I don't get it. Why is SF E2L more efficient than L1?

Ok, below is the table for each lumen and runtime. The last column list the % of total lumen hour emitted from the torches in reference to the E2L. Why is the E2L ran so efficiently? My understanding was that the lower the lumen, the more efficient it will be as there is less energy lost through heat. Hence, I expected the 10 Lumen from the L1 would kill the E2L in efficiency but no, it was surprisingly the other way around.

The L1 uses a resistor in the tailcap to produce the 10 lumens. I'm not sure that that accounts for everything, but that might be part of it.

Someone on CPF did a runtime plot for the E2L and it was more like 9 hours regulated. Using that figure should make the numbers a bit closer.

matthewdanger said:

Someone on CPF did a runtime plot for the E2L and it was more like 9 hours regulated. Using that figure should make the numbers a bit closer.

Click to expand...

Yes, I have just saw that post. Those figures I got were from SF website. However, as you said, it only make the numbers a bit closer.

It makes the numbers comparable (if there is a voltage drop resistor in the L1 for low) if you use 9 hours rather than 14.

E2L 45lumens 9hours = 405 (LxR) vs 320 for L1 low

making L1 low 80% of E2L, with a few rounding errors this isn't so bad

LA OZ,

Based on the chart, you're assuming linearity between the L1 and the E2L for 2-cells. The relationship isn't linear, however. When you run two cells together, you get a more efficient, balanced run than running one alone, then running another alone. This is why some people describe 2x123A incans like the G2 and 6P to appear "regulated."

Two batteries together will almost always give more total runtime than two batteries used up one at a time. To check me on that, see the runtime graphs on FLR for the second-gen Lux KL1. It runs brighter on the E2 body than the E1 body, but it also runs in a more solid regulation, stays in regulation longer, and would give more total runtime than two 1-cells run one after the other.

Depending on the circuitry it is often more efficient to run an LED at a higher voltage.

OK, it all makes sense now.

One must use caution when using manufacturer ratings to compare products.

In my opinion the best way would be for users to record output/runtime charts for the products and compare the results.

since the beams are the same, one could use the area under each line to compare efficiency...

Perhaps the E2L really puts out "only" 45 torch lumens... On the other hand, the L1 seems to be right in the same ballpark of the 100+ lumens high powered Cree lights.

I don't know about the circuitry in either light, but some of the difference in efficiency could be explained by what are known as I-squared-R losses.

To maintain the same power in a given load with a lower voltage, you must use increased current. (power = V * I = I squared R)

Then the contact resistance, switch resistance, wire resistance, and other resistances become more significant in accounting for loss. There is a higher voltage drop across the series resistances when you draw more current through them.

Remember Ohms law: V=I*R

Manufacturers tend to tell you the peak output and the total runtime, with the total runtime including any "moon mode". This doesn't mean that the light produces the peak output (or, necessarily, anywhere near the peak output) for the entire run time period. So a simple multiplication of the two numbers gives you a meaningless result unless the lights both have a very well regulated output and no "moon mode". For a fair comparison of any other lights you'd have to integrate the light output over the runtime. LEDs become a lot more efficient at lower output levels (until they reach a very low output level, where the efficiency abruptly drops), so a light that spent more of its runtime at a lower (but not too low) output level would have an additional efficiency advantage, all else being equal.

There are a few lights which have a very well regulated output and no "moon mode" -- that is, the output stays constant until the battery is drained, then the light abruptly shuts off. The Mini-Mag LED is one light I know of with this characteristic. If both of the lights you're comparing have this characteristic, then simple multiplying of the runtime and output is a reasonable comparison -- with the realization that if the regulators are equally efficient, the light putting out the least light should deliver a better product because of the increased LED efficiency at low levels.

One additional factor to consider when doing this kind of comparison is battery energy. This isn't a big factor here, since both lights use lithium cells. A NiMH or lithium (primary or rechargeable) cell has a reasonably constant total energy content over a fairly wide range of discharge currents. It will deliver more total energy, however, at lower current, giving the lower output light an edge. But if you're ever comparing lights where one or both use alkaline cells, the one drawing less current can have a big advantage because it'll get more energy out of the battery.

c_c