This actually one of the reasons the Lishen 4500mAh 21700 cell would actually be preferred for the GT FC40, since it can draw quite a bit of current, particularly as the cell discharges
I can’t comment on the cell but i can link you to a datasheet:
https://www.fogstar-wholesale.co.uk/wp-content/uploads/2021/09/N21700CG-SPEC.pdf
I’m not a battery expert either and would normally be wary of the 15A rating too but the datasheet (section 5.4.1) claims 85% capacity at 3C which is a 15A discharge so they may be capable of it, but i can’t disagree with BlueSwordM’s opinion that it will reduce battery life.
Nice to see the datasheet but a no name company’s datasheet does not carry much weight for me.
I have dealt with too many knockoffs in my time and find the big players are more honest since they can’t easily get away with making up numbers. Not to say they are squeaky clean either but the ability to hold them accountable makes them less dishonest.
@Bort, Bak is far from a no name company.
Besides, their cells have been tested a lot 
We have the same opinion for most 5Ah 3C cells: their 3C rating is a bit much, and we prefer to underrate them at 2C.
This is the first time i have heard of them.
BYD, CATL, Lishen, these are companies i know.
And you seem to agree with me that 3C is a bit much. Hence their datasheet is unreliable.
? No, it’s just that most of us think about that for 5Ah cell, the Samsung 50EX series, Molicel M50A, Sanyo NCR21700 5Ah, etc.
Only the LG M50LT and the Lishen 5Ah cells seem to be well rated for 15A without any real downsides to cycle life.
Again making my point.
I guess you're talking about the Lishen LR2170SD...
I could have ordered that for even less money than the BAK. 
Life is a school, such things happen to all of us.
Awesome.
For once I'm glad that the better item is sold out. 
I got 4 of them from there also. Didn’t have any problems. Can’t use them for hot-rods.
Bak stuff is decent.
I would use them if i didnt have tesla model 3 packs to tear down for cells.
Last year a regular bought a bunch of the bak 5000mah 21700 for me to put in his harmon kardon speakers.
A cycle a day since and no capacity fade noted.
I decided to switch from Samsung 30s over to Molicel 4,200 mah 45 Amps. Not quite up there with 5,000 but much better runtime for my lights and noticeable increase in power
less internal resistance on the lower capacity cell. this is why finding the right battery for your application is so important.
i use a 5000mAh battery, but only draw 500mA of current most of the time, and 1A at most.
That’s really interesting I didn’t know that
Can you elaborate more on “this is why finding the right battery for your application is so important.” for me please?
Ive raised concerns here and there a few times, about not understanding whether a certain cell and flashlight are an “appropriate combination”.
I basically just get told to not really worry about it.
i’ll try. note, i’m not an engineer or an expert in anything.
first you need to know how many Amps are drawn on the battery, not on the Emmitter leads for the light in question.
then i suggest looking at battery capacity curves at different currents. you may find you get more capacity with a lower mAh cell at a given load.
some want the highest output and least voltage drop from a cell when fully charged and capacity be dammed.
also, MANY cell ‘manufacturers’ and re-wrappers lie about Amp, and capacity ratings.
the two places i use that in my opinion are very reputable for battery test data.
Mooch (AKA Battery Mooch on youtube) his content is aimed at batteries for Vape Mods etc, but his testing is thorough and relevant to us.
make sure you read his notes when he tests cells.
https://www.e-cigarette-forum.com/blogs/mooch.256958/
HKJ (from EEVblog)
https://lygte-info.dk/info/indexBatteriesAndChargers%20UK.html
i really like Mooch’s approach and showing just how many companies and re-wrappers lie about ratings.
you should check him out on youtube. very informative about choosing the right cells.
or, you could ‘not worry about it’
CG battery is good，equal to 50E-2
@Dudeman
Thank you.
I’ve clearly been using my lights like a moron this whole time.
I don’t know anything about electricity let alone taking it that far.
I no longer feel safe using these lights/cells. And probably never should have been in the first place being so uneducated. Thanks for possibly saving my life and others. Or at least from wasting any more money.