An upgrade kit and method for converting a typical monophasic defibrillator into a biphasic defibrillator is provided. The upgrade kit is easily connected to a typical monophasic defibrillator and uses control signals that are commonly available in most monophasic defibrillators. Such control signals include the signals for transferring the defibrillation pulse to the patient, and for dumping unwanted energy from the energy storage capacitor. In addition, the upgrade kit avoids the need for information as to the charge level of the storage capacitor by using a discharge method that allows the production of proper biphasic defibrillation pulses regardless of the initial energy settings of the storage capacitor. More specifically, the upgrade kit determines the desired length of the biphasic defibrillation pulses according to two measurements which are taken during the defibrillation pulse. These two measurements are converted to an index referenced in a look-up table to determine the desired lengths of the two phases of the biphasic defibrillation pulse.