How do I test NiCd cells in a battery pack?

[holabr]

I have a number of 18 volt battery packs for DeWalt cordless tools that no longer hold a charge. I had read somewhere that often only one or a few of the cells may actually be bad causing the entire pack to be useless. I would like to see if I could make a good pack by using the good cells from several bad packs. The problem is how do I determine which cells are good (and rechargable) since all cells are showing fully discharged ( 0 volts on my multimeter)?

 

[BradtheRad]

Yes, often it is just one cell that goes bad.

You will have to measure each cell individually. A single voltmeter is sufficient.

You'll have to open the housing. If it is hard plastic then be careful how you open it up. A sharp tool can slip.

Look for any cell which is oozing or crusty. Do not continue to use it. Discard it. Use hazardous disposal as needed.

Apply DC voltage to the battery pack (charging voltage is fine). Take a volt reading on each cell. Stick pins through the wrapping tape to connect to the terminals if necessary.

The cells may need to charge a while before they show anything on the meter.

A cell might stay low while the others rise. That one is letting current pass (has low internal resistance which is good). But it is not taking a charge.

A cell might have high internal resistance, which is bad. It will cause the pack to charge slowly. The bad cell might take a charge and show proper volt level. But as soon as you connect a load, its voltage drops.

You can experiment with each cell if you wish. Charge one, test it with a low-ohm resistor, at however many amps the tool draws.

When you remove a cell, try to leave the metal connecting tabs attached to the good cells. You want to solder the ends of the tabs, because heat can damage batteries.