reviving NiCds - Page 1
Forum section: electronics / general

author: djacobow
author_rank: Super Contributor
timestamp: July 06, 2016, 02:17:22 am
content: I've read about this over the years and, not being an expert on battery chemistry, I never really figured out if any of the various things you can do to "revive" a dead NiCd are totally BS, largely BS, partly BS, or maybe even true.Well, I'm about to toss some power tool batteries into the bin at Home Despot. They only last long enough to screw in a few screws.Is there any last thing I should try to breathe a little life into these guys? I have access to power supplies galore.

author: BradC
author_rank: Super Contributor
timestamp: July 06, 2016, 02:48:45 am
content: No. Just toss 'em.If you were *really* keen, you could dismantle all the batteries, test the cells and combine the good ones into a working battery.Honestly after spending years doing this I realised there's really no point. Weak cells die first, but the rest certainly follow. So you might get another couple of cycles out of a re-built battery with scavenged cells, but not enough to justify the time spent doing it.The best thing you can do with NiCd batteries is look after them from the word go. As soon as the tool starts to show signs of flagging, stop using the battery. Once you reach that point the chance of cell reversion (reverse charging the weakest cell) skyrockets, and at the current power tools use this causes considerable damage every time you do it. Don't try to "just get that last screw in".Every couple of cycles, run them completely flat and then give them a slow charge and equalization cycle. By run completely flat I don't mean with the tool. I mean hook up a load that draws less than 0.1C of the batteries capacity and let it die completely. I use a small light bulb, and when that dies out I have a led wired up with a 10mA constant current source. Cells will tolerate a small reverse charge current as well as they will tolerate a small overcharge current. So you can use this to completely discharge a set of cells. The slow charge helps the cells with smaller crystal growth, and the equalization makes sure all of them are fully charged.I've got a couple of power tool NiCd's that are in excess of 10 years old now while my father (with the same tool) in on his third battery. Mine takes a bit more effort to maintain, but then I've saved 2 batteries from landfill by comparison.Honestly though, all my newer tools are Li-ion and there is no contest. Even the cheapest Chinese drill I have is 7 years old now and still on the original pair of batteries. No care and attention, just use them until they cut out and put them on the charger when I get around to it (which can be months after they were used). I'm astonished at how durable they are and completely resistant to abuse. When these ones die, I'll just re-cell them and they'll be good for another however many years of carefree abusive usage!The only advantage I've had with NiCd is when I was out on a job and left my headlights on. I hooked the 18V NiCd up to the car battery and used it to put a surface charge on that would start the car. The 18V Li-ions would not do that as their overcurrent protection kicked in.So that was a long winded way to say in general once a NiCd is dead, it's dead enough to be mostly useless.

author: helius
author_rank: Super Contributor
timestamp: July 06, 2016, 03:29:08 am
content: Back when NiCd was the only game in town, approaches at revival were sometimes tried. One of the problems with NiCd cells is that they are susceptible to dendrite growth. Nickel dendrites grow from one terminal to the other, and when they reach the separator they pierce it. Eventually they short out the cell so it can't be charged. A brief high current pulse (200A ?) can vaporize the dendrites, but the cell is still damaged from the holes that they made.A lot of jiggery-pokery was associated with "the memory effect" and schemes that promised to prevent it. But memory was only observed on older space-rated (sintered electrode) NiCd cells and were never a real issue in consumer devices.

author: ez24
author_rank: Super Contributor
timestamp: July 06, 2016, 05:08:53 am
content: I have revived NiCds. It burns out "whiskers" that are shorting out the cellsSo it worked for me. But that was for individual cells, not battery packs.

author: djacobow
author_rank: Super Contributor
timestamp: July 07, 2016, 02:30:47 am
content: Thanks, everyone. This is about what have heard before Maybe tin whiskers, but basically, trash.

author: BradC
author_rank: Super Contributor
timestamp: July 07, 2016, 05:45:55 pm
content: Yeah, once a dendrite punctures the insulator it just grows back faster next time. Zapping them does take out the dendrite, but the damage is pretty much done.

author: ez24
author_rank: Super Contributor
timestamp: July 07, 2016, 07:19:33 pm
content: Quote from: BradC on July 07, 2016, 05:45:55 pmYes this is true - the ones I saved, died soon after. So I stopped doing it.

author: macboy
author_rank: Super Contributor
timestamp: July 08, 2016, 05:48:09 pm
content: If you have two identical old tired packs, you can probably construct one good one from it. It will take time, so decide what your time is worth.I had some with bad cells that would self-discharge within hours or days (probably due to nickel dendrites etc.). Finding those is simple: charge the pack and let it sit a few days, then measure cell voltages under a small (10 mA) load. After finding those bad cells and shorting them out with wire (they are already at zero voltage so this is "safe"), discharge slowly while measuring voltages of the remaining cells. Weak, dried out cells will discharge early while the good ones are still strong. Mark weak/bad cells. Good ones will survive a discharge to near-nominal capacity. After finding the best cells from both packs, disassemble and reassemble one good pack.After realizing how long the above takes, I bought some new-old-stock 18 V packs, and used them as a source of new cells for rebuilding old packs. I get all-new cells and spend much less time, so I felt it was well worth the small cost of the replacement cells. (FYI:http://www.onlybatteries.com/showitem.asp?ItemID=18830.30&cat1=&uid=, note this store also goes by megabatteries.com, no affiliation other than as a one-time customer.)I modified the charger of one drill with a switch that activates a very slow (20 mA or so) trickle charge which I keep on the pack 24x7. After charging, I flip the switch to trickle. That drill is always ready to go with a full charge on all cells. It's been about 2 years now and working great. Without this trickle, some cells might self-discharge before others, so when you go and use the drill/etc., you end up reversing those weak cells, killing them.RECYCLE the discarded cells. Please don't throw cadmium in the landfill.