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How to restore and charge nicad batteries

How to restore and charge NiCad batteries

Nicad batteries have to be charged with current rather than a fixed voltage because they have very low internal resistance.

Various charge rates are used such as trickle charge which takes 15 hours. We call this rate C/15 where C is the capacity of the battery e.g. if we have a 1500mAh battery, we would trickle charge it with 100mA for 15 hours.

At higher charge rates we risk overcharging the battery which will cause a reduction in capacity. To detect the end of charge, the minus delta V method is commonly used. This is where the cell voltage peaks and then starts dropping when fully charged.

Another indication of the end of charge is a rise in temperature of the battery. This charge termination technique is more difficult to implement electronically but simple for us humans!

A very simple fast charger would be a resistor from a 12V car battery feeding say an 8.4V NiCad battery. The user would periodically touch the battery to see if it had become warm and disconnect it. A crude safeguard against overcharging would be a timed cut-out switch.

The recommended minimum discharge voltage for NiCads is 1V per cell. This helps to reduce the problem of cell reversal where if one cell in a stack is completely discharged, it is not reverse biased by the other cells which would cause damage.

NiCads tend to grow crystals if charged very slowly or are never more than partially discharged. This gives rise to reduced capacity often referred to as the memory effect. Sometimes these crystals known as dendrites can short out the plates of a cell making it dead. The number of dead cells may be determined by the drop in voltage of the battery from the expected value of 1.2V per cell.

To recover a dead battery you can zap it i.e. pop the dendrites like fuses with high current. My technique is to charge a low impedance capacitor via a resistor from a 12V power supply. Then tap it on the battery terminals to cause a high current pulse to flow through the battery. Keep the charging supply connected. Monitor the voltage across the battery to check for recovery. You may need to apply several current pulses to restore the battery. Allow the internals of the battery to cool between pulses so it is not damaged by overheating. After recovery take the battery through a full charge (C/3 rate) /discharge (C/5 rate) / (C/3 rate) charge cycle.

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