Have you ever had any problems travelling with the batteries "inside" the flashlights?
I’ve brought like 4x 66Wh Lipos in my backpack before through several international flights.
No problems.
I have had a cell inside a light on a few occasions, never had an issue with that either. Even though I have lockout in my own firmware that I trust, now days I tend to pack cells separately in those little plastic bags for liquids. I leave ’em in the carry on bag during security screening though, never had any comment about them.
Next time I travel I think I may keep the cells separate just in case they make problems. This way they can confiscate the cells but not my actual flashlight, but from all your travels they don't seem to mind so it should be fine.
I fly several times a year and I’ve never had had problems flying with 18650’s.
I had a similar concern back in early January. I received a new laptop battery pack while visiting a relative over the holidays. I decided to harvest the cells from the bad pack, so that it would be easier to take back home. I wrapped them in individually in plastic, and seated in a plastic bag. Put in carry-on. No problems at all. Not even a TSA delay. I would avoid putting the batteries in checked luggage. Someone here mentioned that it could be under greater scrutiny since bags in cargo hold cannot be accessed in flight (e.g. fire).
Powerbanks up to 26800 mAh are allowed. Even slightly higher I think (100Wh)
You are allowed to carry many of such powerbanks.
So a 40000 mAh power bank would be forbidden, but 10x 26800 mAh powerbanks are fine.
Your 18650’s are way below 26800 each, so no problem.
I travel a lot and the batteries I take with me on all flights:
1x 26800
10x 3500 mAh 18650
1x 5500 mAh 26650
1x 4200 mAh 20700
1x 4400 mAh 21700
1x phone with a battery that’s bigger than your 18650
1x laptop with a battery that’s way bigger than your 18650
My baggage is openend about 40% of my trips for extra inspection. Never ever a single word about my cells.
I suppose it depends on the location.
I have not been to that many countries (only a few from Southeast Asia or nearby areas where I’m located).
But so far I know and have personally experienced that the China security inspector (exiting out of China, the “security inspection” part after immigration checking, just before boarding the plane). I’ve seen a number of times (I’ve been to China a few times over the past many years), and have seen (and heard other people’s experiences) powerbanks get thrown in the trash bin very often by the security inspector.
Basically if the powerbank does not have a proper label, then it will get thrown out (the reason is that a non-visible/faded label or no label or incorrect capacity label) is over-capacity. So I’ve seen a friend of mine who brought a smallish powerbank (probably less than 5000mAh actual capacity), but has a “98000mAh” label, get thrown in the trash bin by the security inspector.
However, what irks me very much is the time I got some genuine 18650s from Neal when I visited China about 1.5 years ago. So I got several batteries from him and placed them properly in my handcarry bag. I haven’t even used those batteries yet (a pair each of button-top Samsung 30Q, button-top Sony VTC6, Sanyo GA). On the last leg of the trip, to exit China and come back to my country, the security inspector asked me to take out the batteries in my handcarry bag, which I dutifully did so, I had a Keeppower 18350 battery which has a capacity label, which were not confiscated. But upon looking at the various 18650s — the inspector said these batteries have no label (genuine OEM Samsung 30Q/VTC6/Sanyo GA do not come with any labels), and just threw them in the trash bin — the reasoning is that they have no capacity label and are considered “over-capacity”. Hard to argue with their security inspector… even as I mentioned that those 18650s are 3000mAh and 3500mAh each in capacity… 
(After this experience, I wasn’t able to source any other button-top 30Qs for a long time, until I bought Banggood’s MF01S with the button-top 30Qs bundle (and sold the MF01S)…
I have never had a problem on Transatlantic or domestic USA flights.
Carefully follow the guidance on your airline’s website and also the national guidance.
I individually insulated the cells and then packed them in a 4-cell storage case. I also carried a 26650 cell inside a light.
No issues at LHR, LAX, SLC or FCA. My bag was hand searched at Heathrow, possibly because of the batteries. There wasn’t much of anything else in my carry on beside clothes and some earphones.
TSA:
Loose lithium batteries are not allowed in checked bags.
https://www.delta.com/en_US/traveling-with-us/baggage/before-your-trip/restricted-items
Delta:
US Federal Guidance
https://www.faa.gov/hazmat/packsafe/more_info/?hazmat=7
UK CAA guidance:
I have been traveled with a torch in the hand luggage and been asked to make sure the torch is in “locked” position or there is mechanical interlock of accidental torch turn on. Security guy must be experienced flashaholic 
Sometimes carry few 18650 in hand luggage with no questions. There are same batteries in your laptop, and similar in your phone. Shouldn’t be a problem within certain limits.
You should avoid it. It is, as Andrew_Debbie pointed out, not permitted. I have been asked during check-in weather I have any items with batteries in my check-in luggage.
Actually you are required to take Li-Ion Batteries up to 100 Wh in your carry-on baggage - you must not check them in. Above 100 Wh total you need Airline Approval.
One Li-Ion 18650 Battery has usually not more than 14 Wh, so 4 of them are 56 Wh, so you are safe.
Detailled example Lufthansa
Olles
If you don’t want to or can’t open a pdf:
So couldn’t one print out a “faux” label and apply to the power bank casing over the original? That way you make it look passable. I’m not surprised about China being very strict, though. I’m sure all kinds of electronic items of questionable manufacture challenge their security all the time. So yeah, put on whatever label you want mimicked from a commercial source, then sheath the cell in clear sleeves to look authentic.
The restriction is per cell.
You can definitely take more than 100Wh in separate cells.
Never had a problem with any single lithium ion on cells. I’ve carried up to 10-12 in carry on at a time. This is in past 2 years on flights to Europe from the U.S. I did however just have a 166wh battery for a battery operated chain-saw get taken away 2 weeks ago; which was totally my fault as I thought a single contained battery limit was 300wh.
Glad to know that, was about to limit my cells when travelling.
Not per cell, per unit.
For example you are allowed to carry 10x 3500mAh = 35000 mAh as separate cells. => 10 units
But if those same cells are inside a powerbank then it counts as 1 unit of 35000 mAh, which is above the allowed limit.
The rule is, what the most outside label says. When in a PB they won’t look into the PB.
It’s quite similar to bottles of water. Even if the bottle clearly has way below the limit in it, but the label says otherwise it’s forbidden.
You can take 90 ml in a 100 ml bottle but not 5ml in a 200ml bottle.
BTW the rules on water are changing. At my local airport they upgraded their scanners over a year a go and big bottles of water as no longer banned.
According to delta, spares can be brought in carry-on as long as they are in a proper container, like the box or plastic carrier they came in.
No, Your thinking is reversed—That is exactly what you are NOT ALLOWED TO DO.
If there is a battery fire in the passenger compartment, then it can be detected and counter measures taken to put it out;
If there is a battery fire in the luggage compartment, then it would not be detected and could destroy the plane.