the lie of rapid NiMH self-discharge

I've never had problems with the supposed rapid self-discharge of NiMH cells. And now I have proof.

Almost two years ago, I visited a friend in Gold Bar WA for Christmas. I took some camera equipment, including freshly charged NiMH cells for the flash. I didn't take any flash pictures, so the cells remained unused in the camera case -- which I just got around to unpacking yesterday. (Really.)

Four of the cells were 2700mAh Sanyo AAs. They all measured about 1.23V, rather lower than the 1.4V NiMH cells commonly charge up to, but close to the "nominal" 1.25V of NiMH and nicad cells. Contrary to Urban Legend, they were not completely discharged.

I put them in a Canon 580EX II, and the flash fully charged up in less than two seconds. I fired off some shots. The first few recycled "instantly". The last two took about a half a second. I didn't run down the cells, but it was obvious that two years of sitting on the shelf had no rendered them unusable.

The belief that NiMH cells rapidly self-discharge is utter malarkey. Where it came from, I don't know.

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"We already know the answers -- we just haven't asked the right
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Reply to
William Sommerwerck
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My phone, my wife's phone, my Dremel nail grinder, and the eight Sony AAs I bought before I knew about NiMHs, for a starter. These all are replacements for or successors to products with NiCads, which by comparison held a charge until needed.

Reply to
spamtrap1888

Where

Then why can four NiMH cells sit for two years and still be able to properly power a high-drain device?

Reply to
William Sommerwerck

so you got dead battery performance and maybe 3 flashes and therefor batteries don't self discharge?

Reply to
Cydrome Leader

You seem to be drawing a sweeping conclusion based on one data point. Not always a reliable approach?

Likely possibility: the NiMH cells that you installed two years ago use an internel chemistry which is superior to that used in early-generation NiMH cells.

I definitely did observe the "rapid self-discharge" problem in the first lots of NiMH cells I purchased and used, some years ago. They simply were not reliable for storage times of more than 2-3 months... they'd be down to less than half of their rated capacity after that time.

Newer ones are much better. The ones advertised as "low self discharge rate" (e.g Immedions, Eneloops, and the like) have been available for several years, and I believe that the improved low-self-discharge technologies have begun appearing in more "mainstream" NiMH cells which aren't specifically advertised in this way.

As I understand it, there's a tradeoff in NiMH cell chemistry: capacity vs. self-discharge. Different metal alloys and structures materials have different hydrogen-bonding characteristics... put crudely, a tightly-binding structure will hold less total hydrogen (i.e. less total charge storage capacity) but self-discharges at a lower rate and thus holds its capacity for a longer time. Cells with higher storage capacity use an alloy/structure which packs in more hydrogen, but doesn't bind it as strongly and thus tends to self-discharge more easily. [Again, this is a crude explanation based on my own in-expert knowledge of the chemistry]

In good-quality NiMH AA cells currently on the market, cells down in the 2000-2100 mAh range seem to be representative of the "lower self-discharge rate" variety, and have a useful standby lifetime not terribly different from NiCd cells. NiMH cells up in the 2400-2500 mAh range seem to suffer more self-discharge. At least, that's my own experience over the past year or two.

So, my guess is that the batteries you bought two years ago, were ones at the lower-self-discharge end of the design range (even if not explicitly advertised as such).

--
Dave Platt                                    AE6EO
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Reply to
Dave Platt

the

they

than

The

was

Where

Where did I say ANY SUCH THING?

I'm going to jump down your throat on this one, because I find most people don't understand plain English. If I say "Some people have trouble getting along with their bosses", most readers interpret that as "All people have trouble getting along with their bosses." Really.

The cells WERE NOT DEAD. After two years they were at the nominal voltage for a nicad or NiMH cell. Furthermore, they operated the flash without any difficulty.

Reply to
William Sommerwerck

It's not the only data point. I've never seen any NiMH cell "rapidly" self-discharge. At least, not over a period of a month or two, which is the general claim.

Furthermore, the claim is that ALL NiMH cells "rapidly" self-discharge. I've seen both MAHA and (now) Sanyo cells hold their charge.

properly

Unlikely. These are not low-discharge-rate cells.

These cells are about three years old. And they're high-capacity -- 2700mAh. To the best of my knowledgte, low-discharge cells have a lower capacity. I think.

PS: I will not gainsay anyone's claim to have seen rapid NiMH self-discharge. It's just that I haven't seen it. And I've been using NiMH cells for almost five years.

Reply to
William Sommerwerck

To truly know whether (or how much) the cells had self-discharged, you'd need to actually run them down to exhaustion and see how much useful charge had been retained.

NiMH cells, like NiCd cells, have a rather flat discharge curve. Their terminal voltage, and their ability to deliver current on demand, don't change very much as they discharge... until they get down to somewhere around the 10% charge level, at which point they "fall off of a cliff" quite rapidly. They tend to work perfectly well, until they die.

So, the fact that your batteries had a nominal terminal voltage, and were able to charge the flash quickly and take a few photos, is useful information, but by no means complete enough to diagnose whether (or how much) the cells had discharged during storage.

--
Dave Platt                                    AE6EO
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Reply to
Dave Platt

Your test was unscientific, undocumented, anecdotal, incomplete. You are certainly entitled to draw any conclusions you wish and base your personal decisions on those conclusions.

Your blanket statement about NiMH based on your sample-of-one anecdote requires extrapolation beyond reason. "Proof" is not a word I'd have used to describe your result.

I can say that my personal experience differs from yours. And I get along just fine with my boss. Really!!

Reply to
mike

sorry you got 100 full power flashes before the batteries died.

people that talk about jumping down throats really need to shut the f*ck up and not be talking about how to get along with a boss. Really.

There's nothing quite like the no load battery test.

yeah, for 3 flashes until the flash was unable to even recycle anymore.

Reply to
Cydrome Leader

The local outlet of the Lidl store chain sometimes has NiMh cells at a very tempting price so I bought loads - unfortunately they're no good for the majority of occasional use items or clocks because they need tediously frequent recharging.

As someone else pointed out there are advanced chemistry types - but more expensive and harder to find.

Reply to
Ian Field

"Ian Field" wrote in news:hquFq.145226$ snipped-for-privacy@newsfe02.ams:

I bought a 4 pack of Eveready NiMH cells for my hommade 2 watt bicycle headlight,it outputs 200 lumens for .65A draw,and I go a long time(months) before having to recharge,unless I use the light heavily.Of course,my design does not draw any current when OFF.

some items have a draw even when not in use. like my low cost Polaroid digital camera.

"Pre-charged" NiMH cells have a lower self-discharge rate,but they don't seem to be all that more expennsive than regular NiMH. they are slightly lower in mAH capacity.(2100mAH vs 2300mAH)

I'd ask the OP how many flashes or shots he got from his stored NiMH cells before they needed recharge. seems to me that 1.2V is pretty much "discharged" for NiMH cells. ISTR that 1.1V is the low limit.

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Jim Yanik
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Reply to
Jim Yanik

I didn't run down the flash. I will do so tonight or tomorrow.

1.25V is considered the nominal operating voltage of nicad or NiMH cells. (The point another poster made about the relatively flat discharge was well-taken.) The "discharged" point is, as it is for cells of most chemistries, 1.0V.
Reply to
William Sommerwerck

You're determined to deliberately misread what I wrote, aren't you?

Reply to
William Sommerwerck

Go slam some doors or something, you might be better at that than testing batteries.

Reply to
Cydrome Leader

What was the load current for your voltage measurement?

Reply to
mike

Shall I send you some glorious AA examples that never held enough charge to operate a Canon Powershot A200 a week after a full charge? I have them in both Duracell and no-name.

And no, it's *not* due to any shortcomings in the charger or camera. Another brand of cells now delivers, months after charge.

Reply to
who where

NiMH cells do NOT totally discharge themselves. They discharge to about 50% of capacity (by self-discharge) and then just sit there.

For example, here are some curves I ran for an Energizer 2300ma-hr cell:

and for an Duracell 2050ma-hr cell:

There were brand new cells used to test the assertion that NiMH cells need to be charge cycled a few times before they reach full capacity. Basically, that's true.

Note the voltages (under 1C load). For the Energizer, it starts off at 1.35v and starts to die at about 1.10v. If you put a load on your Sanyo cells, I'm fairly sure your 1.23v no load voltage will drop to something around 1.18v. In other words, your Sanyo batteries self-discharged until they were almost dead, and stopped.

I recently had a similar experience to what you found. I have two old Norelco rotary shavers. I received both with dead batteries and replaced them with NiMH cells of dubious origin. I only charge them when needed, except this time, when a dry squall dropped a few trees through the power lines.

When I dug out the shavers, the LCD indicator showed a nearly full charge (about 80%). However, when I tried to use it, both lasted less than 60 seconds. My guess(tm) is that I hadn't charged it since last winter (9 months).

That's fairly close to what you observed. There was sufficient voltage for the LCD indicator to proclaim a nearly full charge, but not enough stored energy to do much useful work.

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
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Reply to
Jeff Liebermann

Hmmm... I haven't done anything with the batteries I used for this test since May 29, 2011. They've just been sitting on the shelf. I recall recharging them after I was done with the test. I'll run the same 1C discharge test again in a few days and see how much capacity is left after 7 months. This should be interesting. 1C is a heavy load, but does yield fast results.

Note: I don't have phone or DSL at my house at this time and have dragged my home computah to my office. This test will need to wait until AT&T puts fixes things. Patience.

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# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
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Reply to
Jeff Liebermann

Thanks. That would be interesting.

Reply to
William Sommerwerck

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