the lie of rapid NiMH self-discharge

[William Sommerwerck]

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.

 

[Phil Allison]

"William Sommerwanker"

Canon's specs for the 580EX II flash appear to be based on nicad or NiMH
cells starting at 1.25 volts.

** 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.

As I explained, when the unloaded voltage was
at 1.21V,

** 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

 

[William Sommerwerck]

Several points... The cells had sat for two years, but delivered at least

" 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.

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

 

[Phil Allison]

"William Sommerwanker is Full of Shit "

100 is for a full-power flash.

** 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

 

[mike]

" 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.

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.

 

[William Sommerwerck]

"William Sommerwanker is Full of Shit "

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 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.

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.

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.

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..

 

[William Sommerwerck]

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.

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.

 

[Phil Allison]

"William Sommerwanker a a LYING **** "
<

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.

** MASSIVE LIE !!!!!!!!

The spec was for alkaline cells.





.... Phil

 

[mike]

I shouldn't have said anything about the voltage.

and you shouldn't have jumped off the deep end and used words like "lie"
and "proof"

It's not a lie and you disclosed no proof.

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.

You have either a mistake or a serendipitous occurrence that you've
extrapolated to draw unwarranted general conclusions.

Unwarranted conclusions are your right. But people here are disagreeing
with you.

I'm unlikely to use old technology NiMH cells in an application
where I expect full functionality after two years of storage.

Repeating your anecdote is unlikely to change that.

 

[William Sommerwerck]

and you shouldn't have jumped off the deep end and used words like "lie"
and "proof"
It's not a lie and you disclosed no proof.

The fact that I have at least one set of conventional NiMH cells that sat
for two years, yet still correctly powered a device is proof that what is
said about rapid self-discharge is wrong.

flashes.

You have either a mistake or a serendipitous occurrence that you've
extrapolated to draw unwarranted general conclusions.

I made no mistake. And if something is said to be generally true, one
exception disproves it.

I'm unlikely to use old technology NiMH cells in an application
where I expect full functionality after two years of storage.
Repeating your anecdote is unlikely to change that.

I'm not asking you to. The original claim was the NiMH cells repaidly
self-discharged over a period of several weeks. It simply isn't true.

 

[Phil Allison]

"who where"
"William Sommerwanker Fuckwit TROLL"

Your grasp of logic is concerning.

** But the grasp he has on his tiny penis is staggering.




..... Phil

 

[William Sommerwerck]

No, it is proof that it appears wrong IN ONE CASE.

Which disproves the whole. Where did you learn "logic"?

One exception does NOT disprove some thing said to be generally true.
It only proves it is not universally true.

Which is exactly the point.

Your grasp of logic is concerning.

Your insistence on ignorant empirical data is frightening.