"NICAD BATTERIES - FACTS AND FALLACIES " Published on Radio              
Communication May 1988, TT.                                              
 J.FieldiZSJF,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 potassiumcarbonate, 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 20 hours 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. (about 100mA for a 4Ah cell and        
PRO-RATA). This enables the battery to remain fully charged.             
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* ZS5JF lists 7 safety points:                                          
*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 incase 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 can 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.                                
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