NiMH batteries have a high energy capacity rating in terms of milli-ampere hours (mAh). They are used in many applications such as digital cameras and flashlights. These batteries are rechargeable and can be reused for a long time, which makes them a good alternative to non-rechargeable alkaline batteries. The battery life cycle can be extended by properly charging them and avoiding overcharging. Overcharging can lead to a reduced capacity and cycle life, as well as possible damage to the battery.
When a cell is overcharged it begins to build up pressure and elevated temperatures. The overcharge is caused by the chemical reaction of the negative electrode releasing hydrogen into the electrolyte. The hydrogen ions then move to the positive electrode where it is recombined with nickel hydroxide. This produces a gas that is evolved through the safety vent in the positive terminal. Excessive overcharging can result in the loss of electrolyte within the separator, which prevents the transport of hydrogen to and from the electrodes, resulting in diminished capacity and reduced cycle life.
Overcharging NiMH batteries can be avoided by using a charger that is designed for these types of batteries and following the manufacturer’s recommendations for charge times and current levels. The best chargers will also have the ability to detect and monitor individual cells for overcharge conditions. They will have a feature that allows the user to set the desired end of charge voltage for each cell in the pack. This feature is known as a timed or temperature-based charge termination. The temperature-based termination is the most reliable, but it requires that the charger include a thermistor to monitor the battery’s internal temperature during the charge cycle.
In addition to proper battery chargers, a proper selection of battery capacity is necessary for NiMH batteries. It is recommended that each battery in the battery pack be matched for capacity within 2%. This eliminates the possibility that one cell will reverse polarity during a charge before reaching the voltage cutoff point.
The slow charge method is an effective way to avoid overcharging in most NiMH battery packs, but as the demand for higher capacity batteries has increased, fast charging methods have become more popular. The use of these methods, however, requires careful consideration of the battery chemistry and capability, as some NiMH cell chemistries are better suited to faster charging rates than others. In the case of slow charging, proper charge terminations are required to protect against overcharging. Excessive overcharging can lead to reduced cycle life and capacity, as well as damaging the internal structure of the cell. During the slow charge cycle, a battery can be safely charged to a depth of discharge (DOD) or state of charge (SOC) up to about 60%. Beyond this, the battery should be placed in storage mode until it is needed again. It is important to note that while the new generation of NIMH batteries are not susceptible to memory effect, they still must be fully drained and completely recharged before use.