Articles forum: Recovering shorted battery packs



#12

Hmmm...

I'd recommend wearing a helmet during this procedure. Simply ask somebody working for the fire brigade :-)

I used a Personal computer power supply in earlier times and restored the accus (_accus_only_ , no batteries) one after the other.

Seems, that the power supply didn't measure a short during this action. After letting the accu (remeber - no batteries) in the 5V circuit without anything else (voltage at the battery is 1.2 V then) the power supply gets really loud and the battery gets warm, at least. - sometimes it gets hot after a few seconds (That's the part, where I'd wear the helmet)

I tried this method sometimes with some sets of batteries, but found out, that the freshening effect doesn't hold very long and I had to recycle the accus anyway.

NOTE: I tried this with NiCads, not with NiMh accus. I really fear, that the accus might explode when such a treatment lasts too long, but yes, if you're in real need of a working accu, this method will work


#13

Reinhard,

A PC power supply may be overkill, because it can put out a _lot_ of amps at 5V.

What I use is a simple generic wall adapter (say, 6V DC 500 mA). I hook it up to the bad battery cell while watching the voltage on a voltmeter. With a shorted pack, the voltage will drop to near zero, but it'll start to slowly climb. When it climbs up to around 1.2V, I stop the procedure. (The battery cells do _not_ get warm during this procedure.) Once I recovered all the cells, I start charging the pack the "conventional" way.

As you suspect, the recovery effect is temporary at best; once shorted, a NiCd cell will never again work like new. It'll not hold a good charge, and it'll loose the charge in a matter of days, or even hours. But at least if you do things my way, the darn thing will be less likely to blow up in your face (still no guarantees though, so do wear that helmet :-))

Viktor

#14

From my own experience and Steve's article. I think the idea is not to charge the batteries with a high current, but to discharge a large capacitor (zap!) to blow small metallic deposits that made internal short circuits. So the instantaneous current is high, but only for a fraction of a second. All precautions apply !


#15

Yes, high current, very short duration.

The capacitor I mentioned (160uF) charged to 200 volts stores a charge of 1/2 * C * V^2, which in this case is about 3.2 Joules.

That is the same as using a 6V 500mA power supply across the battery for a tad over 1 second.

The amount of energy is quite small.

I still urge caution (mostly because the bite you can get is pretty nasty.

Oh, and the capacitor will probably discharge at between 30 and 50 Amps (conservatively) in well under 1000th of a second.

A common fault in NiCad batteries is the formation of small dendrites that bridge the battery. A small high current burst will vaporise them. Lower currents may melt them.

I have had success with voltages as low as 50V (0.2J) but usually use higher voltages. Even 330V (which is about as high as they go) will give you only 9 Joules, but the sound is very dramatic and will scare animals and small children (and wives, neighbours, and the local police :-)

I repeat that the energy is very low, and the battery has so much thermal inertia that nothing large enough will get hot enough to rupture a battery.

Here's the voltage on 4 batteries that were zapped over a week ago, then charged for between 3 and 5 hours... 5.17V, 5.20V, 5.14V, and one with new cells (the old ones had vomited) 5.38V.

Now one that had been on charge overnight 0.55V

One belt at 118V

In the charger for a minute... The cells are .1v, .12, .12, 1.2V. Even though I'd probably give them another belt for good measure (the voltages on some of the cells are not stable), I'll charge these for a whaile and give you the results later.

Oh, and notice that the cells are not in an even state of charge. I'd be tempted to discharge the cells individually later on and see how they charge up again.

(oh, and 118V is 1.1J, or that 6V 500mA power supply for less than 1/3 sec ;-)


#16

It's already up to 5.5V :-)

#17

>A common fault in NiCad batteries is the formation of small dendrites that bridge the battery. A small high current burst will vaporise them. Lower currents may melt them.

I knew that. I heard of such a method years ago when I was in school, but I heard I'd need a voltage source with a low internal resistance, so I simply took a PC supply. I measured the voltage across the battery terminals and it was (naturally) 1.2 V .

I was some kind of shocked, when I found out, that the PSU didn't switch the supply down during this experiment. Seems, that a considerable amount of voltage dropped across the wires coming out of the supply.

I did this experiments once or twice, before I considered, that this method is simply too dangerous in the way I did it. The experiments should be abt. 12 years ago, so I simply didn't know better. I didn't think about recovering NiCads anymore, when I found out, that they don't last very long (they held their capacity abt some days, sometimes even hours) anymore. I decided to give up on this topic and to recycle such accus properly. I simply guess, that it's not worth the efforts anymore, especially, when you can get processor-controlled charging stations for some bucks - this will raise the lifetime of accus considerably.

BTW: When I (or something else) finds the time, we could design a uP controlled charging station for HP accus. I'd prefer charging batteries of my 21 - 29 series externally. I just don't like the imagination killing a valuable machine, just because an accupack is dead.

additionally the packs could get charged even faster and we could use NiMh accus with our calculators (which seems to be a little strange together with the old HP charging circuit).


#18

> I knew that. I heard of such a method years ago when I was in school, but I heard I'd need a voltage source with a low internal resistance

A capacitor has a very low internal resistance :-) And that is what I use. The reason to use a voltage source is to prevent overcharging. i.e. a 1.2V source can not overcharge a NiCad because the nicad will rise to over 1.2V in a fully charged condition.

The cap can not overcharge the nicad because the total energy is so small.

> I found out, that they don't last very long (they held their capacity abt some days, sometimes even hours) anymore. I decided to give up on this topic and to recycle such accus properly.

Well, these cells were at 5.75V after about 8 hrs of charging, 5.5V about 8 hours after that, and right now (a few days later) at 5.38V

This compares with new cells which seem to settle at about 5.35V and other recovered packs which settle down to around 5.15V average after about a week to 10 days (after charging).

Oh, and by the way these cells have a date code of 1985. The packs I have are dated between 1983 and 1987, with the rebuilt pack 1999.

I'm sort of contemplating putting something together to safely and easily ZAP batteries (i.e. without risk of overzapping the batery or zapping yourself). Anyone interested?

I may do some capacity tests soon. Damn! the DMM that connects to the HP41 is in storage for 6 months. Can you wait that long? :-)

I mean, you've really got to use the HP41 to do the measurements (grin)

I'm sort of contemplating putting something together to safely and easily ZAP batteries (i.e. without risk of overzapping the batery or zapping yourself). Anyone interested?


#19

The battery has now dettled to about 5.22V, so it's not self discharging at an accelerated rate any more.

That's after a week.

Is anyone interested in watching the voltage further, or should I try some load tests with it?

Just for the hell of it I may load test all of my batteries.


#20

A load test might be interesting. Recently I received a vintage APF calculator that had three internal "AA" size NiCds. The cells appeared to be in perfect shape except that they were _soldered_ together with wire as opposed to being spotwelded. (And it looked like this was done in the factory; the original owner also confirmed that no attempt was made to replace the cells.) No corrosion, no signs of outgassing, nothing. When I charged them up they showed the proper voltage, but when I turned the calculator on, the voltage dropped significantly. This is the first time I saw a NiCd that did _hold_ a charge but was no longer able to provide sufficient current due to increased internal resistance.

Viktor

#21

I have been watching this thread with some interest. The capacitor low internal resistance is certainly the right approach. Some further speculation on my part is as follows. It would be very interesting to measure the actual battery internal resistance. If you could implement a low side switch to pulse the battery at a very low duty cycle then the measurement of dv/di would allow you to calculate the internal battery resistance ( I think ? ). Then as you increase the duty cycle and were successful in zapping the dendrites then shouldn't the internal resistance change? How dramatically I don't know. You would need a scope and current probe to accomplish this measurement. I suppose your methods have been trial and error. Since 200 volt supplies are not common and total energy applied is the determing factor, have you any formula or rule of thumb for lower supply voltages and higher capacitance ? Do you zap individual batteries or entire packs ?


#22

I zapped individual NiCd cells (AA size) many years ago, with moderately good results. I think I used 25 Volts DC, with two 3000 uF capacitors connected in parallel (that amounts to 6000 uF).

Given Q = C x V, the charge was 0.15 Coulomb (25 V x 6000 uF)

Again, be very cautious when doing this kind of experiment (it looks safer than 200 V to me), and also keep in mind that Cadmium is toxic.

Today, I think that (except in very specific cases) it would be preferrable to buy new cells, and properly dispose of the old ones; rather than zap the old cells...


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