From: adam@address.invalid   
      
   Robert Riches wrote:   
   > On 2012-07-05, Adam  wrote:   
      
   >> I've found that batteries can usually be discharged within a few days by   
   >> simply leaving the device on without a working charger connected. :-)   
   >   
   > Do that with anything other than a single-cell situation, and   
   > you'll likely get a whisker-shorted cell somewhere in the pack.   
      
   NOW someone tells me. :-)  I assume that would be equally true if the   
   device were left on unintentionally.  But what about devices with   
   built-in rechargeable batteries, like my electric razor (probably with   
   the equivalent of 2 AA cells)?  Should that occasionally be run until   
   the batteries are too weak to power it to avoid the "memory effect", or   
   should it never be allowed to discharge that far?   
      
   > You can tell whisker-shorted from just plain dead by measuring   
   > the resistance of the cell, or by putting some charging current   
   > through the cell for a few minutes and then testing for voltage.   
   > If the resistance is very close to zero or the voltage after some   
   > charging current is still essentially exactly zero, you have a   
   > whisker-shorted cell.   
      
   It took me a while, but I figured out how to do that.  (I'm aware the   
   red probe on a multimeter isn't necessarily the positive voltage when   
   it's in ohmmeter mode.)  The good batteries were essentially an open   
   circuit measured in one direction, and under one ohm measured in the   
   other direction.  The not-so-good ones measured under one ohm in both   
   directions.  My conclusion is that those six cells (two AA, four C, all   
   NiCd) are whisker-shorted, and all the others (some NiCd, some NiMH) are   
   okay.   
      
   >>>>> The cure for such a shorted cell is to charge up a   
   >>>>> substantial electrolytic capacitor (at least a couple milliFarads   
   >>>>> aka a couple thousand microfarads) to around 12 Volts or more and   
   >>>>> _ZAP_ the cell in the positive polarity.   
   [...]   
   > Yes, make sure you charge the electrolytic capacitor in the   
   > correct polarity.  I hear they like to explode if you charge them   
   > backwards.   
      
   So far, I've never exploded a component, only burned out a few.  I can   
   see where proper safety precautions would include some kind of loose   
   wrapping around the capacitor.   
      
   >> So what would you recommend for the NiCD/NiMH batteries that report 0V   
   >> after charging?  Zap them, or just discard them (NiCds are hazmats!)   
   >> because they'll be unreliable soon enough?   
   >   
   > I don't know about zapping NiMH batteries.  I'd be inclined to   
   > NOT attempt it unless I knew it was safe.  For NiCds, I'd try   
   > a few rounds of zapping.  On the other hand, NiCds often seem to   
   > just lose the ability to accept a charge after a few years.   
      
   Some of my NiCds are over 20 years old but okay.  Others have "died" and   
   been disposed of over the years.  This project sounds intriguing enough   
   that I'll buy that 4700 uF capacitor and see what happens with those six   
   "dead" cells.  Thanks for all your helpful advice, Robert!  I think of   
   you as the formally trained Electrical Engineer in this group.   
      
   Adam, who only had two years of a B.S.E.E. program   
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