I asked about it in another thread but put it in a wrong section. If I expect the light to draw some 10A at most (not certain of this either), how would I select the most beneficial battery for it - e.g. Vapcell has the whole page of them that seem to fit… Why so many?
I would look for a battery rated above 10A but not too far, you don’t need one rated for 35, and then look for the highest capacity.
The Vapcell F60 is rated at 12.5A and 6000MaH, that looks to be the highest capacity at just over 10A.
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The Vapcell batteries usually have discharge profiles shown. I imagine that as little sagging as possible under close to the expected loads would be preferable? But what about regulated and not-that-regulated flashlights? Should one look for relatively flat discharge profiles if the flashlight is not well-regulated?
Wow, 6000mAh!
My best 21700 cells (Samsung 50S) only have 5000mAh, but I get fairly inexpensive cells. 
I was surprised too, but that’s what Vapcell claim.
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Mooch measured the F60. At 5A it has an e-score of 14.2, at 10A 12.9.
As a comparison, a Samsung 50S has 14.3 and 13.7, so even at a mere 5A load the 50S will already run for longer than the F60. P45B has 13.1 and 13.0. Despite only being 5000 mAh (50S) and 4500 mAh (P45B) they both outperform the F60 at 10A.
Maybe in a lower powered light it could show its potential, but in most lights I’d rather grab a 50S, P45B or cheap 50E (14.0, and 12.2, at 5A and 10A, respectively). I would not get it for anything running more than 2-3A through the emitter.
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Nice!
I wasn’t aware of that.
This is where I got my Samsung 50S if anyone wants a store recommendation…
They charge shipping, but even with shipping it’s a pretty good deal, in my opinion. 
I think that’s one of the best sources in the states. In EU it is nkon.nl. That’s where I got mine. Shipping was rather slow, but price was excellent.
I have 50S in two M21B, one with XHP70.3 (6V 5A boost), one with SFT40 (3V 8A buck).
I was told the 50S are a bit sensitive regarding being stored while full, that is apparently deteriorating the cells quickly. So if you wanna store them, discharge to 3.7 or something, and ideally do not put full cells in lights you rarely use.
But in the end, batteries are there to be used. One can be super cautious and waste hours on being gentle with them, or just use them and replace when they eventually break. I try avoiding obvious and simple ways of damaging the cells (storing at 3.7 or 3.8V only), but do not go out of my way to keep em safe.
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I must have gotten my Samsung 50S cells on sale because I paid $4.65 each, before shipping.
Good luck catching a deal as sweet as that (in the states.) 
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That’s excellent pricing indeed, I’ve seen people pay more for 50E 
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18650BatteryStore.com has had pretty good sales on certain holidays in the past, so it pays to be patient (if you can.)
What’s the ‘e-score’?
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A metric mooch uses to classify runtime of cells.
This pic (measured and published by mooch) explains it 
Basically: Higher number = longer runtime
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Where can I find those tests and tables?
Google for “Mooch” + cell name.
21700: 20700 + 21700 Battery Ratings and Performance Table | E-Cigarette Forum
18650: 18650 Battery Ratings and Performance Table | E-Cigarette Forum
Mooch is also a member of BLF, and I think not all tests they did are in those rating-threads (F60 for example is missing) - therefore googling for “mooch” and the cell names sometimes leads to benches not in the tables.
Did I get it right: it’s the battery’s energy content in Wh computed by integrating the discharge curve between max voltage down to 3.2 V, as tested at different constant continuous loads?
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Yes, that’s how I understood it as well. Which is actually a pretty good metric, being directly related to the amount of energy you can draw from the cell before it’s empty.
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