mikey@trsvax (05/27/85)
There was a contsruction article in the old Popular Electronics about the spring of 1976 for a NiCad Zapper. Basically, they used a charger of about 50 ma but about 25 vdc to charge up a 3300 or so ufd cap and "zap" a shorted nicad cell. After the zap, the cell would continue getting charged at the 50 ma rate. NiCads ususally short from reverse voltage. "But honest, I didn't put the batteries in backwards!" You don't have to. Running NiCads to a partial discharge does more than lead to the potential for a "memory charge". Lets say that you have a new pack of multiple cells fuly charged. You run it down to about 30%. Since any two batteries are not EXACTLY equal, maybe one is at 28% and another is at 32%. If you don't fully charge the cells up to 100%, ie you let the 28% cell only charge to 95% while the rest are charged to 100%, the 95% cell will be the first to go dead. With one cell dead and the others still pumping power, the curent will effectively be trying to charge the dead cell "backwards". NiCads will actually take a small reverse charge, but the internal construction of the cell leads to small deposits when this happens and these are what "shorts" the cell. How can you avoid this? Well, the easiest way is to always make sure that you give your pack a FULL rcharge. The standard .1C (usually 45ma) rate can be kept up almost indefinately with out cell damage. Running the full 14 hours certainly won't hurt the cell. As for a simple "zapper" I used a 24 vac transformer, full wave bridge, current limiting resistor for about 50 ma on my 8 unit pack, and 3300 ufd cap. Don't try to use a switch on the cap to connect the cell, the pulse is SEVERAL amps. I just slip the cell into the holder. I used to get all my NiCads for free, or almost free, by hitting the local Radio Shack and Lafayette stores. They would give me the units that customers returned as "bad". Also, you can do more than one cell at a time, but cells in series will limit the current pulse through the bad cell. If zapping a pak doesn't fix it, break the pak down and zap the bad cell by itself. Mike Yetsko KA5MJQ mikey at trsvax