the battery is charged by the coil when car is operational, not when it is stationary. Also, do not allow the key to remain in the ignition when the car's main battery dies (like listening to radio for couple of hours) as this may de-synchronize (is there such a word??) the key from the car, and at this point the key becomes a paperweight!!!

The keys are molded shut, so when you cut it open make sure you reseal it rather well, so it does not fall apart in your pocket... How do yo know it is dead? maybe, like Rob said, it is just not programmed into the car?