From: avery@scruffy.neuroinformatik.ruhr-uni-bochum.de (Avery Wang)
Subject: Re: NiCad batteries again -- useful hints
Organization: DSO, Stanford University
Date: Fri, 13 Nov 92 09:24:39 GMT

Here's something I pulled off the net over 4 years ago -- hope it's useful!
-Avery
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" NICAD BATTERIES - FACTS AND FALLACIES " Published on Radio Communication
May 1988, TT.

Richargable nickel cadmium batteries, have, with reasons, become a popular 
source of power for portable and handportable equipment. They can provide 
reliable service over many years if due account is taken of their 
peculiarities. Yet it remain true that many amateurs are failing to appreciate 
not only the full capabilities but also the limitations of nicad cells used in 
battery packs.

J.Fielding,ZS5JF,in "Nickel cadmium batteries for amateur radio equipment" 
(Radio ZS september 1987,pp4-5) provides a useful survey of the facts and 
foibles of nicads.The following extracts from his article attack some of the 
common myths and also provide some safety hints.

1) "Rapid charging causes a decline in cell capacity".
NOT TRUE provided that the charge is always terminated at a safe point.

2) "You should not charge only partially discharged cells as this causes a 
loss in capacity."
NOT TRUE. It is not necessary to discharge fully nicad batteries before 
charging. In fact, THE OPPOSITE is true. Repeated partial charging gives an 
increase in the number of charge/discharge cycles compared with 
full-discharged cells.

3) "White crystals growing on the tops of nicad cells mean that the seal is 
faulty and the cell should be scrapped."
NOT TRUE. The electrolyte (potassium hydroxide) is extremely searching and can 
penetrate the seals used in minute quantities. These crystals are potassium 
carbonate, which is harmless and can be removed with soap and water. The 
action of the carbon dioxide in the atmosphere reacts with the electrolyte to 
form the crystals. After removing the crystals, it is recommended that a smear 
of silicon grease is applied to slow down the growth of new crystals.  The 
amount of electrolyte lost in this way is insignificant.

4) " I have a cell which appears to take a charge, but after the normal 
charging period the open circuit voltage is very low. I have been told I 
should throw it away."
NOT TRUE. The reason the cell won't take a charge is usually due to minute 
crystalline growth across the internal electrodes, caused by prolonged 
storage. A cure that nearly always works is to pass a very high current for 
very short time through the affected cell. This fuses the internal "whisker".
Discharging a large electrolytic capacitor is one method of doing this. But 
note that in a battery the faulty cell MUST be isolated from the other cells 
since zapping the complete battery will not usually result in a cure. Charge 
the capacitor to about 30v and then discharge it through the faulty cell. 
Several attemps may be required to clear a stubborn cell.

5) "A battery contains a cell with reversed polarity. The only cure is to 
replace it".
NOT TRUE. The reversed cell ca usually be corrected by a similar technique as 
that given for 4). After re-polarising the cell, the complete battery can be 
recharged in the normal way. Full capacity can be regained after about five 
cycles.

6) "A nicad battery should be stored only in a discharged state".
NOT TRUE. It can be stored in any state of charge. Due to its inherent 
self-discharging characteristics it will eventually become fully discharged 
after a sufficiently long period of storage. To recharge the battery before 
returning it to service, a "conditioning" charge of 20h at the normal charging 
rate is recommended. Afterwards charge normally; full capacity can again be 
expected after about five cycles.

7) "It is not advisable to keep a nicad battery on permanent trickle charge as 
this causes permanent degradation of the cells".
NOT TRUE. So long as the trickle charge current is adjusted correctly, the 
charge can continue indefinitely without loss in cell capacity. The safe 
current can usually be obtained from the manufacturer's data, but 0.025C is a 
reasonable guide (ie. about 100mA for a 4Ah cell and PRO-RATA). This enables 
the battery to remain fully charged.

ZS5JF also lists seven safety points that should be considered by users:

1) DO NOT short circuit a fully-charged battery. This if prolonged, can cause 
excessive gas production with the danger of possible rupturing of the sealed 
case.

2) Nicads contain a caustic electrolyte: this is perfectly safe as long as 
common sense is used in use and handling of the cells.

3) A nicad can supply a very high current for a short period (a 4Ah cell can 
supply over 500A for a few seconds). Sufficient thought should be given when 
selecting a fuse between the battery and the equipment. The connecting wire 
should be capable of passing enough current to ensure the fuse blows quickly 
in the event of a short circuit.

4) DO NOT use partially-discharged cells with fully-charged ones to assemble a 
battery. Assemble the battery with all the cells discharged and then charge 
them as a battery.

5) DO NOT carry a fully- or partially-charged battery on an aircraft without 
taking proper safety precautions. A short-circuited battery pack ca be a time 
bomb in such situations. Consult the relevant IATA regulations or ask at the 
airline check-in.

6) DO NOT subject battery packs to very high or low temperatures. Never 
dispose of a battery pack in a fire or throw it out with domestic waste.
If it cannot be disposed of properly it is probably best to bury it in the 
garden in a safe spot.

7) DO NOT discharge battery packs below about 1V per cell, otherwise there is 
a possibility of cell reversal.

ZS5JF provides a good deal of other information on charging nicad batteries, 
and gives as a reference a Varta publication of 1982 "Sealed Nickel Cadmium 
Batteries" from which some of his notes may have been derived. (G3VA)

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