How does a defibrillator work?

An ICD?: If you are referring to an implanted cardioverter defibrillator it works as follows: it detects fast heart beats above a threshold set by the cardiologist and programmed into the defibrillator and delivers a shock or ATP - pacemaker override, depending on the heart rate. It will deliver subsequent shocks depending on the heart rate it detects.

Defibrillator : An external defibrillator can be automatic or manned either way either a computer or the operator can determine the patient is having a lethal arrhythmia and electrical energy is delivered to the chest wall and the rhythm can be terminated.

Bad battery: Usually it would be that the battery has worn out. Or one of the elebrodes could have a break in it.

Debirillator: A defibrillator is an electric device and subject to all the failure modes of any electric device. Assuming the device has a good power source and all components are working and it delivers the shock appropriately, there is no guarantee the heart will respond. If the cardiac status is too far gone electrophysiologically the rhythm won't convert.