Rejuvenating Nicad batteries.

[PaulUK]

I have a cordless drill and one of the batteries has died,will not charge and does not move the drill motor.I have read that they can be revived and have tried using a car battery charger,just touching the contacts for a second or so. I then put the battery on charge and it charged for a short while then stopped so I put the battery in the drill and it went but very slow.Does it have to be pulsed to get it back to "normal"?.I should add this a 9.6 volt battery.If I hold the charger cable on the battery using 12v it puts in 6amps and at 6volt, 2amps.I read that Nicad over time grow whiskers between the plates and short them out making charging impossible.I could use a car battery but I am not sure if this will be too high an amperage to put through as I don't want to blow it up.Any thoughts would be welcome.
Paul.

[mael]

Well. I'm not an expert, but I've had quite a lot of experience with Nicads.

The bottom line is that although you might get the battery pack to work again, it will be living on borrowed time.

I don't think a normal battery charger being zapped on the pack will work very well. The idea behind that is to 'vanish' the dendrites, which are basically shorts which have managed to pierce the separators inside the cell(s).

The main problem is that they grow back again very quickly.

Take the screws out of the pack and get access to the cells. Use an arbitrarily 'LARGE' capacitor which is charged to a few tens of volts and 'zap' the cells a few times each.

The cells which are OK will have a voltage over or around one volt - hopefully 1.3 or so volts. Those won't need zapping at this time. The cells which are bad have voltages in the milli or micro volt region and are basically shorted. - So zap those and when (if) they respond by registering a voltage you can cross your fingers and try to charge the batt pack so the cells are as evenly charged as circumstances permit.

The trouble is that they most likely won't last long. They'll do a job for you, but the next week when you want to use it again you'll find the same problem has returned.

Perhaps I'm being overly pessimistic, as sometimes they last a long time after you've zapped them. One 'trick' is to keep those 'zapped' batteries fully charged, as those dendrites seem to come back on a discharged battery much more than if it is topped-up.

The cells you fix will also probably have a higher-than-normal rate of self-discharge, so that's another reason to keep the pack in the charger if possible.

* I suspect those really ancient Nicads from twenty years ago which were rated at 400 mA are still often good - just look after them. And those Nicads used in telephone handsets rated at 600 mA also seem more robust than newer ones. But I'm not absolutely sure of this.

* And I suppose it is certainly true that the battery packs for cordless tools do have to provide a lot of power and can get hot, so if they last a year of almost daily work then they have done well. - The bigger the available capacity the less cycles it will have to go through, and I think it means the larger ones will last longer.

* I direct-wired my old cordless tolls to a car battery when I couldn't get the packs to work any more.

Clever me has managed to burn-out the internal connections of a few Nicads, by using too many amps for too long. Be aware this is possible - esp if you use something like a car battery to directly zap the cells.

[SeaMonkey]

You've got the right idea Paul. To clear the dendrites from the NiCd cells takes a very high current pulse of at least 50 Amperes for a very brief period of time, no longer than 1 second.

If you have a battery charger (12V) that is capable of high current, it may be used such as you've already tried. Or, a 12 Volt battery, or even two in series, may be used with caution.

To avoid damaging the battery terminals, connect heavy wires to them with clips or some other means, and do the 'pulsing' with a 'switch' consisting of the current carrying wires held in your hands. Strike the wires together to make contact, in a sweeping motion, to keep the wires from fusing together and do this for several pulses about 5 seconds apart. Then check the battery to see that it isn't hot and measure its voltage.

Once the battery voltage, after pulsing, reaches about 12 to 13 Volts (for a 9.6 Volt battery) it may be ready for use. Put it into the charger to top off then test it with the device to see if it has come back to life.

If it seems to lose charge too quickly after the first 'pulsing' rejuvenation treatment, repeat the 'zapping' as above as needed.

It may take several attempts, if there are multiple dendrites, to clear them all but they will in time.

As long as the 'pulses' are applied very briefly and in a succession of 5 to 10 pulses at a time the battery should not be damaged. Feel it after each 3 to 4 pulses to find if it is getting hot. If warm, let it rest until it cools before continuing with the pulsing.

[mael]

And be aware there is a device inside the pack which cuts the battery power. Some I've examined have a current-sensitive device which trips.

* Best to do the cells individually, and yes. Pulse-charging is a good way to bring shorted Nicads back from the abyss.

[SeaMonkey]

[PaulUK]

[mael]

[mael]

[PaulUK]

[mael]

[PaulUK]

This shows the parts that hold the battery in the drill, the clips halfway along the battery.


This shows what looks like a join.

Different lighting, the clips are part of the sides so breaking them off would not allow me to take it apart.
I have taken the battery off the car,it is a 70 amp hr diesel battery so will have to proceed with caution when zapping, will have to work out how to get the contact right, I have some jump leads but the clamps may be a bit too big to fit.

[PaulUK]

Forgot to add,if you click on pic it will enlarge in new window.

[SeaMonkey]

[mael]

[SeaMonkey]

Good points Mael.

There are several configurations and cell arrangements for the various battery packs. Once you've opened one successfully to see the lay of the land you may devise a most effective procedure to open up the cases.

Here are some example links: