How should I look after my rechargeable radio batteries?

The failure to manage rechargeable batteries correctly can result in a gradual decline in performance. This decline very often goes unnoticed by the user and can lead to the failure of the equipment at a critical point, resulting in the battery being returned to the supplier as defective.

Memory effect
The gradual degradation in performance is often attributed to “memory effect”. Memory effect is caused when the battery is recharged after the equipment has been used for a short period. In these circumstances, the battery builds up a memory of its capacitance. Recent improvements in battery technology have virtually eliminated this condition.

Changes in crystalline structure
The main cause of reduced battery life is due to changes in the crystalline structure of nickel and cadmium, the component elements of most rechargeable cells.

Under normal operation, where the battery has been discharged fully and charged correctly, the crystalline structure remains finely divided. However, when the battery is incorrectly managed, crystals grow that dramatically reduce the surface area of the cells, leading to a decrease in the cell voltage. Subsequently, due to the low voltage per cell, any available cell capacity cannot be delivered effectively.

How to improve battery performance
To reverse the gradual decline in a cells performance, it is necessary to fully discharge the battery to less than 0.6 volt per cell. This is best achieved by using a battery analyser, designed specifically to cycle the battery correctly. Alternatively, exercising the battery, by leaving the equipment on with the battery connected or discharging the battery fully using a load, will improve the performance, but this method may not reduce the cell voltage below 0.6 volts.

To overcome the degradation, Nickel Cadmium (Ni-Cd) and Nickel Metal Hydride (Ni-MH) batteries should be cycled periodically to maintain the best performance. This process is sometimes referred to as conditioning.

Some manufacturers provide fast chargers that discharge the battery before charging. Although this will prevent crystal degradation, the disadvantage is that the battery life could be reduced. This is because each battery cell has a finite lifetime in cycle terms and conditioning the battery each time uses one of these ‘lives’. To achieve optimum performance Nickel Cadmium batteries that are in regular use should be conditioned once a month, whilst Nickel Metal Hydride batteries need conditioning just once every 3 months.

Fast or slow charging?
Most products are provided with a slow (or trickle) charger that takes about fifteen hours to fully recharge a discharged battery. Clearly, this time is unsuitable for some applications and a fast charger option is usually available to recharge the battery in one or two hours. Fast charging reduces the risk of changes in the cells crystalline structure.

Statistically speaking…
A study carried out by GTE Government Systems in America produced some interesting results. A sample of 2500 portable radios and batteries, used by military personnel, were monitored over twelve months. The standard practice was to “charge-use-charge” and this resulted in a battery failure rate of 45%. However, by “exercising” the batteries, this rate was reduced to 15%. The use of battery analysers reduced the failure rate further, to just 5%.

06/11/2003