I have not read all the replies, but NiCad batteries can delivery very
high currents compared to NiMh or other batteries. If used in a device
that assumes limited current capability from the battery could be a
problem.
Usually the circuits for electronics is regulated with switching regulators
but the flash circuits may use a direct connection to the battery. Since
the flash voltage needs to generate high voltage, there is no reason to
use a regulated supply, but simply go direct to the battery. The switching
circuits of the flash charge pump probably assume a limited battery
impedance. As such, in order to work with NiMh batteries, the circuits
are optimized for higher battery resistance. If a low resistance battery
is then used, the switching circuits could be damaged as they are not
designed to handle the lower batter impedance and higher surge currents.
I won't go into the details, but simply put, the switching control circuits are
minimal in design and control in order to minimize cost.
That is, the designer assumed NiMh and optimized cost for that battery.
Wayne - former Linear Tech analog designer.
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