Resurrecting a 'bad' rechargeable cell?

I think I read a tip here a while ago that it was sometimes possible to get a duff NiCd or NiMH cell working again by some way of applying a voltage to it; however I now can't find the post.
Does anyone know? I have two or three NiCd and NiMH 1.2V cells which just display "BAD" on my allegedly intelligent charger and won't charge - anything I can do to kick-start them? (other than binning them, erm, I mean taking them to the local recyling centre)
David
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Put them in the freezer before charging, this increases their internal resistance and makes the charger think they're in better condition than they are.
Rgds
Andy R
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I've a Goodmans charger with microprocessor which is fickle and won't charge any cells that starts at 0v. So if the cell is real badly discharged I have to charge it in an old fashioned non-intelligent charger, ie. at 14 hr rate, for a few minutes to get some cell volts approaching 1v, then the intelligent charger thinks the cell's OK and does its stuff. Only takes an hour or so to charge after that. This is for perfectly healthy cells. There used to be another story about burning micro whiskers off NiCad plates by putting a very high current into them, but don't know much about it. john
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john wrote:

Fab - just tried it and that's done the trick. (Andy - thanks for the freezer tip: maybe I'll try that next time!)

That possibly sounds like what I was thinking about originally. I like your method better though!
David
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The theory goes that these 'whiskers' short the cell so that it not only produces no voltage but it also appears to be a dead short. To 'repair' it one discharges a large capacitor through the cell which is supposed to vapourise the whisker and allows the cell to be charged.
However I'm not sure how realistic this is, I tried it once on a very dead 'D' cell and had no success at all.
--
Chris Green

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On 25 Jan 2005 16:11:25 GMT, snipped-for-privacy@isbd.co.uk wrote:

This does sometimes work, but success is variable - I had a set of AAAs that lasted over 5 years of daily walkman useage with re-zapping every year or so. Others die almost immediately afterwards. It's worth a try before binning them.
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I used to do this regularly back in the days of LED calculators, when NiCad packs were expensive, works fine but the cell often went bad again after a few months, meaning another zap was needed. Used to be fun resurrecting someone's dead calculator with a big flash and a bang.
--
bof at bof dot me dot uk

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Lobster wrote:

You can try 'zapping' them - a very high current applied fo a very short time. The idea is I think that it burns out micro-shorts.
Sort of 10,000uF capacitor charged to 12v applied across the cell...
Sholdn't be excessivly dangerous, and might fix em - nicads anyway. General experience with NiMh is its crap to star with and gets much worse and dies in under 18 months anyway, regardless of how treated...

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This comes from the web, generously submitted by those mor
knowledgeable than yours truly on the subject. Use at your own risk. good smart charger seems to be the best answer to all of this, alon with keeping things cool. Nothing lasts forever, and it is not wort getting burned or losing an eye over a battery. Of course, nobody like to be ripped off.
Case in point: Bosch sells two types of chargers, a smart charger and standard charger. They include the standard with thier drills, you ca buy the smart for about 50 bucks. The smart supposedly increase battery life by a factor of four. If they are interested in th environment, why dont they just include the smart charger with th drill? Cost of parts? not hardly. They want to sell batteries. That where they make the real moola. why do you think inkjet printers are s cheap?
Before doing any of this, consider what your time is worth. Most of u boomers do this because we come from a world where things could b repaired. That world is gone.
Zapping NiCds to Clear Shorted Cells
Nickel-Cadmium batteries that have shorted cells can sometimes b rejuvenated - at least temporarily - by a procedure affectionatel called 'zapping'.
The cause of these bad NiCd cells is the formation of conductiv filaments called whiskers or dendrites that pierce the separator an short the positive and negative electrodes of the cell. The result i either a cell that will not take a charge at all or which sel discharges in a very short time. A high current pulse can sometime vaporize the filament and clear the short.
The result may be reliable particularly if the battery is unde constant charge (float service) and/or is never discharged fully. Sinc there are still holes in the separator, repeated shorts are quite likel especially if the battery is discharged fully which seems to promot filament formation,
I have used zapping with long term reliability (with the restriction identified above) on NiCds for shavers, Dustbusters, portable phones and calculators.
WARNING: There is some danger in the following procedures as heat i generated. The cell may explode! Take appropriate precautions and don' overdo it. If the first few attempts do not work, dump the battery pack
Attempt zapping at your own risk!!!
You will need a DC power supply and a large capacitor - one of thos 70,000 uF 40 V types used for filtering in multimegawatt geek typ automotive audio systems, for example. A smaller capacitor can be trie as well.
Alternatively, a you can use a 50 to 100 A 5 volt power supply tha doesn't mind (or is protected against) being overloaded or shorted.
Some people recommend the use of a car battery for NiCd zapping. DO NO be tempted - there is nearly unlimited current available and you coul end with a disaster including the possible destruction of that battery your NiCd, you, and anything else that is in the vicinity.
OK, you have read the warnings: READ THE WARNINGS, DAMMIT!
Remove the battery pack from the equipment. Gain access to the shorte cell(s) by removing the outer covering or case of the battery pack an test the individual cells with a multimeter. Since you likely trie charging the pack, the good cells will be around 1.2 V and the shorte cells will be exactly 0 V. You must perform the zapping directly acros each shorted cell for best results.
Connect a pair of heavy duty clip leads - #12 wire would be fine directly across the first shorted cell. Clip your multimeter across th cell as well to monitor the operation. Put it on a high enough scal such that the full voltage of your power supply or capacitor won' cause any damage to the multimeter.
Wear your eye protection!!!
Using the large capacitor:
Charge the capacitor from a current limited 12-24 V DC power supply.
Momentarily touch the leads connected across the shorted cell to th charged capacitor, + to +, - to -. CAUTION: Polarity is critical - d it backwards and you will make the problem worse, probably terminal There will be sparks. The voltage on the cell may spike to a high valu - up to the charged voltage level on the capacitor. The capacitor will discharge almost instantly.
Using the high current power supply:
Turn on the supply.
Momentarily touch the leads connected across the shorted cell to the power supply output, + to +, - to -. CAUTION: Polarity is critical - do it backwards and you will make the problem worse, probably terminal. There will be sparks. DO NOT maintain contact for more than a couple of seconds. The NiCd may get warm! While the power supply is connected, the voltage on the cell may rise to anywhere up to the supply voltage.
Now check the voltage on the (hopefully previously) shorted cell. If the dendrites have blown, the voltage on the cell should have jumped to anywhere from a few hundred millivolts to the normal 1 V of a charged NiCd cell. If there is no change or if the voltage almost immediately decays back to zero, you can try zapping couple more times but beyond this is probably not productive.
If the voltage has increased and is relatively stable, immediately continue charging the repaired cell at the maximum SAFE rate specified for the battery pack. Note: if the other cells of the battery pack are fully charged as is likely if you had attempted to charge the pack, don't put the entire pack on high current charge as this will damage the other cells through overcharging.
One easy way is to use your power supply with a current limiting resistor connected just to the cell you just zapped. A 1/4 C rate should be safe and effective but avoid overcharging. Then trickle charge at the 1/10 C rate for several hours. (C here is the amp-hour capacity of the cell. Therefore, a 1/10 C rate for a 600 mA NiCd is 50 mA.)
This works better on small cells like AAs than on C or D cells since the zapping current requirement is lower. Also, it seems to be more difficult to reliably restore the quick charge type battery packs in portable tools and laptop computers that have developed shorted cells (though there are some success stories).
My experience has been that if you then maintain the battery pack in float service (on a trickle charger) and/or make sure it never discharges completely, there is a good chance it will last. However, allow the bad cells to discharge to near 0 volts and those mischievous dendrites will make their may through the separator again and short out the cell(s).
And this one from Tom Lamb:
Measuring NiCd capacity - I use a very simple/effective system. Put a 2.5 ohm resistor across the contacts of a cheap travel analog clock, which will time the rundown. It is quite consistent for good cells. A good typical AA NiCd will run one hour.
NiCd zapping - I use a 1 ohm power resistor in series with a car battery, though a series headlight will also work. I charge for about 30 secs or until warm, which will clear the whisker and put in enough charge to see if the cell is salvageable.
--
jagans


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