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Lobster
 
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Default 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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Andy R
 
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"Lobster" wrote in message
...
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)


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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john
 
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"Lobster" wrote in message
...
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


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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Lobster
 
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john wrote:
"Lobster" wrote in message
...

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)


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.


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

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.


That possibly sounds like what I was thinking about originally. I like
your method better though!

David
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Lobster wrote:

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.


That possibly sounds like what I was thinking about originally. I like
your method better though!

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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The Natural Philosopher
 
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Lobster wrote:

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)


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...


David

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Junior Member
 
Posts: 1
Talking

This comes from the web, generously submitted by those more knowledgeable than yours truly on the subject. Use at your own risk. A good smart charger seems to be the best answer to all of this, along with keeping things cool. Nothing lasts forever, and it is not worth getting burned or losing an eye over a battery. Of course, nobody likes to be ripped off.

Case in point: Bosch sells two types of chargers, a smart charger and a standard charger. They include the standard with thier drills, you can buy the smart for about 50 bucks. The smart supposedly increases battery life by a factor of four. If they are interested in the environment, why dont they just include the smart charger with the drill? Cost of parts? not hardly. They want to sell batteries. Thats where they make the real moola. why do you think inkjet printers are so cheap?

Before doing any of this, consider what your time is worth. Most of us boomers do this because we come from a world where things could be repaired. That world is gone.

Zapping NiCds to Clear Shorted Cells

Nickel-Cadmium batteries that have shorted cells can sometimes be rejuvenated - at least temporarily - by a procedure affectionately called 'zapping'.

The cause of these bad NiCd cells is the formation of conductive filaments called whiskers or dendrites that pierce the separator and short the positive and negative electrodes of the cell. The result is either a cell that will not take a charge at all or which self discharges in a very short time. A high current pulse can sometimes vaporize the filament and clear the short.

The result may be reliable particularly if the battery is under constant charge (float service) and/or is never discharged fully. Since there are still holes in the separator, repeated shorts are quite likely especially if the battery is discharged fully which seems to promote filament formation,

I have used zapping with long term reliability (with the restrictions identified above) on NiCds for shavers, Dustbusters, portable phones, and calculators.

WARNING: There is some danger in the following procedures as heat is generated. The cell may explode! Take appropriate precautions and don't 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 those 70,000 uF 40 V types used for filtering in multimegawatt geek type automotive audio systems, for example. A smaller capacitor can be tried as well.

Alternatively, a you can use a 50 to 100 A 5 volt power supply that doesn't mind (or is protected against) being overloaded or shorted.

Some people recommend the use of a car battery for NiCd zapping. DO NOT be tempted - there is nearly unlimited current available and you could 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 shorted cell(s) by removing the outer covering or case of the battery pack and test the individual cells with a multimeter. Since you likely tried charging the pack, the good cells will be around 1.2 V and the shorted cells will be exactly 0 V. You must perform the zapping directly across 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 the cell as well to monitor the operation. Put it on a high enough scale such that the full voltage of your power supply or capacitor won't 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 the charged capacitor, + to +, - to -. CAUTION: Polarity is critical - do 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 value - 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.
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