NIMH Cells Zero Volts Shorts Whiskers

For years, I've read that nicad cells develop shorts from crystalline(?) whisker growth penetrating the insulating layer. I've encountered many of 'em.

Do NIMH cells develop the same faults? I realize the Ni represents nickel.

I acquired a used 14.4V NIMH battery pack and the voltage is maybe 35mV for the entire pack. It's relatively difficult to open the pack case (have already done an identical pack), but I suppose I'll end up doing that.

There's no indication of the pack's age, although it was military surplus, and could be as old as about ten years. The battery pack is for a Xenonics NightHunter II short arc long range flashlight, as mentioned in some of my earlier posts.

Still haven't found a source for the 4/3-A NIMH 4000mAh cells with a length of only 63mm (normal consumer cells are about 67mm in length).

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Cheers, 
WB 
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Reply to
Wild_Bill
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Mee Too. However, I usually see them with one shorted cell in the pack, not an entire pack of shorted cells as you seem to have found.

I don't think so. Here's my logic.

The positive electrodes are identical in NiCd and NiMH. The whiskers come from the cadmium in the negative side of the NiCd cell. The NiMH negative electrode is an alloy of various rare earths known as AB2 and AB5 alloys. AB5 is LaNi5 and AB2 is TiN2. Neither alloy is known to produce whiskers. For example, I could find no mention of whiskers in the military study of NiMH batteries, which seems cover everything else:

Don't you find it odd that the entire battery pack would simultaneously develope whiskers at the same time?

Sorry... I don't have a source. Also, are you SURE that they're NiMH and not NiZn? Count the cells: With 0.4V per cell more than NiCd, no toxic substances, high peak current, and the ability to run them down to zero, there are advantages to using NiZn.

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Reply to
Jeff Liebermann

A recommended repair at one time was to open the cell, pull out the center electrode, then re-insert.

As I write that, I'm aware of just how stupid it sounds. But I remember reading it. (Of course, that doesn't prove it actually worked.)

Reply to
William Sommerwerck

Thanks, Jeff.. the packs are marked NIMH and 4.1Ah. There are 12 cells in each pack. The size of the 4/3-A cells is about 18mm dia x 63. NICD cells are limited to lower capacities.

The Wiki write-up states the NiZN are only available as sub-C and common AA, AAA etc. I dunno what the NiZN capacities are, but it would be interesting to see greater energy density than NIMH. And then there would be the need for different chargers.

I've seen NICD packs which had all cells shorted, typically a flea market item that had probably been sitting around unused for years.

This is my first discovery of DTADN deader-than-a-door-nail NIMH cells.

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Cheers, 
WB 
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Reply to
Wild_Bill

You might have already been pointed at powerstream.com , but the 4/3-A batteries they have are all 67 mm. They seem to have some kind of relationship with a manufacturer in China, so if you want to order a bunch of the 60 mm cells, they may be able to have them made.

One manufacturer of sealed lead-acid batteries refers to a "DIN overdischarge recovery test". They don't mention the DIN standard number, but the procedure is apparently:

  1. Discharge at 20 hour rate to 1.70 volts per cell.
  2. Connect 5 ohm resistor across battery and leave it there for 28 days.
  3. Remove resistor, recharge at a constant 2.25 volts per cell for 48 hours.
  4. Capacity test.

(See page 12 of

formatting link
). Obviously some of the numbers would be different for NiMH, NiCD, or other chemistries.

A hit from another battery manufacturer suggests that the relevant standard might be DIN 43539. This has apparently been superseded by both DIN EN 60896-1 and DIN EN 60896-2. All of these standards are secrets.

Standard disclaimers apply; I don't get money or other consideration from any companies mentioned.

Matt Roberds

Reply to
mroberds

Yep, Matt.. there are some strange testing procedures for rechargeable cells.

I've read numerous datasheets, and they may do a specific test for the first

50 recharges, then change the parameters for later test cycles. I guess that means something to someone who specs cells.

The 4/3-A cells aren't popular like the other sizes used in the RC hobby, and there aren't many suppliers that sell the high capacity cells.

I could modify the flashlight battery space although it's not just a matter of trimming away some plastic, as it generally would be in a consumer grade device. The 67mm length cells are about 1/4" too long to fit in the original battery pack plastic case, so I'd need to fabricate cases too.. still, not extremely difficult, but quite a bit of trouble for a light that uses a $450 lamp.

Alternatively, these lights have a power cord connector on them, so using a corded battery pack which hooks onto a waistband or pocket would be an option.

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Cheers, 
WB 
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Reply to
Wild_Bill

Well, there's nothing wrong (in my opinion) using the too short A battery size cells.

The specs seem to have been written by battery manufacturers looking for very high numbers to make their products look better. I normally use high discharge rate cells at ummm.... high discharge rates. Yet many of the capacity tests are done at C/20 or less, resulting in highly inflated numbers. All I can suggest is try to find the data sheet and determine how a cell was tested before accepting the number printed on the garish shrink-wrap coverings.

You may be thinking of the common LiIon 18650 cell, which has a disk shaped PCB on the negative terminal end to protect the cell.

Diversion: I just love the names given to the various 18650 batteries such as Trustfire, Truefire, Ultrafire, Fireworm, Marsfire, etc. Somehow, such names fail to inspire confidence in the safety of their products.

I've never seen any per-cell protection on a NiCd or NiMH battery or pack. There are some PCB's found in radio battery packs that are coulomb counters, but I don't think there are any that offer short circuit protection for the individual cells.

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Jeff Liebermann

That voltage is perfect for lithium conversion. 3S LiPo is 14.8V nominal.

Even if you can't effectively make use of the space, you might still get close to the original capacity, but at much less weight.

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RoRo
Reply to
Robert Roland

whisker growth penetrating the insulating layer. I've encountered many of ' em. Do NIMH cells develop the same faults? I realize the Ni represents nick el. I acquired a used 14.4V NIMH battery pack and the voltage is maybe 35mV for the entire pack. It's relatively difficult to open the pack case (have already done an identical pack), but I suppose I'll end up doing that. The re's no indication of the pack's age, although it was military surplus, and could be as old as about ten years. The battery pack is for a Xenonics Nig htHunter II short arc long range flashlight, as mentioned in some of my ear lier posts. Still haven't found a source for the 4/3-A NIMH 4000mAh cells w ith a length of only 63mm (normal consumer cells are about 67mm in length). -- Cheers, WB .............

I would try to burn the apparent shorts open by putting the power pack acro ss a 12V battery, being carefule to avoid melting wires burning something u p. Nothing really to lose, as long as the 12V source battery can take a mo mentary high-short-circuit load. I would sort of brush one lead across the source battery a few times while monitoring the powerpack voltage, and see if it comes up at all as you brush the clip lead onto the 12V source batte ry. Really do exactly the same as you would for a NiCad battery pack short .

Reply to
hrhofmann

I recently suggested opening the cell, pulling out the anode, then reinserting it.

No one has commented on this, possibly because it seems absurd. But I remember reading it in the PPC calculator journal over 30 years ago.

Reply to
William Sommerwerck

For mechanical palliation of the whiskers, what about a small paint shaker, such as

I've never tried it (for batteries) but maybe it would work. Beats prying open the cell casing.

Reply to
Rich Webb

Thanks.. that seems to be a fairly popular although fairly desperate easy fix-it method for nicad cells. If I were to try this method, I'd use a safety barrier of some sort between the batteries and myself, as the results could be unpredictable.

I might try this after I open the pack case to see if there are other issues, leaking, etc.

I was more curious about anyone's knowledge of the whisker/short failure mode in NIMH cells, because I didn't remember hearing about it, and thought that I'd read that this would not be a failure mode in NIMH cells.

In this instance replacing the cells, which would be my usual choice, isn't easily accomplished unless I can find a source for the 63mm length cells.

-- Cheers, WB .............

I would try to burn the apparent shorts open by putting the power pack across a 12V battery, being carefule to avoid melting wires burning something up. Nothing really to lose, as long as the 12V source battery can take a momentary high-short-circuit load. I would sort of brush one lead across the source battery a few times while monitoring the powerpack voltage, and see if it comes up at all as you brush the clip lead onto the

12V source battery. Really do exactly the same as you would for a NiCad battery pack short.
Reply to
Wild_Bill

Often, the battery pack cases include insulator material for mechanical shock, so 'percussive maintenance' with the cells still inside the case might be complicated.

These particular packs use silicone glue and the thin DST double sided tape (the membrane type) to hold the cells in place within the packs.

I dunno how severe the G-forces are for a paint shaker, but probably low compared to some other relatively common machines. I have some lab agitator mixers, but they're fairly mild compared to a commercial paint mixer.

Your suggestion caused me to remember a small pneumatic tool (inline scraper of sorts, nothing as severe as an autobody panel cutter) I saw years ago, and fast, repeated mechanical shock may be a worthwhile experiment at least, and may produce positive results. I'm visualizing a heavy duty housing for an individual cell which holds the cell securely, but prevents any physical deformation/damage to the cell while transferring the forces. If the holder were subjected to rapid-fast rise time mechanical shock(s), I would imagine that salt whiskers could be persuaded to break, although any holes/penetrations in an internal separator layer would likely still exist.

Another device happened to occur to me.. the armature in one of those transformer-type motors such as the vibrating motor used in Wagner paint applicators, hair clippers and a variety of other common devices. If the cell were firmly attached to the vibrating armature, a sustained buzzing vibration may be a worthwhile exercise.

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Cheers, 
WB 
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Reply to
Wild_Bill

Thanks for the suggestion. I don't have many devices which use lithium chemistry batteries, so I'd prefer to utilize NIMH, which I already have numerous universal, multi-voltage chargers for.

NIMH seem to be less problematic than others, and have high capacities in compact sizes.. simple to recharge, and AC or DC (mobile/field) chargers are inexpensive.

--
Cheers, 
WB 
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Reply to
Wild_Bill

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