The traditional story about damaging your flash with "high-performance"
batteries goes like this:
Cheap flashes use small, inexpensive transistors that can't tolerate abuse. The
oscillator is therefore designed so that the internal resistance of ordinary
alkaline cells is the limiting factor in how much current can flow. Substitute
nicad or NiMH cells, and the oscillator draws so much current it overheats and
fails.
This story has been repeated for many years. I don't know if it's true, but
it's _theoretically_ possible.
NiMH cells have very low internal impedance and can rapidly pump a lot of
current into a demanding load. That's why using them provides such a
performance boost -- and why they're de rigeur for high-drain devices, such as
digital cameras.
The faster the flash recycles, the hotter the oscillator transistors get. But
unless you're firing the flash rapidly, at or near full power, the transistors
have time to cool off. So the faster recycling provided by NiMH cells should
not be, in and of itself, a threat to your flash.
The real danger is to the flash tube. Continuous firing at or near full power
will cause the head to overheat. If the head is hot to the touch, STOP!
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