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.