At the low drain rates of LED modules when driven by 3 or more CR123s, the available watt-hours of CR123s does start to overcome li-ion cells displacing similar space, however... There are some flaws above...
True watt-hour capacity, and estimated are different things. CR123s don't deliver an average 3V into most loads, it's usually a little less. Also, it depends what brand you compare to, as some are far better performers at low drain rates, while others are better at high drain rates, while others pretty much stink at all drain rates.
The "750mA" power consumption of the M60 is based on 6V input. When you run higher voltage configurations (3xCR123/2xli-ion), the power consumption should drop to ~0.5A.
According to SilverFox's CR123 shootout, at 0.5A, the best performing CR123, a Sanyo, delivered ~4.1WH. For 3 of them, that's 12.3WH. Panasonic cells delivered ~3.8WH.
If you moved up to 4 CR123s, obviously the M60 would not work, but say you were using a module with similar power consumption that was compatible with higher input voltages, we could extrapolate that the total WH capacity of 4 cells would be closer to ~18WH assuming the best available cells.
SilverFox's li-ion shootout doesn't have 0.5A tests for the 18500 and 18650 size cells, but here's some rough extrapolations:
[email protected] : 5.2WH per cell, or 10.4WH for 2 cells.
[email protected] : 8WH per cell, or 16WH for 2 cells.
One more important note, the discharge testing for CR123s drags them all the way down to below 1V per cell. So on the 3xCR123 test, a portion of that discharge and available stored energy, is not powering the module in regulation. The discharge testing for li-ion cells cuts off at ~2.5V per cell, so every WH of stored energy on paper is available at a voltage output that is high enough to maintain regulation.
So as I see it, the comparison is 10.4WH vs 12.3WH for 2x18500 vs 3xCR123, and 16WH vs 18WH for 2x18650 vs 4xCR123.
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While the comparison above may be fun to make, it's a moot point. If you use your flashlight on a regular basis, then the cost of operation on those li-ion cells is way lower, even if you do pick up a bunch of extras as spares.
Cost of 6x18500 + Pila IBC charger. $110. This would replace literally thousands of CR123s. Even if you only cycled one set of the cells, 100 times, over a few years, the cost of operation here would drop to the cost of running CR123s.
$110 buys ~70-80 CR123s, give or take.
The WH comparisons are a difference of ~10-15%, the cost of operation for li-ion cells drops to literally a fraction of CR123s with regular use. Taking a 10-15% hit on runtime, to reduce cost of operations by as much as 95%, is a pretty easy decision to make for most people.
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