Why does my battery completely drain within one day?

Charge the battery and disconnect it, then check it for a charge the next day to eliminate it as the problem. Then pull the fuses on the radio and turn signals and see if it still drains. Next is to trace the radio and turn signal wires down for shorts.

Do you have anything like a gps or something like that plugged into the 12 volt thing(drawing a blank on it's real name...)? Our 2004 Chevrolet silverado 1500 doesn't kill the 12 volt when the truck is shut off, which means if we forget about the radar detector, it will stay on until will either unplug it or the battery dies(hasn't died yet).

"How about, your engine actually does not charge the battery? i have had similar problem with my old car. what turns to be bad was i hope i say it right ""alternator""(dynamo). it produces elektricity and stores unused in car battery. if it does not work, engine takes power for spark plugs from battery, so it dies quickly...

worth testing it, i think..."

I had this problem. I discovered that the nut which clamped the wiring harness to the alternator was loose which caused intermittent connection from the alternator to the car's electrical system including the battery. Tightening the nut down fixed the problem.