FreeLists / homeclinic / [homeclinic] Re: monthly quiz- Nickel cadium batteries

[<homeclinic-owner@xxxxxxxxxxxxx>]

 A rechargable Nickel cadmium battery pack from a cord-less phone was
determined to be bad. Upon farther inspection with a voltmeter and
ohmmeter it was determined that one cell of the three cell battery
pack(sub miniature A type)
was shorted. True Or False. The battery pack or single cell that is
shorted must be replaced. It can not be repaired.

We have two winners. Terry Gaudet & Glenn George. Good!
The answer is False. Most Ni cad's can be repaired by a process called
rejuvenation.

Rejuvenating Ni-Cad batteries is possible depending on what is wrong with
it in the first place. If the cell is not chargeable due to too many
recharging cycles then "no" is the answer (electrically warn out). If it
received a short reverse charge (typical of single cells when a battery is
discharged too much) or has formed a bridge of conducting salts between
the plates and is discharging rapidly (not holding a charge for very
long), then the answer is yes.  The way this is done is to zap it with a
short burst of voltage and current many times the normal charging rate.
Typically, what is done is to charge a large electrolytic DC capacitor to
say 40-150 volts then discharge it through a small 1.5 volt cell in the
charging direction. This zaps and sometimes burns out the conducting
bridge. sometimes itdoesn't work if the bridge is too big or too spread
out. If done properly the result is the battery will hold a charge longer
than it did before zapping it. The cell really goes back to near new
leakage. Another way is to quickly arc it across a 12 volt car battery.
One caution do all this rapidly if too much heat builds up too fast the
cell could theoretically explode before the bridge burns out.

If you do this be cautious. Wear gloves and safety glasses and stand back.
There are times when the bridge is not conductive enough to break the link
and no change happens to the battery. The method does work well and can
save you lots of money, especially on multi celled battery pacs.


Ken(list owner)

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