Like other re-chargeable batteries, Ni-MH batteries have characteristic properties particular to their chemistry.

These properties can be grouped under the following headings:

• Charge
• Discharge
• Shelf life (Storage)
• Charge - Discharge cycle life
• Safety Precautions

The charging of Ni-MH is both temperature and charging current dependent. The battery voltage rises with charge current and at lower temperatures. There are ideal conditions for the batteries to reach their full operational capacities. Ideal temperature ranges from 0 oC to 35 oC. Above or below this range, the battery capacity deteriorates rapidly especially at high temperatures.

Similarly, an ideal charging current situation exits, with the preferred charge being around 0.5 C~ 1C for maximum stored capacity. At higher charging currents, the internal temperature and pressure rise and may cause the installed automatic safety vent to open for releasing the pressure. The ideal battery charger has to be able to charge the cells rapidly without damaging them. However, a Ni-MH battery cannot tolerate rapid charge all the time. The charger must cope with the following conditions.

Towards a fully discharged cell, an ordinary charger will send a high charging current since the voltage difference between the charger and the cell is high. This causes rapid charging which cannot charge the cells to their maximum capacity, causing an increase the internal temperature. The ideal charger will send only a trickle charge to a fully discharged cell until the cell voltage rises.

When the cell voltage exceeds 1.1V, a rapid charge may be applied. The cell voltage will continue to rise reaching a possible maximum of 1.8V. If a cell reaches this voltage, the charger has to be able to switch back to trickle charge in order to control the cell temperature and avoid damage.

Rapid charge is also to stop if the cells start losing their peak voltage by 5mV ~ 10mV( V). The charger will revert back to trickle charge in this case

A dedicated thermistor usually monitors the battery temperature. As already stated, there is a prescribed ideal temperature range for battery charging. The charger will only provide a trickle charge if the actual temperature of the battery is outside this range. Moreover, the rate of change of temperature within this range is also monitored. When the rate of temperature change dT/dt is higher than that tolerated (feedback from the thermistor) the charger reduces the charging current to a trickle value. Typically, rates of temperature changes of the order of 2oC/min are allowed.

A quality charger usually has other timing features over and above the functions mentioned above. For instance, there is circuitry installed to limit the maximum charging time, an initial delay in order to correctly sense V, and a rapid charge timer. Since the properties of battery packs vary from one pack to the other, specific chargers for specific battery products are normally designed.

Summarising ideal charger properties:

At New Energy Ltd. there are certain battery applications like bicycle battery packs which require very careful attention to the above parameters. During assembly, sophisticated computerised equipment test these parameters. Moreover, for this type of application, where several D-type cells are utilised, initial matching of the cells is required for consistent and equal performance of all the cells during their application.

Similar to charge, discharge characteristics are also temperature and discharge current dependent. With regards to voltage characteristics, at the nominal voltage (1.2V) a flat curve is obtained which is similar in shape to what would be expected from a Ni-Cd cell. The discharge efficiency decreases with temperature drop or rise in demand current (as one would expect). It is to be remembered that the high current discharge ability of Ni-MH is not as good as that of Ni-Cd equivalents, thus making them unsuitable for certain applications where high current dissipation may be required.

Repeated cycling of the batteries, under high discharge cut-off voltage conditions of over 1.1V per cell causes a drop in discharge voltage and possibly a drop in actual capacity. Lowering the discharge end-voltage to less than 1.)V per cell restores the batteries.

All Ni-MH cells lose part or all of their capacity over time. The initial drop of a fully charged cell is high (~10%) but after this initial drop, the cells discharge less rapidly to lose a further 10% over say a one month period. Elevated temperatures increase the rate of self discharge. One advantage of Li-Ion cells over their Ni-MH equivalents is that Li-Ion cells have better shelf life characteristics, thus requiring less user-attention.

Typically, 500 or more cycles can be expected from Ni-MH cells. After this, the useful capacity of these cells deteriorates rapidly, making them unsuitable. Much depends on the care that the cells receive during their lifetime. If optimal charge and discharge conditions are not applied during the battery cycles, the number of cycles tolerated and hence the lifetime are much reduced.