Battery Chargers

When fast charging Nickel based battery packs, a charge control is required within the NiMH battery charger to properly charge the batteries. Relative voltage control using the negative delta voltage (–dV) and / or rise in temperature with respect to time (dTdt) principles are proven, reliable charge termination methods.

The -dV termination technique is based on the fact that there is a slight drop in voltage in NiMH / NiCd when they are fully charged, provided they are being charged with DC current. The NiMH battery charger senses voltage reduction when the battery voltage has reached a certain level in relation to the maximum recorded value and terminates charge.

The dTdt termination technique utilizes the cells’ natural rise in the temperature when nearly full to terminate charge. When charged with sufficient current (usually charge rates of C/2 and higher), the temperature of a NiMH / NiCd cell will increase at a rate of >1°C / minute when it has been charged to its full capacity. As the charger senses this elevated rate of temperature increase (via a thermistor within the battery pack), charge will be terminated.

If neither –dV or dTdt occurs, the charger has a built-in safety timer which interrupts the charging after a set period of time and prevents the batteries from overheating.

The NiMH chargers also have a start time which prevents premature charge termination during the first few minutes of a charge cycle. This applies to batteries that have not been charged for extended periods of time, and the natural voltage drop occurs during the first stage of charging.

All chargers are equipped with a computer programmable microprocessor, allowing the charger to be programmed to the specific battery in use.