Shocking NiMH Cells - Page 1
Forum section: electronics / beginners

author: metrologist
author_rank: Super Contributor
timestamp: December 06, 2024, 04:02:35 pm
content: This is related to My friend's dad was an engineer...As kids we had NiMH technology for our RC car batteries. Not everyone had new batteries and when the packs started to underperform, the dad would measure each cell and then "shock" the bad ones briefly using a car battery and jumper cables. He explained that crystalline structures would grow between the "jelly roll" layers and short them out, and the brief shock would vaporize them. It seemed to work and the batteries would perform like a good used pack again.When I was kid, I had a stubborn cell so I left the current on too long and the battery blew up. It launched straight up and glanced off my forehead.Now here I am later in life with a bunch of old AA and AAA MiMH cells, either loose or in packs for things like 2-way radios, and the cells will not hold a charge, so I am thinking about doing the same thing to them and see how it goes. Is there any truth to this phenomenon?

author: jpanhalt
author_rank: Super Contributor
timestamp: December 06, 2024, 04:17:31 pm
content: "Shocking" was done with NiCd cells to restore capacity. It presumably has no positive effect on NiMH. Are you sure they weren't NiCd?

author: David Hess
author_rank: Super Contributor
timestamp: December 06, 2024, 04:36:49 pm
content: For both NiCd and NiMH, dendrites grow and puncture the insulator. Applying a strong pulse clears the dendrites which are causing shorts and restores operation for a short time. I always used a capacitor bank to clear these shorts.

author: coppercone2
author_rank: Super Contributor
timestamp: December 06, 2024, 04:51:56 pm
content: is this what obrien and scotty were doing in star trek??reroute warp core to the reciever waveguides on C deck. It got shorted out by a falling girder.I can see them like pumping a megawatt into some broken waveguide to get the signal to get through to a subspace antenna, output power is 50uW.

author: metrologist
author_rank: Super Contributor
timestamp: December 06, 2024, 05:19:10 pm
content: Now that you mention it, I'm sure they were NiCd.Thanks for the Capacitor tip. I might have some big soda can-ish sized caps. I had also thought of one of those impulse battery spot welder but that tends to blow off the metal surface.I'll give it a zap and retest...

author: Gyro
author_rank: Super Contributor
timestamp: December 06, 2024, 05:26:34 pm
content: Quote from: David Hess on December 06, 2024, 04:36:49 pmI thought dendrite growth was specific to NiCds and their absence was one of the selling points of NiMh. As far as I know, the major failure cause of NiMh is choosing too low an end-point voltage for a cell stack, causing cell reversal of the weakest cells.I don't think I would ever risk using a car battery to blow out dendrites. The time differential between removing a dendrite and boiling the contents is too short. Capacitor discharge is, by far, the better choice, but yes, once you've got dendrite sized holes in the separator, it's all but over.

author: LinuxHata
author_rank: Frequent Contributor
timestamp: December 06, 2024, 08:28:38 pm
content: I have used that technique on NiMH cells and it works (verified multiple ways). But once battery started "growing dendrites" or whatever it is, it's already reached end of the life and such measures will regain it's powers only for short period of time.

author: golden_labels
author_rank: Super Contributor
timestamp: December 06, 2024, 09:26:51 pm
content: While David Hess is right, a note to not get a wrong picture from the description. It is not like a puncture insulator would behave in a capacitor. Electrodes are not electrically insulated in an alkaline battery after all. And removal of dendrites would leave the holes behind.The electrolyte provides a chemical reaction with electrodes. That reaction creates and maintains voltage potential, with electrons produced on one side and absorbed on the other. Because of that voltage potential the electrons can’t “move back”, regardless of insulation in electric sense.A dendrite disturbs that balance. Provides a path for electrons to move back during electric energy production (causing self-discharge), and a path for electrons from charging to simply flow across the battery instead of revesing chemical reactions. So yes, it is a kind of a short, but the mechanism of action and importance is somewhat different than in components based on electric insulation.

author: coppercone2
author_rank: Super Contributor
timestamp: December 06, 2024, 09:38:39 pm
content: it might leave a blast that kind of cauterizes the surface around it (think of a under ground bomb, it glasses the surface of the rock to make a stable 'cave' bubble) maybe its more stable then you think.The capacitor might just do that. You might get a thin layers of zinc that is somewhat polished/corrosion resistant from plasma depositionA plasma drill apparatus makes a self supporting cave because it melts the rock around it. Similar to making a concrete shell inside of a tunnel. This might have a similar but uncontrolled effect. Its not like you just scooped it out, you are talking compaction, plasma deposition and possibly reactions that exhaust the reactive surfaces prone to corrosion and reactions. And the foil might get a kind of "lip" around the spark leading to a geometry that is not conducive to further dendritic growth, and the vaporized plastic might infiltrate the shell to give it a tiny coating, the plastic should condense from the plasma at a later time then the metal, I think its ultimately related to vapor pressure, not sure how matter crystalizes from plasma, I assume its kind of like freezing, where high melting points freeze first.I am thinking the impulse of a cap might be the ticket to trying to get some of these favorable results.Perhaps a cosmologist can give insight. If zinc and plastic are both vaporized under the pressure, will the metal condense out first and get coated with plastic?A practical experiment might be detonating a small nuclear bomb placed deep under solid rock inside of a large zinc container that is surrounded by alot of plastic.https://en.wikipedia.org/wiki/Plasma_deep_drilling_technology

author: CaptDon
author_rank: Super Contributor
timestamp: December 07, 2024, 02:34:19 pm
content: I think the reality is that the 'shock' flashes those dendrites into carbonized leakage paths. It seems once you do this procedure the batteries develop a large amount of internal electrical leakage and simply go dead again within a couple days. They may maintain if left on a float charger but I have seen them suddenly go internal dead short and the battery gets very hot until dead but may also kill the charger wall wart. Once a NiCad or NiMh is in this 'not holding a charge' or 0.00vdc it is simply trash. Time to buy new and not risk disappointment with dead trash. Usually by this point with NiCad cells you will see the white corrosion around the positive end of the battery, a sure sign it is done for.

author: David Hess
author_rank: Super Contributor
timestamp: December 07, 2024, 08:52:52 pm
content: Quote from: Gyro on December 06, 2024, 05:26:34 pmBefore I discovered low discharge NiMH cells, all of my "standard" NiMH cells failed of apparent dendrite growth and shorting after a couple years of light usage. Clearing the shorts as I would with NiCd had the same effect in restoring operation for a limited time. I never reversed any of my cells.For both NiCd and NiMH cells, switching to low discharge varieties, which have a thicker separator, stopped shorting. For NiCd cells, these were better known as high temperature and low leakage cells. Since the behavior, cure, and solutions were the same for NiCd and NiMH cells, I assume the cause of dendrite growth was the same also. I may have read something about dendrite growth in NiMH cells, but it would have been long ago.

author: coppercone2
author_rank: Super Contributor
timestamp: December 07, 2024, 09:28:53 pm
content: Quote from: CaptDon on December 07, 2024, 02:34:19 pmdarn I forgot that it will carbonize. I think what I wrote is possibly true but I forgot about carbonization of organics. Plastic often decomposes into carbon instead of phase change.Some stuff its possible to vapor coat with, like parylene

author: Gyro
author_rank: Super Contributor
timestamp: December 07, 2024, 09:59:47 pm
content: Quote from: David Hess on December 07, 2024, 08:52:52 pmI'm not sure the dendrite growth mechanism is in fact the same. As I understand it, the dendrites in NiCd were Cadmium, as referenced in this NASA study...https://nepp.nasa.gov/whisker%20/dendrite/2006-cyganowski-paper.pdf. I'm not sure what the equivalent material would be in NiMh (I've never been that clear on what the metal is in metal Hydride).Low self-discharge has anecdotally helped cell life considerably. Unfortunately it trashed the one advantage that NiCd brought to the party, lower internal resistance than NiMh - Not that it is of any consequence these days, with the environmental considerations and arrival of main stream Lithium.

author: Analog Kid
author_rank: Super Contributor
timestamp: December 07, 2024, 10:11:30 pm
content: Somewhat related, so hope you don't mind this somewhat tangential intrusion into this thread, but years ago when I had a remote that ran on AAA cells, I used to recharge them. Even though they weren't rechargeables (actually the ones I used weren't even alkaline but carbon-zinc), and despite all the dire warnings about "don't even think about trying to recharge non-rechargeable batteries! they'll blow up! you'll lose an eye!".I used a little homemade charger the guts of which were made for, I think NiCds that I put into a "handy box" (single metal junction box made for a switch or receptacle), with a 2-cell holder. A cell that would no longer operate the remote would be usable for a couple of months after "recharging" (several hours to overnight), and I never had a single cell leak, let alone explode.Of course the current draw from the remote was minimal.

author: CaptDon
author_rank: Super Contributor
timestamp: December 08, 2024, 12:17:34 am
content: I did multiple experiments with recharging non-rechargeable alkalines. I tried very slow under 10ma. charge and several other techniques. The final outcome was every one of them leaked, some overnight and some within 2 to 3 days. We did recharge carbon zinc cells 'back in the day' usually through a small miniature lamp bulb in series with the charger. It worked, but not well. We did this with the big 'ignitor' batteries used with model airplanes. Eventually even they would rot through and leak.

author: themadhippy
author_rank: Super Contributor
timestamp: December 08, 2024, 01:49:08 am
content: Tandys flogged a charger for there bog standard batteries,cant seem to link it ,but its on page 142 of the 1989 catalogue.Also one of the mags around the same time published a charger circuit ,seem to recall it used some sort of AC or pulsed DC to break down/reverse the chemical action.Another old skool trick was to put your batteries in a warm oven for a bit to get another bit of use out of them

author: golden_labels
author_rank: Super Contributor
timestamp: December 08, 2024, 02:00:29 am
content: Standard zinc-manganese batteries (usually called “alkaline”) are not reliably rechargeable. The electrodes are irreversibly destroyed during operation. This is why the battery acts differently during the bounce test. You may see Osenberg et al. “An X-ray Tomographic Study of Rechargeable Zn/MnO2 Batteries” (doi:10.3390/ma11091486) for X-ray photos (Fig. 3d). Unlike with rechargeables, which are usually rolled from two flat electrodes capacitor-style, those batteries are just two bulky blocks: not something that can be easily rebuilt. Chemistry is also not designed to produce a “clean” reaction, leaving a lot of byproducts. Both dissolved in the electrolyte and as gas build-up (hydrogen). This is again different from alkalines that capture stray hydrogen (NiMH), or batteries designed to produce reasonably clean reaction.So while you may attempt to reverse the reaction and get some positive results, this is never going to work too well or be reproducible across different manufacturers/batches. The rupture risk is also there, because of hydrogen.

author: Analog Kid
author_rank: Super Contributor
timestamp: December 08, 2024, 05:57:56 am
content: So how is it that I was able to (at least partially) recharge carbon-zinc cells? The "recharge" was weak but it definitely occurred, and without any damage (at least none evident).

author: metrologist
author_rank: Super Contributor
timestamp: December 09, 2024, 08:12:53 pm
content: I found my "big" capacitor - it's more like the size of two D cells.Charged to 15V and zapped a few AA cells a few times. I was not noticing any spark - and it had no effect on discharge tests.I will try more voltage and if that fails, I'll try again the old fashioned way.BTW, I picked up an AA/AAA package of ZITHION-X lithium batteries with the USB-C charging port, $20 on sale at Costco for an 8 pack. Holey smokes those are expensive.

author: tooki
author_rank: Super Contributor
timestamp: December 09, 2024, 08:16:42 pm
content: Quote from: CaptDon on December 08, 2024, 12:17:34 amAnyone remember the semi-rechargeable alkalines they sold for a while? I say “semi” because they were only intended to be recharged something like 10 times (I forget exactly).I never used them. Were they any good?