Any ideas of Brand and where to buy?
Putting it into an L21A torch.
Best: Molicel P45B for high current and IMO overall, Molicel P42A as a slightly smaller, cheaper, and lower powered P45B, Samsung 50S for high capacity.
Cheapest: Samsung 50S are fairly low priced batteries that will work with at least moderate performance for almost all lights (although Molicels are competitive in TCO due to higher cycle life), the very cheapest battery probably isn’t something you’d want to buy 
Where to buy:
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Sound advice.
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Yup.
I usually order from 18650batterystore, but those three stores are three of the best.
I think I should order some Samsung 50S cells.
They’re currently on sale. 
Thanks 
Somewhere I saw to use Samsung 40t?
What amp should they be?
Thought I was going to be spending 
$20. Per each, but since I bought the torch 
price seems to have come down 
.
What emitter did you choose? SBT90 would benefit from a cell like the P45B (or the Samsung 30T/40T) due to the FET driver. I believe most other emitter options are limited to 8A drivers, so any of the other cells mentioned would be fine.
I’ll be hands on soon after I go get it at storage.
In the meantime I had a coversation with 18650 batteries, who said, 21700&18650 cells were never made to be a stand alone system, only made for battery packs.
INTERESTING 
Everything that’s in a battery pack is in the flashlight
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I mean like in a Tesla.
Racks of batteries.
Wolfgirl42 is spot on. Your light is only drawing 9-10 amps on turbo. Molicel P45B, Molicel M50A or Samsung 50S and you’ll get the most out of your light.
Darn it:joy:
I just ordered Samsung 40T batteries.
Am I going to regret it?
Samsung 40T is one of the best hi current 21700 battery with fairly high capacity.
I use Samsung 40T, Samsung 50E, Sony VTC6A, Sofirn 4000,Sofirn 5000 and Liitokala 50 batteries in Convoy L21B and M21A, Sofirn C8G and SP35 and for this flashlights Samsung 50E works the best.
It is because this lights pulls maximum 10 amps from battery.
Samsung 40T will show its full potency in high current fet driven flashligts with sbt 90 diode or similar.
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Is 50e that much better to change my order of 4 count 40t?
I can give you example:
My L21B SFT40 works on medium(10%) mode about 6 hours on 40T, with 50E it is about 7 hours.
Maybe on turbo(100%) 40T will give some better result but this flashlight have 8amps driver and stepdown after initial heating on 35% mode where current is about 2.5 amps.
So for lower power flashlights like this I think better is higher capacity battery.
If your flashlight has sbt90 or xhp70.3 led and You use it more in turbo mode than 40T can give benefits against 50E because than You will pull more than 10 amps which is limit for constant current for 50E by datasheet.
I think that it is more important to see from datasheet which battery has better cycle life?!
Or You can try both and give to us Your conclusion after tests because that batteries are fairly cheap.
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I’m learning
Are you an electrical engineer?
Yes I am.
Which led is in your L21A?
I’ll have to check 
on it.
Right now it’s sitting in my gear box in storage. It outperformed my big Catapult and became my favorite, then it went crazy and stopped working.
It’s basically brand new, so I’m really trying to get it up and running.
Been a FLASHAHOLIC since 1993, when all we had was SUREFIRE, and no LED torches at all.
Its good to have a few different high drain / high capacity cells on hand. But sometimes there are physical incompatibilities as well. For example, I have a few lights from Hank and Firefly that like to crush positive terminals in P45B, etc, but the VTC6A has a stronger positive terminal design and is on the shorter side. I have never had a VTC6A crushed in any light. Yet, the VTC6A isn’t a great fit for some lights like various Acebeams which easily lose contact with unprotected / flat top batteries because the springs don’t have the same reach / tension.
I have a great ACE BEAM protected with a charge port that’s unusable due to length. A waste of $20.
Convoy says to use the Samsung 40t cell.
40T is a great cell. Last year this time they were on sale for $4 during black friday.
As an aside, I wish all flashlight companies would list, among the specifications for the light, what the minimum CDR should be of any cell you would use with it.
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