Prolonging NiMH cells by discharging them completely - Page 1
Forum section: electronics / general

author: Analog Kid
author_rank: Super Contributor
timestamp: April 28, 2025, 03:09:30 am
content: I'd like to get some opinions on the matter of prolonging the life of NiMH cells by periodically discharging them totally.I have an electric toothbrush (Braun) which runs on NiMH that I've been doing this to regularly for a few years now. The toothbrush is now 7 or 8 years old and still works as well as when it was new.I've looked this up online, and have read up on it atBattery Universitywhich seems to be one of the better places to get battery info. But there's a lot of what seems like folklore and mythology out there about this practice, and lots of conflicting information. Some say that it's a good idea to do what I do--I run the toothbrush completely down, until it won't start, twice a year--while others say that it's actually bad for the cell.My understanding is that by completely discharging a NiMH cell, one thing it does is to dissolve any dendrites that may have formed, which can discharge the cell prematurely.My own empirical evidence is that after I do this and recharge the toothbrush (it sits on a magnetic trickle-charging stand), it has a lot more pep than before the run-down, and the cells appear to be in very good shape after a number of years, so I would say that running them down is a good thing.

author: BrianHG
author_rank: Super Contributor
timestamp: April 28, 2025, 03:35:49 am
content: A complete controlled full discharge and full recharge of NiMH as well as some led acid batteries, like car batteries, can roll back the clock a bit and give you some rejuvenation. This means a lower series resistance, better peak current output. However, if the battery is damaged, this wont work. And, especially for car batteries, only do this something like one a year.I've stretched my car battery her in Montreal's freezing winter weather for around 6 years, parked outdoors overnight.Once I got to the point where I had to completely cycle my battery twice in one winter, it was time to replace the battery a month later.

author: bdunham7
author_rank: Super Contributor
timestamp: April 28, 2025, 03:50:35 am
content: I don't think modern NiMH cells have any signficant memory effect, but overdischarging can and will damage them. This is especially true in packs where cell imbalance will lead to electrode reversal on the lesser cells. Discharging to near zero isn't nearly as bad. The good news is that your typical device will not overdischarge your cell or battery as it will shut off first.IDK about dendrites in NiMH cells, I don't think that is actually much of an issue--unlike both Li-ion and NiCd.As for toothbrushes, as long as we're sharing anecdata, I have had two different types and I've never noticed any change in performance at all regardless of battery condition or charge level--they either work or they don't. I've torn down two of my old models as I wanted to see how the little transformer system works (it's cute, but I'm not sure i can use it for anything) and I've found that they use 3x AAA NiMH cells and on both of them the batteries are in good condition despite being 8-10 years old and basically sitting on their stands forever. I haven't tested the charging system to see how it manages the batteries yet.

author: David Hess
author_rank: Super Contributor
timestamp: April 28, 2025, 05:23:31 am
content: Quote from: bdunham7 on April 28, 2025, 03:50:35 amBefore low discharge NiMH cells became available, all of my NiMH cells failed with shorts after a couple years of use. They could be restored to operation for a while with a high current discharge, just like NiCd cells which had failed with dendrite shorts.

author: amyk
author_rank: Super Contributor
timestamp: April 28, 2025, 05:42:51 am
content: NiMH/NiCd can be stored at 0V.

author: Benta
author_rank: Super Contributor
timestamp: April 28, 2025, 09:53:24 pm
content: A toothbrush is difficult, it's usually low-current contactless/inductive charging.That's exactly the issue with dendrite build. It happens at low-C charge.High-current well-controlled charge (0.5...2C) eliminates dendrites completely.

author: Analog Kid
author_rank: Super Contributor
timestamp: April 28, 2025, 09:57:55 pm
content: Quote from: Benta on April 28, 2025, 09:53:24 pmOK, but does a deep discharge help solve that problem?As I wrote, my toothbrush is alotpeppier after I completely run the battery down. So it seems to be the right thing to do.

author: edpalmer42
author_rank: Super Contributor
timestamp: April 28, 2025, 11:40:13 pm
content: First, a couple of definitions. A 'battery', AKA a 'pack' is made up of more than one 'cell'.Running a cell down to zero volts is okay. If your battery is made up of only cells in parallel, i.e. 1V2, that's also okay. If your battery has cells in series, i.e. 2V4 or higher, DO NOT run them down to zero or anything close to it. In that case, one or more of the cells will be driven into reverse polarity and that will damage them. If the voltage on a pack has dropped really low because it was left sitting disconnected, that's equivalent to a bunch of cells and the 'reverse polarity' issue doesn't apply.I have a couple of battery chargers/exercisers/rejuvenators that will charge/discharge/charge NiCd or NiMH cells. They do a good job of rejuvenating tired cells. I've found that cells that were left sitting for an extended period require a rejuvenation before they're useable.Most devices need more than 1V2 so typically, you're dealing with 'batteries' rather than 'cells'.

author: Analog Kid
author_rank: Super Contributor
timestamp: April 29, 2025, 12:25:25 am
content: Quote from: edpalmer42 on April 28, 2025, 11:40:13 pmYes, I think most of us know that.QuotePretty sure my toothbrush'sbatteryconsists of 3cellsin series.And as I've stated a few times here, I've been running these down to zero for a few years now with, apparently, no ill effects. No damage here.Dang: there still seems to be so much mythology and folklore surrounding this subject.Doesn't anyone know for sure how this all works? Any good online references to this?

author: wraper
author_rank: Unknown
timestamp: April 29, 2025, 12:43:14 am
content: One of the fastest ways to screw up NiMH cells is to trickle charge them constantly. For example we had some wireless Motorola barcode scanners at work. Batteries failed in about 1.5 years with developing very high ESR, they seemingly still held the charge but could not provide the current. They were barely used and sat on a docking station almost all the time. So after 2nd battery replacement I said to just leave them nearby and put on the docking station once a month. Problem gone.

author: Analog Kid
author_rank: Super Contributor
timestamp: April 29, 2025, 12:51:00 am
content: Quote from: wraper on April 29, 2025, 12:43:14 amAgain: my experience seems to contradict what you wrote.My li'l toothbrush sits on a magnetically-coupled charger that undoubtedly trickle-charges the battery. (How much current can such a transformer transfer to the device being charged?)I've had it for 7-8 years and it works about as well as when it was new.

author: edpalmer42
author_rank: Super Contributor
timestamp: April 29, 2025, 12:53:36 am
content: Quote from: Analog Kid on April 29, 2025, 12:25:25 amYou'd be surprised how many people don't.QuoteThe issue is that every manufacturer has their own 'secret sauce'. One brand might have a way to make their cells more tolerant of reverse charging or trickle charging. Another has optimized their cells for low self-discharge. A circuit designer can also use tricks to improve battery longevity. Whatever. You can find some of that info by looking for datasheet for the battery. Another source is batteryuniversity.com . I haven't checked there lately but it might be helpful.

author: bdunham7
author_rank: Super Contributor
timestamp: April 29, 2025, 01:11:30 am
content: Quote from: Analog Kid on April 29, 2025, 12:25:25 amI haven't yet had a chance to finish studying my old disassembled toothbrushes, but I'm pretty sure the answer is that you are actually NOT running these to zero. Most devices that aren't incandescent flashlights or otherwise primitive designs will shut off before they get to the point of cell reversal. This is pretty simple to accomplish, for an N-cell pack, you shut the device off at (N-1) * 1.2V.QuoteI'm pretty sure there are people that understand it, especially the ones that implemented NiMH-powered hybrids and EVs. But cell technology has advanced and I don't think all of that knowledge is all that accessible. I've started accumulating some interesting random research papers as I come across them. I have an idea that the current "wisdom" of rapid-charging NiMH batteries may not be completely accurate--but I'll expound more on that someday when I have some evidence.From another reply of yours:QuoteYou don't know for sure what the charging rate is or if it is continuous or not. The transformer on my version has an input of about 80V @ 75kHz with a ferrite rod going to the secondary, separated by the thin plastic. That could result in a fair amount of power. It charges 3S AAA cells and IDK the current or profile yet.Edit: I discharged the battery until the device would not work and then connected the charger. I'm getting charging current of about 50mA. The battery cells are 700mAh, so about C/14. I'll let it charge overnight and see if that current ever reduces. These were used for perhaps a decade and have been sitting for 3-4 years. They were not completely discharged even after sitting that long. During their decade of use they would have been in the charger, plugged in 24/7.

author: bdunham7
author_rank: Super Contributor
timestamp: April 29, 2025, 01:19:05 am
content: Quote from: wraper on April 29, 2025, 12:43:14 amI'm not sure your conclusion is as generally valid as you might think. If you overcharge the cells, yes you'll cook them. And if you have a simplistic, non-terminating charger design that is going to charge the battery up in any reasonable time, it probably will cook the battery as you have seen. That's a bad compromise. But from a previous post where you mentioned the very low self-discharge of some NiMH cells, I don't think the self-discharge rate is the limiting factor for a "safe" trickle charging rate, I think it is more likely the recombination rate. If you trickle charge cells at a rate where they do not heat up, that's probably a lot better and maybe even OK in the long run. I'm saying maybe because I don't know.

author: bdunham7
author_rank: Super Contributor
timestamp: April 29, 2025, 09:53:44 pm
content: After 18 hours of charging and 1 hour of rest, the 3S pack has 4.267V OCV. I'm pretty confident it is fully charged. The charging current is still about 40mA and the voltage under charge is about 4.4V. The circuitry appears to be quite simple and I'm reasonably sure that there is no timer control or anything like that. I'll give it another 24H continuous on-time to be sure. The batteries do not get noticeably warm, no more than a few C above ambient.So it looks like this design uses a continuous ~C/20 charge current and presumably cells appropriate for that. However, the reason I stopped using these is that they would intermittently not start up--they'd just pulse one jolt and shut off. Now that I've charged this one, it won't start going and it looks like the voltage is dropping to about 3V when I try and turn it on. Maybe the cells are cooked, that's a 0.7V drop for about 1.5A current, so ~500mR internal resistance. Hmmm.

author: Benta
author_rank: Super Contributor
timestamp: April 29, 2025, 10:50:36 pm
content: Quote from: bdunham7 on April 29, 2025, 01:19:05 amBest way ever of creating dendrites.

author: thm_w
author_rank: Super Contributor
timestamp: April 29, 2025, 11:26:18 pm
content: The cheaper nimh Sonicares definitely are weaker and don't last as long as the lithium versions. Surprising that it has lasted 8 years. Did you check the manufacturing date on it?Quote from: Benta on April 29, 2025, 10:50:36 pmCitation?This is all I found from a quick search: "DOD is the most important factor affecting the cycle life of the Ni-MH battery in HEV conditions."https://www.sciencedirect.com/science/article/abs/pii/S2352152X2031834X

author: Analog Kid
author_rank: Super Contributor
timestamp: April 29, 2025, 11:42:58 pm
content: Quote from: thm_w on April 29, 2025, 11:26:18 pmThe brush is an Oral-B made by Braun, so a very good brand.There's a model # on the bottom as well as a production code, but it's not a date code. I suppose I could contact Oral-B if I cared to get the date of manufacture.It has definitely lasted 7 years, probably actually 8. Yes, I find that surprising too.

author: bdunham7
author_rank: Super Contributor
timestamp: April 30, 2025, 12:51:28 am
content: Quote from: Benta on April 29, 2025, 10:50:36 pmI can live with a 10-year lifespan, although even that sort of bites when the batteries are not replaceable.I don't want to dismiss NiMH dendrites outright even though AI says they don't exist. It's entirely possible that they were a problem early on but later manufacturing techniques have eliminated them. I have not had a shorted NiMH cell that I can remember and I haven't had a shorted NiCd cell in many decades. I've had plenty of bad NiCd cells recently, but they weren't shorted. Leaking, very high ESR, you name it, but none that could be recovered with a large current pulse like I remember doing earlier in the late 70's or so. I have some partly-bad NiMH cells as well, a fair number that leaked and some that had the typical failure of just not holding very much charge and high ESR--but not shorted. I did have one NiMH tool battery (cheap replacement for a Makita saw) burst into flames on me, some sort of runaway--I suppose that could have been a short but it happened during a high rate discharge. I don't remember having many earlier NiMH cells but I didn't test every product I owned that died.In any case, my old toothbrushes clearly don't have dendrite shorts and they've been overcharged for 8-10 years or so. I use a lot of NiMH cells and I'm adapting a few old instruments to NiMH from NiCd. I'm interested in figuring out how to do this effectively and there's little possibility of implementing a high-rate charging profile such as an individual cell charger would use. In addition, I'm starting to think that series-charging a pack (without a BMS or balancing) is a bit of a different ball game than charging individual cells.

author: thm_w
author_rank: Super Contributor
timestamp: April 30, 2025, 01:01:59 am
content: Here is some info from Energizer, if they can be trusted:https://data.energizer.com/pdfs/nickelmetalhydride_appman.pdfQuoteAgain no issues with slow charging, the problem is temperature and fast charging:Quote

author: bdunham7
author_rank: Super Contributor
timestamp: April 30, 2025, 01:12:29 am
content: Quote from: thm_w on April 30, 2025, 01:01:59 amThere was another one I saw from Duracell recommending C/300! I suppose it depends on the exact cell involved.

author: thm_w
author_rank: Super Contributor
timestamp: April 30, 2025, 01:25:21 am
content: Quote from: bdunham7 on April 30, 2025, 12:51:28 amYou reached 1.47V per cell, at 40mA. What is the cell capacity, 700mAh or something? So 0.06C, quite low, I doubt its stressing the cells much. You can probably measure the temperature and it would tell you for sure.At high charge/discharge rates, yes series charging could overstress cells due to temperature, low charge rates it shouldn't be an issue though.If I search for Braun photos I can see some using a FDK made in Japan nimh cell, which is going to be of high quality (they manufactureeneloop cells). The cheap brands definitely don't last as long, if the cause is supposed dendrites or low quality materials, I don't think it matters, just don't buy them.

author: tooki
author_rank: Super Contributor
timestamp: June 27, 2025, 11:47:27 am
content: Quote from: Analog Kid on April 28, 2025, 03:09:30 amOne aspect that hasn’t been mentioned in this thread is that in some devices, there’s a completely different reason for occasionally fully emptying, then fully charging a battery: letting the “fuel gauge” learn or recalibrate what the battery’scurrentcapacity is. As rechargeable batteries age, their total capacity slowly drops, as does the maximum current it can deliver (this aging is often summarized as the “state of health” of the battery, or SOH). An accurate battery gauge needs to know what the up-to-date values are, so they use various algorithms to learn or update the battery capacity and maximum current numbers used for charge level (“state of charge”, or SOC) calculations. Otherwise, you can get things like unexpected shutdowns, where the device suddenly turns off because it has actually run out of power, but the fuel gauge still thought it had power left.NiMH discharge curves are highly nonlinear, so voltage alone is a very poor indicator of SOC. Consequently, accurate SOC monitoring uses more complex approaches, including coulomb counting and internal resistance calculations (by monitoring battery behavior as loads change), etc.I have no idea whether a toothbrush bothers with this or not, since they just trickle to keep it always full, and probably don’t expect you to frequently, if ever, actually run it without charging after each use. But recalibrating the fuel gauge is definitely one of the reasons why manufacturers of more complex devices (phones, laptops, etc) often recommend fully draining and recharging the battery once in a while.

author: Ranayna
author_rank: Super Contributor
timestamp: June 27, 2025, 01:15:51 pm
content: Regarding the trickle charge thing. Aren't there two different chemistries used for NiMh Cells?I remember reading about that when i looked into why there are specific cells designated for cordless phones.These have a significantly smaller capacity than normal NiMh cells, IIRC 800 mAh for an AAA cell.Supposedly these use a different chemistry as the higher capacity cells, that can be trickle charged, which most cordless phones do when the phone is in the charger. I would expect that cells like these would generally be used for devices that spend most their life in a charger, like toothbrushes.

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 27, 2025, 02:27:43 pm
content: Just because it sits in a charger doesn't mean it's continuously trickle-charged.

author: David Hess
author_rank: Super Contributor
timestamp: June 27, 2025, 06:51:18 pm
content: Quote from: Ranayna on June 27, 2025, 01:15:51 pmThere were a bunch of NiCd variations, and some handled trickle charging better than others.My favorite were the low discharge ones intended for higher temperature operation, which were equivalent to NiMH low discharge cells now. They lasted a lot longer.

author: Analog Kid
author_rank: Super Contributor
timestamp: June 27, 2025, 07:22:18 pm
content: Quote from: SiliconWizard on June 27, 2025, 02:27:43 pmTrue; however, in the case of my li'l electric toothbrush, the charging stand appears to have no real "smarts" in it, so I'm guessing continuous trickle charging.I suppose I could test this: what's the easiest way to detect if/when the charging coils are active? Compass?

author: SteveThackery
author_rank: Super Contributor
timestamp: June 27, 2025, 07:50:32 pm
content: The charging stands on both my Philips and Braun toothbrushes are both just a coil across the mains - nothing more. I guess any smarts will be in the toothbrush.I don't suppose a compass will respond to a 50Hz field, will it?

author: Analog Kid
author_rank: Super Contributor
timestamp: June 27, 2025, 09:16:59 pm
content: Quote from: SteveThackery on June 27, 2025, 07:50:32 pmNo, but itwillrespond to a 60Hz field.[just joking]

author: Peabody
author_rank: Super Contributor
timestamp: June 27, 2025, 09:24:46 pm
content: Quote from: SteveThackery on June 27, 2025, 07:50:32 pmI would be surprised at that. These are relatively small coils on both sides, and I would think the frequency would be much higher than mains frequency. The Qi standard even provides for communications between both sides to negotiate things. I don't know if either Philips or Braun is Qi, but I would think they would have some smarts even in the charger base.

author: thm_w
author_rank: Super Contributor
timestamp: June 27, 2025, 10:20:39 pm
content: Quote from: Peabody on June 27, 2025, 09:24:46 pmIts switched to 80kHz yes, not mains. But its always active as he says, I don't think there is any intelligent charging like with Qi.https://www.righto.com/2016/09/sonicare-toothbrush-teardown.htmlhttps://newscrewdriver.com/2023/09/22/fun-with-philips-sonicare-charging-base-hx6100/https://www.powerelectronictips.com/teardown-whats-inside-a-phillips-sonicare-electric-toothbrush/https://www.facebook.com/photo/?fbid=930923735043498&set=pcb.2329769810545674