I'm going to try to address the original question, but please bare with me this could get confusing very fast, and I've had several beers at this point..
A CR123 cell discharges most of it's energy while maintaining above ~2.25V into most loads, as it approaches being "dead" the voltage starts to rapidly fall.
Under the load of a P60L, a pair of CR123s start to sag down down in voltage after a couple hours coming up close on their point of being "dead"...
When the voltage drops to about 4.5V total, or about 2.25V per cell in a 2 cell light, the LED module drops "out of regulation" and starts feeding less current to the LED. the more the voltage drops per cell, the less current flows through the LED, and the more the runtime is "dragged" out from the cells.
The voltage continues to fall slowly, and as the voltage slowly falls, so does the current flowing across the LED. The "drain rate" from the cells keeps slowing down more and more.
The result is a light that runs for a few hours at full output, followed by many hours of diminishing output.
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When you have 3 cells driving the same module, the module stays in regulation, running at about 2W all the way down till the cells have been drained to about 1.5V each. In this case, by the time the module drops out of regulation and starts reducing it's power consumption, the cells have all been very heavily depleted. So the total runtime is less because the cells spend more time keeping the module in regulation.
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A 3 cell light driving a P60L will have more useful regulated runtime than a 2 cell light, the SureFire runtime ratings on these lights are very misleading and they should revamp the way they are rating these lights.
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does this make any sense at all?
Eric