From: spamtrap42@jacob21819.net   
      
   On 2012-07-05, Adam  wrote:   
   > Robert Riches wrote:   
   >> On 2012-07-03, Adam  wrote:   
   >>>> NiCd cells must be discharged or they develop the memory   
   >>>> effect   
   >>>   
   >>> Isn't just leaving them alone for a few months effectively discharging   
   >>> them?  I read somewhere that NiCds lose about 1% of their charge per day   
   >>> when they're not used.   
   >>   
   >> That would work, if you let them sit long enough.  If   
   >> self-discharge is linear (1 percent of full charge per day), then   
   >> three months and ten days would suffice.  However, if the   
   >> self-discharge is exponential (1 percent of _current_ charge per   
   >> day), it would take quite a bit longer to get them close to fully   
   >> discharged.   
   >   
   > The 1% was only a general, rough figure, but I believe it was linear.   
   > The author's point was that NiCds didn't stay charged like most   
   > disposable batteries, and needed to be recharged periodically even if   
   > not used.  That was news when consumers were only familiar with   
   > disposable batteries, which generally have a shelf life of several years.   
   >   
   >> Letting a fairly expensive investment sit idle for   
   >> months just doesn't feel right.   
   >   
   > 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.   
      
   >>>> you run the risk of reverse   
   >>>> charging the weakest cell in the chain.  Reverse charging a NiCd   
   >>>> cell results in a dead short caused by metal whiskers inside the   
   >>>> cell.   
   >>>   
   >>> How does one detect that?  Will the cell actually have a negative voltage?   
   >>   
   >> The whiskers form pretty quickly (I think less than a minute or   
   >> so) when it goes negative.  Once the whisker has formed, voltage   
   >> is exceedingly close to flat zero.   
   >   
   > I've just been recharging all the NiCd & MiMH batteries I could find   
   > around here (AAA, AA, C, 9V).  After charging, most reported acceptable   
   > voltages (using my DMM's "battery test" mode), but a few reported   
   > exactly zero.  How can I tell whether that's from metal whiskers, or   
   > just a battery that's reached its end-of-life?  All my chargers handle   
   > pairs of batteries, and the other of each pair reported something   
   > reasonable, so I don't think it's the charger.   
      
   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.   
      
   >>>> 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.   
   >   
   > Sounds straightforward -- I can get a 4700 uF electrolytic capacitor at   
   > Radio Shack, and charge it from any of my AC adapters that outputs at   
   > least 12V DC.   
   >   
   >>> So the +ve terminal on the charged capacitor would go to the +ve   
   >>> terminal on the battery?  For how long?   
   >>   
   >> A very small fraction of a second is sufficient.   
   >   
   > Would the +ve terminal on the capacitor first go to the positive output   
   > of the AC adapter, and then to the +ve terminal on the battery?  I   
   > assume it would take only a few seconds to charge the capacitor.   
      
   That capacitance and voltage should do the job.  If it doesn't do   
   it the first time, I would try a couple more times before giving   
   up on the cell.   
      
   Yes, make sure you charge the electrolytic capacitor in the   
   correct polarity.  I hear they like to explode if you charge them   
   backwards.  Come to think of it, another student in high-school   
   electronics class had a bright red spot on his forehead one time   
   after a fairly small electrolytic capacitor decide it didn't like   
   what he was doing with it.   
      
   Just measure the voltage on the capacitor to see when it has   
   finished charging.  Charge time will depend on several factors.   
      
   >>>> The only problem with the zap cure is it reduces the capacity of   
   >>>> the cell slightly, and it was already the weakest cell in the   
   >>>> chain.  The weakest cell gets weaker.   
   >>>   
   >>> And, I assume, eventually won't hold a charge at all.  Is that the same   
   >>> thing as a NiCd cell failing (won't hold a charge, even when charged   
   >>> separately) from normal use, after numerous charge/discharge cycles?   
   >>   
   >> As I understand it, it's not so much that they won't _hold_ a   
   >> charge but that the cell's capacity shrinks so much that it's   
   >> effectively worthless.   
   >   
   > 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?   
   >   
   > Adam   
      
   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.   
      
   HTH   
      
   --   
   Robert Riches   
   spamtrap42@jacob21819.net   
   (Yes, that is one of my email addresses.)   
      
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