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Ni-Cad Zapper

Ever had a Ni-Cad cell that won't charge or hold it's charge? Well, the reason for it may be the fact that it has been left in a discharged state for a long time and crystals have grown between the plates. The cell is effectively shorted out.

Help is in hand. To repair the cell we need to blow the short circuit safely. A lot of people said "I just place the cell across a car battery for a few seconds." This can work but can be very dangerous as the cell can explode very easily. The best way to do it is to make a capacitor discharge system. Sounds complicated? Well it's not.

With just a resistor, capacitor and a switch we can make a very simple system. Look at the circuit below.

If you are not sure what you are doing seek help from a competent

person before you start.

C = 68,000 uF Working voltage Greater than the supply voltage.

R = 1k 1/4 w

Sw = Single pole Double throw (Change over switch).

The supply voltage was a commercial bench supply.

As you can see, the capacitor is charged by the series resistor (1k) via the switch.

Throw the switch, this causes the charged capacitor to discharge rapidly through the dud cell blowing any shorted spots on the plates. Charge up the cell in the normal way at 1/10th of the discharge rate for 14 hrs.

ie. 650ma cell you would charge for 65ma for 14 hrs

Notes

If you are not sure what you are doing don't attempt this project.

Only zap a cell and not a pack of cells as the good cells

in the pack absorb the jolt when the capacitor is fired.

Caution is to be observed with respect to polarity of the cells and of the capacitor