between the battery post and the removed battery cable terminal. You may need to do a little scraping to get clean contacts. If there is a short in the car's wiring, or some appliance left "on", the bulb will light somewhat. If lit, this is how much current is being pulled out of the battery, and can run it down overnight.
Check to see if your belt is loose.
Get a good digital voltmeter, and see that the battery voltage rises to more than 14.1 volts after the engine has run for a while, which is full charge. Good Luck! Regards, Larry.