Do this simple test to measure the parasitic drain on the battery:
- Turn car off with key removed and doors closed - just like it would be if it were parked
- Disconnect neg battery cable
- Using your multimeter, connect the positive probe to the neg battery terminal and the negative probe to the neg battery cable
- Divide the measurement (in milliamps) by 1000 to get amps
- Post the number of amps here ______.
If I had to guess, your battery should have somewhere around 30 Ah (or more of capacity). You can find this by looking up the "C10" capacity of your battery from the manufacturers website.
If the C10 capacity is 30Ah, then divide 30Ah by the number of amps that you measured above to get the number of hours to draw the battery down to zero charge.
Using the example numbers from above, it would take a 3 amp parasitic loss to drain the battery completely in 10 hours (30 Ah divided by 3 amp = 10 hrs). This is what PW meant when he said that it takes a significant draw to drain a battery overnight. 3 amps is a lot of current in a car that is turned off!
But the battery probably needs something like 30% capacity in order to start the car, so a draw of 2.1 amps would still be enough to drain the battery to the point where it wouldn't start the car (30 Ah - 30% = 21 Ah and 21 Ah divided by 10 hrs = 2.1 amps).