The primary vs secondary debate depends on usage patterns and personal preference. Most of us will have a smattering of both, with rechargeable options for daily carry or heavily used tools and primary backups for emergencies.
As for the Lumen-Hour/$ concept goes. Here's some thins that I started thinking about:
Very low levels on many flashlights are not actually all that efficient. The driver in many cases consumes more than is fed to the LED on many of the ~1L modes on some lights. Also, high flux LEDs (actually all LEDs) do have a sort of "cut-in" point where they ramp up to their peak efficiency. Lower drive levels do not continue to result in increased lumen/watt conversion all the way down to nothing, I'm not sure where the efficiency peaks on a modern cree, but this would technically be a factor for some of the extreme low modes on some lights, that could actually be operating the LED at a pretty low point of efficiency. These issues is partially offset by the fact that primary cells benefit most from slow drain rates. The Lumen-Hours/$ would still probably calculate out to be relatively poor on some of the low modes out there.
Medium or "normal" modes of many LED flashlights will probably have both the driver and regulator running in pretty good efficiency ranges, while high modes (or "turbo") will take an efficiency hit usually at both the driver and LED.
Generally speaking, buck regulation is more efficiency than boost (buck is often as high as 99%, however, non-flickering designs will take a few percent hit). Extreme boost like from 1x1.2V cell up to an LED is usually the worst efficiency, sometimes as bad as 50% but usually closer to 75%.
All of these factors and more come in to play and make such a comparison very hard to make. One starts realizing, that even an extensive chart is not going to properly cover every flashlight out there unless each flashlight is included in the chart and tested.
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With that in mind, how about I'll throw out some constants and some assumptions and we'll run a few numbers and compare a few possible hypothetical lights:
1xAA alk vs 1xAA Lithium vs 1xCR123 LED light with 3 modes (5, 25, 125 lumen):
We'll assign a 100lm/w constant and assume this across all ranges for the LED.
When driven by a single alkaline cell, the input voltage will range from ~0.8-1.5V to make near-full use of the cell. The voltage will steadily decline through this range through the discharge resulting in continuously diminishing efficiency. Driver efficiency will range from ~50-85% across this range. We'll assume an average of 75% efficiency.
Power consumption in each mode will average ~0.067W, 0.33W, and ~1.67W (low, medium, high respectively).
I should point out at this point, that the high mode in this comparison is creating a very inefficient circumstance for an AA alkaline cell, as it will require current demands exceeding 1 amp at all times (probably averaging close to 1.5A). Alkaline cells at these drain rates perform horrendously in capacity tests. I wanted to run this example in this fashion in order to illustrate the strength of alkaline at low drain vs it's weakness at a high drain rate. Lithium chemistry and pretty much all rechargeable cells tolerate these higher drain rates with less penalty.
Since I haven't seen, nor care to take the time at this time to locate test results, I'm going to throw out some extrapolated educated estimates on the capacity we can expect from the cell at these various drain rates:
2.8WH, 2.2WH, 0.6WH (low medium high).
42 hours, 7 hours, 20 minutes. (low medium high).
Assuming $0.40 per cell.
Alkaline
Low Mode: 525L-H/$
Medium Mode: 438L-H/$
High Mode: 104L-H/$
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Lithium AA:
I'll take the same flashlight from above, higher average operating voltage increases average efficiency to 80%.
power draw from unit:
0.063W, 0.313W, 1.563W (low medium high)
Battery capacity in each mode:
5WH, 4.8WH, 3.5WH (low medium high)
Runtimes:
79 hours, 15 hours, 2 hours 15 minutes. (low medium high)
Assuming $2.00 per Energizer Lithium.
Lithium AA:
Low Mode: 198L-H/$
Medium Mode: 188L-H/$
High Mode: 140L-H/$
*Note: Marked improvement in operational costs on high mode, everything else suffers in costs dramatically. This comparison does not adequately cover the effects of temperature, keep this in mind! In very cold weather, the Alkaline is likely to leave you in the dark very quickly!
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CR123:
Another increase in average input voltage results in an increase in average operating efficiency. Lets call it 85% now.
power draw from unit:
0.059W, 0.294W, 1.471W (low medium high)
Battery capacity in each mode:
4.2WH, 4.1WH, 3.9WH (low medium high)
Runtimes:
71 hours, 14 hours, 2 hours 40 minutes. (low medium high)
Assuming $1.25 each
CR123
Low Mode: 284L-H/$
Medium Mode: 280L-H/$
High Mode: 266L-H/$
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but but but.... Yea...
-Eric
