We have a 2005 Chevy Cavalier. About a month ago, we found the battery to be dead. It would charge, but as soon as the car was turned off, it was dead again. We took it to an autozone and they tested it and said that the battery likely was bad and needed replaced. Nothing wrong with alternator. We bought a good battery and everything was fine until this morning when it wouldn't start again. Lights won't come on or anything. We assume its probably the battery again but haven't confirmed that yet. This question is based on the assumption that it is the battery.
The car was last driven either last Wed or Thur, so its been sitting for five or six days. The only thing I can think of that could be killing the battery is the XM radio, which my husband leaves plugged into the cigarette lighter all the time. It stays on even when the car is turned off (so that the stereo in our house will pick up the signal and he can listen to it in here). Even if it is the XM killing the battery, why would it ruin it to where it wouldn't hold a charge anymore at all and have to be replaced? Could it be that the first battery was already bad and so had to be replaced, whereas this one can maybe be charged and hold the charge if we unplug the XM?
Any help would be very appreciated.

This may help:

Parasitic battery drain is the cumulative load produced by electrical devices (e.g. ECMs, clocks, security alarms, and radio presets), when the ignition is turned off. These items continue to operate, when the ignition has been switched off, and are becoming an ever increasing problem. Normal parasitic loads are below 75 milliamps (.075 amps). When parasitic load is greater than 75 milliamps, batteries will drain even faster. Glove box, trunk, and under hood lights that do not automatically turn off when the door is closed, or shorted alternator diodes, are common offenders. Cooling fans, power seat belt retractors, radios and dome lights left on, alarm systems, and electric car antennas can also cause batteries to drain overnight. Leaving headlights on will usually discharge a fully charged car battery, with 90 minutes of reserve capacity (36 amp hours), in a couple of hours.

Two methods are used to test for parasitic load (engine off, under hood light disconnected, accessories off, and vehicle doors closed):

1. The basic approach is to connect a 12-volt bulb across the positive and negative battery terminals to test the bulb and battery. If it glows brightly, remove the negative battery cable and connect the bulb in series between the negative battery cable connector and the negative battery terminal. If the bulb continues to glow brightly, start removing fuses or connections to the positive battery, one-at-a-time, until the offending electrical component is identified by the bulb dimming.

2. The best approach is to use a DC ammeter, connected in series between the negative battery cable connector and the negative battery terminal, or a clamp meter (set to DC amps), clamped around the negative battery cable. Starting with the highest scale (or set multimeter to auto range), determine current load. If load is above 75 milliamps, after the initial surge, start removing fuses or connections to the positive battery post, one-at-a-time, until the offending electrical component is identified by the parasitic load dropping to 75 milliamps.