Originally posted by kaseki Aha! Some have seen the method to my madness. I am actually interested in determining [a measure of] Quantum Turbo Battery life by loading the unit such that the drain on the battery is similar to that which the flash would induce if one wasted flash lamp life going through hundreds of flashes...................
You've got several issues doing it this way. First, the flash uses somewhat more than any given amount of flash energy since the inverter circuit which supplies the high voltage isn't 100% efficient. That same inverter will run even though the flash capacitor is fully charged, eating more battery energy that doesn't go into the rated Ws or Joule output of the flash.
Calculating the flash discharge energy from the scene brightness is wrought with room for errors. As you pointed out, the concentration of flash power by the reflector and Fresnel lens is unknown and is a key factor as to how much of the flash energy is directed out of the flash. There is also the conversion efficiency of the flash tube to consider since it will not convert all of the stored energy to light. Then there is flash duration which is varied by the flash circuit and it's anyone's guess where that is set for a given discharge (unless manual full output is selected). If the full discharge is not used (discharge % unknown), the stored energy value is meaningless. Then there is the inverter efficiency which is unknown.........and there's some capacitor leakage (though small) which eats battery energy.
What this boils down to is that it would be far easier to tap into the battery circuit and measure current drain for a full discharge and determine battery life from there. Even this is pretty difficult, because the current starts high and decreases (as the flash capacitor charges), and battery life varies depending on current (longer for low drains, and shorter for high drains - Peukert's law). (If you were to plot and integrate the voltage & current curves product for a charge cycle, it would give you a close approximation of the flash energy not taking inverter efficiency into account.)
Better to just trip the flash (manually set to maximum flash power) X number of times with fully charged batteries (Quantum Turbos of course), until they quit and average a few trials to come up with average battery life. That still has room for error since the life you determine will depend on the frequency at which you trip the flash (and you might melt down the flash and/or the batteries in the process if that frequency is too high).
Anyway, it's a good challenge but let your neighbors in on what you're doing so they don't think you have a gun fight going on in your house.
P.S. You're probably in the neighborhood of 200 to 500 Joules per flash at full power with the AF500FTZ.