the lie of rapid NiMH self-discharge

ISTR NiCd cells frequently became shorted if allowed to self discharge too often, but I haven't noticed it with NiMh.

No mention of low self-discharge.

100 flashes does seem like an almost fully charged battery. However, with such an intermittent load, it would have been nice if you had a more controlled and more measurable way to test charge level. If what you say is true, then you may be correct that there's no self-discharge for a *NEW* NiMH battery. I'm wondering if it might be a phenomenon that appears later as the battery is used. I'll run my little discharge test when I have time and get my computer back home.

Incidentally, several people have mentioned that you cannot use the open circuit voltage as an indication of state of charge. I agree. It works with batteries that have sloping discharge curves, such as carbon-zinc and some alkaline. However, batteries that have a flat discharge curve offer such a small change in terminal voltage, that the numbers are difficult to distinguish from tolerance and temperature variations. The only way that seems to work for such batteries are coulomb counter chips, as found in many laptop batteries.

More later... bizzeeeeeee

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
# http://802.11junk.com               jeffl@cruzio.com
# http://www.LearnByDestroying.com               AE6KS

"Jeff Liebermann"

** I have several sets of AA NiMH cells - all Sanyo brand and rated at 1700 to 2500 mAH.

Self discharge is a REAL problem, but no worse than with NiCds.

In general, charged cells lose 90% of capacity in about 6 to 8 weeks - taking a full 6 hour charge at 400mA to recover.

In my Canon A430, the uncharged cells will run the back display for a few minutes, let me take a few shots and maybe one flash before the camera shuts down.

... Phil

This might be of interest:

It's an independent test of the Sanyo Eneloop NiMH batteries including some self-discharge tests. Unfortunately, he doesn't compare the results with the non-LSD batteries. Scroll down to the "Self Discharge" section heading and note the self-discharge tables.

50% charge loss after 1 year.

There is also this quote from Sanyo: Storage temperature is of high importance if you measure self-discharge rate. Higher temperatures substantially increase self-discharging. It is best to store Eneloops as cool as possible to keep the charge in the battery. As a rule-of-thumb, every 10°C increase in storage temperature is equivalent to doubling the storage time. Some R/C pilots in Europe put Eneloops in the freezer, with rather good results. So, how were your batteries stored?

Presumably, the non-LSD batteries would produce much worse results, making your miraculous Sanyo HR-3U cells better than Eneloop cells, which seems rather dubious. Since the Eneloop batteries tested were brand new, I don't think it's something related to an aging effect. That leaves your test as an oddity. Are you sure someone didn't charge your batteries when nobody was looking?

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
# http://802.11junk.com               jeffl@cruzio.com
# http://www.LearnByDestroying.com               AE6KS

Ok, I lied[1]. There is a comparison. See:

The graph shows the Eneloop battery to be MUCH better at self-discharge than the conventional NiMH. After 1 year, the Eneloop retained about 50% of charge, while the conventional NiMH retained only 2.6%.

[1] I'm suppose to be doing my end of year billing and bookkeeping. I hate doing billing and am therefore easily diverted. If I go broke, it's all your fault for creating an interesting diversion.

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
# http://802.11junk.com               jeffl@cruzio.com
# http://www.LearnByDestroying.com               AE6KS

But it isn't, of course. The voltage was at the nominal 1.25V when I started, but this is well below the end-of-charge voltage for NiMH cells.

It was "scientific" in the sense of mimicking photographic use. 100+ full-power shots in a half hour, three at a time, is fairly extreme use.

Of course. That's one of the problems with nicad and NiMH cells.

However, the fact that the cells read about 1.25V showed they WERE NOT fully charged. But despite having sat for two years, they were able to give 100+ full-power shots. The point is that their were perfectly usable without having to be recharged. This contradicts belief that NiMH cells rapidly self-discharge. (Rates of 1% or more per day are stated.)

I might disable the flash's auto-shutoff and let the cells run down to 1.0V, then see whether it can still fire more than once. (I have no desire to keep popping the flash.)

PS: Sanyo says their current eneloop cells (rated at 1500 charge cycles) will hold as much of 75% of their charge for 3 years.

1700

shuts

I wonder if the "spectacular" behavior of my 2700mAh Sanyos has anything to do with their "superlattice alloy" construction. These appear to be the only Sanyos using this design. (That's life, I guess.)

Anyone care to explain this article to me?

This is the typical "doubling of chemical reactions with each increase of

10°C" rule.

Right before Christmas 2009, I packed up my camera bag for a visit to Gold Bar, WA. Two packs of NiMH cells were in the bag. When I came home, I put the bag on the living room floor, where it has remained for almost two years. The Pacific Northwest is cooler than the rest of the country. The cells were exposed to "high" temperatures only intermittently, during the warmer days of Summer.

Hmmm... Is there a purple owl on these cells?

Agreed, but we don't know whether the "superlattice alloy" has magical powers.

Well, it was around Christmas... Elves, perhaps? Brownies?

Speaking of which... One of Stan Freberg's less-well-known recordings is "Yulenet", with Joe Friday trying to convince a doubter named Grudge that there really is a Santa Claus. When they visit the North Pole, they're greeted by a brownie from the South Pole (Daws Butler doing a silly mock-Southern voice) who's helping out while Santa is away. I can imagine the flap such a joke would cause today...

Extremely interesting.

I'll contact Sanyo and ask them about the self-discharge of my specific cells.

information

In for a penny's worth; i add that this might be some freaky low self discharge cells for the given process, perhaps at some process corner for the set.

?-)

I'll try to make this quick...

Canon's specs for the 580EX II flash appear to be based on nicad or NiMH cells starting at 1.25 volts. As I explained, when the unloaded voltage was at 1.21V, I had no trouble getting more than 100 full-power flashes, which meets the 100 - 700 flash spec in the book. After letting the flash sit, running, for several hours, the unloaded voltage was about 1.18V. After removing and replacing the cells, the flash charged up once, taking more than 7 seconds. After firing it, it would not recycle.

Several points... The cells had sat for two years, but delivered at least the spec'd number of flashes. Some NiMH cells might self-discharge quickly, but these Sanyos did not. (I was surprised that, throughout the discharge, the cells' voltages were virtually identical, never differing by more than about 10mV. This suggests very tight manufacturing tolerances.)

It was also interesting that the flash "conked out" well before the cells reached 1.0V. This suggests that this flash is /not/ designed to work down to 1V per cell, the traditional "standard" of battery-operated designs.

"William Sommerwanker"

** The manual clearly states that it is based on Alkaline cells.

This implies that the unit will operate with much lower cell voltages and currents than modern digital cameras require.

** It is totally meaningless to quote unloaded voltages of MiNH, NiCd or alkaline cells.

Only when loaded as in the application does terminal voltage become meaningful.

... Phil

" at least the MINIMUM spec'd number of flashes" - which you would expect given (a) the expanse of the range specified (100-700) and (b) the proximity of your cells' voltage to the Canon staring point.

Sanyo is probably the most highly regarded name in both NiCd and NiMH manufacture. You shouldn't be surprised.

100 is for a full-power flash. The 700 refers to a partial-power flash, under auto-exposure.

"William Sommerwanker is Full of Shit "

** The flash energy input is probably about 20 Joules, ie 330uF and 350V.

Allowing 5 Joules for losses, the energy required for 100 flashes is 2500 Joules.

A fully charged, 2500mAH NiMH has a capacity of 11,000 Joules ( 1.2 x 2.5 x

3600 )

Four of them have a capacity of 44,000 Joules

So, your "magic" Sanyo cells had under 6% of normal capacity.

... Phil

Make sure you're comparing apples with apples. The unloaded voltage of a cell is irrelevant. I've found it very difficult to get the unloaded voltage of NiMH below

1.2V. Discharge it down to .8V, remove the load and let it sit and it will creep back up to 1.2V. But it's still dead and can't supply much current. A flash is a VERY high current device. Once the LOADED voltage gets much below 1V, it's too weak for a flash. The ONLY useful voltage measurement is with the intended load.

A useful measurement is internal resistance. Use a square-wave load from

1/2A to 1A. Measure the P-P amplitude of the cell voltage and use that to calculate a resistance dV/dI. Try it at different states of charge.

Calculate the voltage drop from your load current and the ISR. Multiply that by the number of series cells and it's easy to see why high-current loads quit working long before the open-circuit voltage gets below 1.2V.

Phil Allison is a foul-mouthed shmuck.

By the way, in German "shmuck" means "jewelry" or "adornment". Billy Wilder gets a funny gag out of this in "One, Two, Three".

The losses are probably greater than that. When I left the flash running, without firing it, I was surprised that it conked out after about two hours.

I suspect this flash provides more than 20Ws output. But the specs are silent on this.

I can't argue with plausibly-chosen numbers, and I won't. However...

You're still missing the point. We have been told that NiMH cells lose several percent of their capacity every week. (Let's say 3%, and assume it's a linear loss, rather than exponential.) After 102 weeks, the cells should have been dead, dead, dead. They were not. They had no trouble powering the flash to its spec'd number of full-power flashes. What do you want, for heaven's sake?

The point about "voltage creep" of nicads & NiMH cells was well-taken. However, I measured the cells' voltages within about 15 seconds of shutting off the flash. Furthermore, the under-load voltage is arguably not that important if the DUT works as it's spec'd. Which this flash did.

Please note that I actually performed an experiment! I tested cells that should have been useless. They were not. The statement that conventional NiMH cells are generically incapable of holding a charge for extended periods is simply NOT TRUE. Not because "I say so", but because I have empirical evidence.

Yesterday I pulled out my Sunpak 622 Super, a "professional" potato-masher flash. It has four 5500mAh NiMH C cells made by CTA, whoever that is. (They came from Overstock.) The last time I charged them was about six months ago. I flicked the power switch, and unit came to full power in 6 seconds. Full-power recycling was 4 seconds -- not great, but not bad, either, especially for "dead" cells..

I shouldn't have said anything about the voltage.

The point is that the cells "should" have been dead, but weren't. After nearly two years, they powered the flash to its spec's number of flashes.