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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%.

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