Recovered NiCads

[auplater]

Over the weekend, I was on a roll... was able to fix my 2 tractors (electrical problems) and recovered a half dozen NiCads from various portable drills, etc. of varying voltages as follows:

All of these were showing internal shorts (resistance < .8 ohms, no voltage)... so, after tearing one down and working on each cell
as follows:

hooked up as though for charging to 15V 10A power supply, ran current up to 10 amps, watching the voltage on a meter. The initial voltage was quite low maybe .15V for a variable 10 -30 seconds depending on the cell, then would jump to 1.2V as the current dropped to maybe 4 amps. I then reduced current to .4 amps and continued to charge @ 1.3 volts until current dropped to <100 mamps. This process worked on all 12 of the 14.4V pack I d/a'd, so I tried the next pack as one. I had to bridge 2 supplies, as 15V wasn't enough to drive the pack off of low voltage, so using another 25 amp 15v supply, I cranked maybe 15 amps thru the pack until the voltage jumped to ~14 volts or so and the current again dropped. After processing, the cells charged fully and are running fine in my portables under heavy use.

Interestingly, one could watch the individual shorts in the 12 cells melt (dissolve?) away, as the voltage jumps/current reductions occurred in stages... not all at once. Recovered 6 nicads and only blew one up...

... forgot to turn down the current and garage filled with acrid smoke that "got out" of the battery

Not for the faint of heart, but the local recycler wanted $35 to rebuild each one... so I saved a bundle..

John L.

[AndrewT]

Hi,
this sounds a bit similar to another poster that said blast them with a high current from a capacitor to burn out the shorting whisker that forms across some internals.

[auplater]

Yupp... 'cept this can be a bit safer and more controlled. Also probably easier on the battery as you can observe what happens and monitor the repair. I read that post a while back, but being an electrochemist among other things, I HAD to try the theoretical thing...

John L.

[acid_k2]

Originally posted by auplater
Recovered 6 nicads and only blew one up...

... forgot to turn down the current and garage filled with acrid smoke that "got out" of the battery

this method or recovering is VERY dangerous.

- battery may explode
- cadmium is a poison

[Eva]

I employed a similar method to recover shorted Ni-Cd cells and got the same good results. The method is safe as soon as you keep yourself sitting in front of the setup all the time reading the meters and acting in consequence. You can't leave such a setup operating on its own (unless some kind of automatic control scheme is added).

It's true that Cadmium is a bit more poisonous than the stuff used in Ni-Mh or Li-Ion, but Ni-Cd cells can last much much longer than the other types if they are used with care. Li-Ion are particularly short lived, in my experience they may last 2 years at best under normal operation (shame!) despite all the active protections employed to avoid exceeding battery ratings.

Overcharging is the main source of Ni-Cd damage (big shame that most low-cost chargers don't control either time or cell temperature), followed by reverse biasing when one of the cells of the pack becomes flat and you notice less power but you continue using the tool (never do that!!)

[acid_k2]

hi Eva
I agree with you

Originally posted by Eva
Li-Ion are particularly short lived, in my experience they may last 2 years at best under normal operation (shame!) despite all the active protections employed to avoid exceeding battery ratings.

[/B]

A Li-Ion cell can be recharged a thousand times, if you use a good quality cell and a temperature-voltage-current controlled, low current charger. But probably buying a new cell is cheaper then a microcontrolled charger....

[auplater]

Originally posted by acid_k2



this method or recovering is VERY dangerous.

- battery may explode
- cadmium is a poison

No this method is perfectly safe... works like a champ.. as long as you PAY ATTENTION TO THE DETAILS!!

not for people who like to "fool around" and/or are inattentive, nor for anyone who is afraid of DIY.

Yes cadmium, lead, bug spray, gasoline, kerosene, aspirin, salt, water, alcohol, Tylenol, ginseng, fugu, sand, electricity, etc.. etc... are all poisonous at some point if handled or used inappropriately... but are used daily by most everyone w/o consequence unless they do something stupid...

What's your point?

John L.

[dangus]

I kinda like my method better: hook up a normal constant current supply with a voltmeter. Then, VERY briefly apply a high current using a modest gauge of wire (lamp cord, not welding cable) by just brushing against the terminal of a car battery. Watch the voltmeter; if the short is gone, the voltage will increase steadily under charge. For safety, put the victim around the corner and under a box. It usually takes a few swipes to remove the short, which suggests this is not too extreme.

With a battery pack I would do the cells individually; otherwise, the good cells are the ones at risk.

[DigitalJunkie]

I usually use a decent PSU,say 12-15V,and atleast a couple amps,with a 12V light bulb in series,and a big cap after that...and just tap the clip-leads from the cap across the cell a few times.
The lamp will glow as the large cap recharges,and it limits the current a bit.
Basically the cap-discharge method.

[soongsc]

I used to just charge 70V or so caps up, and discharge it though the Ni-Cd batteries. Also tried to build a circuit to do it, but the swich kept fusing short. There used to be an article "Zapp new life into Ni-CD batteries" in Radio Electronics.

After a few discharges, the voltage will go up, and you can put it into a charger to charge it.