rjr@mgweed.UUCP (Bob Roehrig) (05/16/85)
I have had my Kenwood 2500 HT for 3 years and the original nicad just developed a shorted cell. An exact replacement cell is made by G.E. and has a part number of GCL450ST 48F224. The pack is easy to open. Just stick a pointed object into the 2 holes on top and press down on the plastic tabs inside, then pry open, separating the 2 halves. Bob K9EUI
jhs%Mitre-Bedford@d3unix.UUCP (05/22/85)
Re: shorted Nicad cells. It is not always necessary to throw them out and start over! Often, zapping them with a burst of high current, followed by normal charging, will bring them back to life for several more years of service. I do this with a power supply which has internal current limiting to about 1.5 Amps plus a large capacitor across its output. I set it to something above the Nicad cell voltage, like 12 or so volts, and connect it across the dead cell. The capacitor, which is charged to 12 or so volts, zaps much of its charge through the dead cell over a very short time. Judging from the sparks that fly, I would guess that 20 or 30 Amps flows during this burst. I'm not sure what the optimum would be, but 100 uFd at 12 volts seems to bring back most of my dead AA cells. Then the regulated supply goes into current limiting at 1.5 Amps or so and puts some charge into the restored cell. Finally, after 15 or 20 seconds of this, I check the cell with a voltmeter, and it usually reads about 1.3 or 1.4 volts, which, however, is typically dropping off slowly. I then pop the cell into a "normal" charger (50 mA or so for a 450 MAH, "C/10" type cell) and let it soak up a full charge over the recommended 16-hour period. Most of the cells I treat this way come back and work fine for many months thereafter. Supposedly, one of the cell demise mechanisms is the growth of little conductive whiskers inside the cell which short them out. The zap of high current burns out these shorts and allows normal cell operation. I speculate that some of the cells I have thrown out after trying my procedure could also be brought back if I had used a larger zapping capacitor, like 5000 uFd or more. I have found that this procedure works best if applied to INDIVIDUAL 1.2 volt cells. Thus it is still desirable to open up the case to get at the individual cells. However, the procedure sometimes works OK when applied across a whole stack of cells at once, which can be done from the external terminals of a battery pack. 73, John Sangster, W3IKG jhs at mitre-bedford