Nickel chemistry rechargeable batteries exhibit a negative temperature coefficient (NTC) for their voltage characteristics. This NTC characteristic causes a decrease in cell voltage at the charge termination point as follows: At the initial application of charging current to the battery, the voltage will increase from approximately 1.1V / cell to 1.3V / cell relatively quickly. Then the voltage will slowly climb as charge current is converted to chemical energy in the battery. This chemical process is referred to as an endothermic reaction, and causes little additional heat to develop in the battery.

Typical Charging Profile

Some time later, after the batteries charge acceptance has dropped due to saturation, the chemical reaction becomes exothermic. At this point the electrical energy is no longer converted to chemical energy, but is now converted to thermal energy. This causes a sharp increase in the rate of rise of temperature in the cell. The increase in cell temperature causes a consequent decrease in cell voltage due to the NTC effect, the charger looks for this dropoff in voltage, and once detected, initiates a switchback to the trickle charge state. The cell voltage now decreases (due to the reduction in charge current), and the temperature of the cell decreases back towards the ambient temperature. It is important to note that NiCad and NiMH cells have different negative temperature coefficients. In fact the NTC for NiCad is much greater than for NiMH. Consequently, the delta V method of charge termination works much better for NiCad batteries than for NiMH batteries, (since the magnitude of the voltage dropoff is greater). Generally, a delta v charger sensitivity of 10mV / cell is considered good for a NiCad cell. Whereas a NiMH cell calls for a more sensitive delta V detection circuit. The circuits employed by Jerome Industries all employ a nominal 2.5mV / cell delta V detection sensitivity, with a max limit of 5mV / cell. When using the delta V technique for NiMH cells the following precautions should be addressed: 1) Do not allow discharge to below 1V / cell for any cell in the battery pack. In order to accomplish this your system will probably require a discharge cutoff circuit. 2) Make sure the cells are balanced in AHr capacity. Imbalanced Ahr values will cause voltage imbalance at the end of discharge state, consequently causing cell voltages less than 1V. Additionally, syncronization problems can develop for batteries with many cells, at the delta V point. The above two recommendations also apply to NiCad cells, but to a smaller degree. Bear in mind that additional charge termination methods are available to prevent continuous overcharging of the battery. The timeout circuit is standard on all chargers, and will cause the charger to switchback to the trickle charge state after a fixed amount of time. A high temperature cutoff circuit, necessitates the addition of a 10K NTC thermistor to the battery pack. This circuit will cause the charger to switchback to the trickle charge state if the battery pack temperature reaches 50 degrees centigrade.