who where wrote:
On Fri, 16 Dec 2011 05:11:33 -0800, "William Sommerwerck"
wrote:

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.

" 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.

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.)

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

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.

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.