Dead Battery Again
Batteries definitely get hurt a bit each time that they are completely discharged, so you may have damaged your battery. But I doubt the damage would cause the type of overnight discharge you just experienced.
If your battery dropped from 12.5 plus volts to 2.4 volts in a day or two you probably have a serious current drain from the car (hidden glove light on, stuck relay, etc.). Were you able to hook up an ammeter and measure the current draw from the battery? It should be less than 100 milliamps (ie, less than .1 amp). Ideally, it should be even less than that, like 30-50 milliamps. Since someone noted above that the car's computer can be responsible for a large current draw, try to measure the current draw without removing battery power even momentarily from the car, using the technique that I described previously in this thread.
You can somewhat verify that your battery is good by charging it up for 24 hours on an external charger and verifying that it can hold a charge. Measure the battery voltage about an hour after you have charged it up. The voltage should be about 12.6-12.8 volts. Then connect it to the car and start the car up. That will verify that you do not have any weak or shorted cells in the battery. Then disconnect the battery from the car and measure the voltage again (it should still be in the 12.6-12.8 volt range). Let the battery sit for 24-48 hours while disconnected from the car.
Measure the voltage after the sitting period, it should still be about the same as you measured 24-48 hours earlier. This verifies that your battery is capable of holding its charge while not being subjected to an excessive external current drain. Just as another verification, connect the battery to the car again and start the car up. If the car starts okay, then your battery is almost certainly okay.
You can do all of these tests without the hassle of removing the battery from the car and taking it out to be tested at a parts store.
