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November 19, 2011

How to rejuvenate rechargeable NiCd batteries

NiCd battery pack, assembled

NiCd battery pack

This post is about a method to restore (rejuvenate) NiCd (NiCad) battery packs that are failing to charge. I have 2 sets of cordless tools (drill, circular saw, reciprocating saw and more) that use rechargeable NiCd battery packs. Three of the battery packs were failing to charge. Having had some previous success with rejuvenating apparently expired NiCd cells, I decided to do some tinkering, and I was able to restore the packs such that they will now charge to a usable level. I’ve previously used the same procedure, at lower current, to rejuvenate NiCd cells for cordless phones.

These tool sets and battery packs are very common. Mine were MasterCraft brand purchased from Canadian Tire, but there are many similar ones on the market with other branding. I’d guess millions of similar cordless sets have been sold, so maybe someone else can benefit from this post and save a few $ on new batteries. Perhaps more significantly, maybe this will save some battery packs and even the tools themselves from adding to our garbage output; the Cd in the cells is very toxic, so the less of it we put into use and the less that goes into the waste stream, the better.

The following is the procedure that I used. Note that it only applies to Nickel-Cadmium (NiCd) cells, not Li-ion cells. Check the labels on your battery packs about their contents, and don’t proceed if you aren’t sure that you have NiCd cells. As for why this procedure works, please check my note at the end of this post.

Usual caveats first: there is risk to property and life and limb in this procedure. You should be familiar with basic electronics and electrical safety practise. Safety glasses are required, just in case the cells leak or something else goes wrong. It’s probably also worth putting a plywood shield or equivalent between the cells and yourself, during the procedure. While I’ve never caused any fireworks with this procedure, better to be safe than sorry. I take no responsibility for what happens if you try this procedure yourself, but I do wish you good luck!

Special tools required:

1. Safety glasses. Wear them. I’ve never had an explosion or significant leak during this procedure, but better safe than sorry.

2. A variable DC power supply with adjustable current limiting. Needs to supply at least 1.5A and 2VDC max. I used an old Micronta supply that I’ve had for 30 years. If you lack a current-limited supply, an alternative is to use a voltage supply with a series resistor, but be careful with the power rating of the resistor given the voltages and currents that it might experience.

3. Two multimeters, to measure voltage and current. Nothing too special is required but one of them needs to have a DC ammeter that will read at least 1.5A. Any test leads you use to supply current in this work should be able to handle much more than 1.5A. (If your power supply has current and voltage meters built in, you can get away without these external meters.)

The procedure:

1. Open up the case of the battery pack and remove the string of NiCd cells. Don’t disconnect the cells from each other; they are probably connected by welded wiring and you won’t be able to reconnect them without special spark welding equipment (don’t try to solder them!). The procedure does not require the cells to be disconnected from each other if they are in a typical series configuration. See the pictures below (pictures coming soon) for assistance, if your battery packs are anything like mine.

NiCd battery pack, assembled

NiCd battery pack, assembled

cover off

cover off

dis-assembled

dis-assembled

2. Work through the following steps one cell at a time. Note that the typical failure mode of these cells results in the cell being in a short circuit condition. This is where the current limiting of the power supply is important. Start with the power supply set to zero voltage and with current limited to about 50mA. Connect a multimeter (or some equivalent device) configured as ammeter in series , and supplying current to one cell. Power supply polarity connection should positive to the positive terminal on the cell, and negative to negative. Measure the voltage at the cell terminals with the other meter. Set the voltage limit to 1.5V.

Circuit and Setup

Circuit and Setup

3. Gradually increase the limit current while observing the voltage measured at the cell, until you see the voltage increase rapidly. Wait until the cell voltage exceeds 1.2V, and then disconnect the cell. You should not need to exceed 1.5A current  if the cell is actually one that can be recovered.

4. If step 3 succeeds, follow the same procedure with each remaining cell, until all the cells have shown some ability to hold a charge.

5. Reassemble the battery pack, and try charging it with the normal charger that came with the tool set. You should find that it charges normally again.

Why this works:

The discussions I’ve seen on the common failure mode of these cells that this procedure works on is that the cells develop shorts between the plates from conducting deposits out of the electrolyte, thus the apparent short circuit state of the failed cells. Once the short exists, it is of course impossible to charge the cell. The high current, which is significantly greater than the normal charging current, serves to obliterate these conducting paths, perhaps by local heating. The procedure I’ve written up above is more controlled than the capacitor discharge method that I’ve seen elsewhere in that the current is raised in a controlled fashion only until the short is removed. The above procedure could be less risky than the capacitor discharge method as a result, although you should be careful to remove the power supply current as soon as the voltage rises. YMMV, but my success rate on my small sample size of 3 battery packs (about 30 cells) stands at 100%.

23 Comments »

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  1. This is good. New dewalt battery pack was $40

    Comment by write on — November 21, 2011 @ 3:19 am

  2. Thank you for this extremely useful blog.

    Comment by Andrew J. Gephart — November 28, 2011 @ 2:21 am

  3. Great post.

    Trackback by Gordon M. — February 12, 2012 @ 4:44 pm

  4. I appreciate you sharing this.Really looking forward to read more.

    Trackback by Abner — February 12, 2012 @ 7:50 pm

  5. [...] I have a pair of 14.4 volt flashlights that were part of rechargeable toolsets that go with these NiCd battery packs. Each flashlight had subminiature bayonet-base incandescent bulbs, for which the filaments had died [...]

    Pingback by Retrofitting Old Flashlights With LED’s « Flounderings — February 21, 2012 @ 11:36 am

  6. Hi
    I happened to come this site and noticed your power supply,do you still have it?Please e-mail me back.

    Thank you
    M

    Comment by Mario — March 28, 2012 @ 5:11 pm

  7. Yes. I still have that power supply. It’s one of the core instruments on my workbench and gets used a lot.

    Comment by admin — March 28, 2012 @ 8:12 pm

  8. Hi again
    Glad you still have it.Is there a way I can ask you more info either thru here or thru your e-mail?Details that I need…
    Thanks
    M

    Comment by Mario — March 29, 2012 @ 12:35 pm

  9. Sure. You can ask any questions here on the blog, and I’ll do my best to answer. If I can’t answer, maybe someone else reading the blog can.

    Comment by Dave Baar — March 29, 2012 @ 12:52 pm

  10. Great article and I’ll keep it handy ’cause I have power packs two and that senerio might happen.

    Now to my question(s) relating to the Micronta.
    It’s model 22-8230?
    If so what year was it made?American or Canadian Radio Shack store?
    it looks something from the ’70’s early 80’s, might be wrong?

    (If it’s not too much to ask,would be very helpfull)

    inside of the Micronta variable power supply there’s a 14 pin chip,can you provide me what’s written on it.Now for the voltage adjustment pot what is written on and what is the value?
    Thank you
    M

    Comment by Mario — March 29, 2012 @ 7:46 pm

  11. Very amazing coincidences that you noticed that supply, for a few reasons. Many years ago, in the late 1970’s, I used to do electronics repairs for Radio Shack stores in Sault Ste. Marie, Ontario. That was one of my first jobs, in my final years in high school. I also just finished a few months ago replacing the output transistors in that supply, and nearly wrote a blog post about that! I should have and maybe now I will. Now, some answers to your specific questions. I just opened up the case to check on what you were asking:

    1. Yes, the supply is indeed model 22-8230, and was from a Canadian store. The store was of the ones I worked at in Sault Ste. Marie. I actually recall paying something like $29.95 for it on sale, although that was a very long time ago.
    2. The IC looks to be MC1723CP, date code 7744 (consistent with late 1970’s date of manufacture). Googling on MC1723CP shows it to be a voltage regulator, so that seems right.
    3. The voltage adjustment potentiometer looks to be a 10K linear pot based on an in-circuit measurement, but I can’t be certain of that from any markings on it. I’d have to do some desoldering and measure it out of circuit to be sure. Let me know if that is critical information for you; if so, I can look deeper.

    Hope that helps. Please ask in case of any other questions.

    Comment by Dave Baar — March 29, 2012 @ 8:18 pm

  12. Thanks alot for your trouble.That’s why I couldn’t find it in the American catalogs.
    The reason I’ve asked for the chip is that for some reason mine is hardly visible(scratched for some reason,bought the Micronta power supply second hand)it’s a Sanyo from it’s logo that’s all I can see.Yes I agree with you,voltage regulator.On the pot for current adjustment there’s some writting on it it might be the same for the pot for voltage(mine is missing that’s why I’ve asked for the value).I don’t want you waste your time to take the voltage pot appart,it’s up to you.It would be nice to know the value but you’ve helped me alot with the info.
    Sorry to ask you something again the manual for that(I know it’s straight foward,but it helps)do you recall if even there was one or just a page?

    “Just to let you know I’ve looked at the picture again and funny thing is your tester is like mine,I bought it at canadian tire.It’s a good one too,it came in a kit with testers for vechicles.”

    Back to the subject I’m glad I came across your site and appreciate your help.
    Thanks again
    M

    Comment by Mario — March 29, 2012 @ 8:42 pm

  13. Glad to be of any help. I just checked that pot out of circuit: It’s actually 18.6K ohms, so probably a nominal 20K, and it’s linear.

    I looked for the manual, but sorry, don’t have it. I actually did find a few old Radio Shack manuals in my files but not that one!

    Very funny re that tester, too. I think I got mine in similar way, from Canadian Tire.

    Best regards,
    Dave

    Comment by Dave Baar — March 29, 2012 @ 9:22 pm

  14. hi
    I will check on that.
    Thanks again

    BTW when I’ve mentioned American Radio Shack catalogues if found them on the net.As for Canadian no luck yet.

    Comment by Mario — March 30, 2012 @ 3:11 am

  15. I’ll be back in the Sault in a few weeks visiting family there. I’ll check to see if I still have any of the old Radio Shack catalogues there. Slim chance they are still around, but I used to keep them.

    Comment by Dave Baar — March 30, 2012 @ 6:39 am

  16. Thanks

    Comment by Mario — March 30, 2012 @ 8:27 am

  17. Here’s a site that a friend just pointed me to, that has many years of complete Radio Shack catalogs:

    http://www.radioshackcatalogs.com/catalog_directory.html

    Comment by admin — June 28, 2012 @ 3:27 pm

  18. Hi
    Thanks that the site I was mentioning in March although no Canadian cat unless I haven’t checked well.

    Comment by Mario — July 1, 2012 @ 4:57 am

  19. [...] unlike NiCd cells, Li cells apparently can’t be rejuvenated when they die. However, all I was looking for here [...]

    Pingback by Harvesting and Re-using Lithium Cells from Laptop Batteries « Flounderings — September 17, 2012 @ 6:44 pm

  20. excellent. Thanks.

    Trackback by louis — March 6, 2013 @ 2:31 am

  21. Hi I got Schematic Diagram for the Power Supply Micranta # 22-8230 & Part List if poeple needs it.

    Comment by Robert — January 11, 2014 @ 2:20 am

  22. good article
    this 1.4amp limit is for small D Nicadsand Nimh types, right? on larger or smaller ones (AA) I guess the “busting current is smaller….any experience with these?
    why cant i solder in a “winged” battery? very carefully spot solder without cooking the battery of course….

    Fred

    Comment by Fred/K1HB — January 20, 2014 @ 8:55 am

  23. Thanks. Those were roughly C-size NiCd, but yes, I’d expect required current to vary like you suggest. Hard to know how much current will be required for any case, so monitoring voltage is helpful to see if short disappears. Soldering OK if v careful with heat, but you might find the case/tab metal to not take solder well without very corrosive flux. Spark welding might work -haven’t tried that.

    Comment by admin — January 21, 2014 @ 4:48 pm

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