Using C batteries in place of AA for Flash units

Started Dec 9, 2004 | Discussions thread
Michael J. Capozzi Regular Member • Posts: 333
Re: It's all about cell impedance...

And what is the impedance of a SLA battery? The one I have looked at is 8 volts/3.5 AH capacity.

RogM wrote:

Michael J. Capozzi wrote:

The Nikon SD-8A battery pack for the SB-800 uses alkaline or Ni-MH
batteries. If you use alkaine batteries then the pack has a voltage
of 9 volts (6 x 1.5). If you use Ni-MH batteries the pack has a
voltage of 7.2 volts (6 x 1.2). Nikon's manual indicate that the
Ni-MH recycle faster than the alkaline but the alkaline will power
more flashes than the Ni-MH. So, my question is if the SD-8A
battery pack can output up to 9 volts (6 alkaline x 1.5) is it
possible to use a sealed lead acid battery of 8 volts as an
alternative? And, would the recycle times be shorter because of
the higher voltage or is there some other factor such as amperage
output that would affect recycle time?

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Recycle time has mainly to do with cell impedance, with both NiCad
and NiMH types being significantly better (lower) in this regard.
And of course, the larger the cell size, the lower this impedance
can be. I use sub-C NiMH types now in my Norman 400 pack, and am
getting about double the shots as compared to a new unit, simply
because of the increased A-H capacity available in the newer cells.
Cell impedance is the most critical factor, since flash units tend
to draw power in short, high-current pulses, rather than a steady
flow (as for a light bulb). For this reason a good share of
available cell capacity can be simply dissipated in heat for
alkaline types, since high pulse currents cannot be delivered
efficiently in this case.

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