Something is draining your battery when the car is off. Are your headlights coming on on their own when the car is parked? Sometimes the spring becomes weak in the switch and the headlight flash comes on and stays on. Try parking the car with the switch in the high beam position, closest to the dashboard. If you have a multi tester, go through the circuits at the fuse box with the keys out of the ignition and see if anything is drawing a lot of power, You will need to remove the fuse to do this.
I had a very similar problem. Battery DISCONNECTED from the vehicle wiring did not discharge over night = battery OK. A fully charged battery CONNECTED to the vehicle would discharge completely by morning. The alternator was the problem. One of the diodes failed and allowed the drain. If you drive the vehicle and after the block has cooled to ambient temperature, but the alternator remains warm to hot to the touch you have found the culprit. The discharge through the diode will generate heat. Take off the alternator and have your local auto parts store spin test it to confirm.
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Believe it or not, the clock will drain a battery, but yours sounds like something else is running besides the clock. But I don't know what. If I was going to be parking it for more than one day, I would disconnect the battery.
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