Size is an issue.
The chargers in 14500 batteries reduce battery capacity to make room for the charger. Or they use oversized extra long cells.
Also, chargers in batteries don’t need waterproof ports to get access to the outside of a light.
@Lumen9000 Yes but complexity is an issue. Have you actually drawn it to see how the connections would actually work? I think not. Adding a charging port ANYWHERE on the fwxxx lights will be a challenge to even those that can engineer
Now that I think about it, here is one way it might be done:
Swap the threads on the head from female to male. Result is the body tube would be much longer, while the inner tube remained short. To prevent the inner tube from coming out of position it would lock at the tail.
With these changes, partially unscrewing the head from the body would reveal threads on the head right over the driver. Just add a slot in the threads for the charging port and it should work fine.
The driver would need a separate sandwich layer for the charger though, and the light might need to be slightly wider to maintain same width in the driver compartment. Figure the entire light might be 5mm longer and 2mm wider at the head, but otherwise look the same from the outside.
@Firelight the driver would need to be smaller diameter plus it’d need to be glue in place since a retaining ring will no longer work (with the inner tube)
Could make the entire head slightly wider rather than the driver narrower. Or just make a narrower driver since the driver would need to be completely redesigned anyways.
The driver does not need to be glued. Press fit into the head would work fine, just like in the Egletac D25a.
However, also like the D25a, some threadlocker would be needed on the bezel threads to ensure the bezel doesn’t accidentally unscrew during battery changes.
Making the entire head wider makes more sense because the driver cannot be any narrower than the inner tube. Something has got to give
Maybe it would be possible to use the signal tube to carry the + input for the charging circuit and put a magnetic port at the tail, with some serious miniaturisation on the driver to fit the charging circuit.
So the sleeve carries (-) for driver UI and (+) for charging…? What happens if you accidentally press the tail button while it’s charging?
Hmm, yeah, I didn’t think about that, it would just short the USB I think ? It seems that a proper USB source should have a short circuit protection but it would not be good practice to rely on that.
Why is there always tube in these kind of lights? Isn’t the tube just for switch signal? It could be any shape that takes less space inside flashlight body. Maybe little groove that takes anodised piece of aluminium from tail to driver. Of course that would need precise machining, but should be doable with modern tools.
How exactly would it contact? Remember at least one end needs to ‘screw’ open to change the battery.
My thoughts are one of two things will happen 1- poor reliability 2- problems contacting outer tube when screwing the light together
Same way as it would with inner tube. Battery would sit in actual flashlight boby and on one side next to battery there would be this separate “rail” in small groove with unanodized heads. Size savings aren’t big, but they can matter with millimetre game.
So it would be point loading the threads? Like tightening a nut and bolt with a pebble jammed in one side? Something like that would also point load the contact board in the tail so a ~0.5mm washer would need to be soldered over the contract on the board to deal with the constant wear.
Something like that. It could even have small spring effect so that it provides better contact. Like spring steel.
That sounds doable. The outside edge on tail cap can screw hard up against the body to keep even pressure on the threads and the contact rod can slide easy but still make good contact. The rod would need to be a little thick for rigidity to hold it’s shape/ position in the groove though which again adds diameter to the body. Alternatively drilling a tiny hole the length of the body would be difficult to machine… but that also adds some diameter
you are certainly entitled to wish for on board charging. Since it is not there, I suggest an alternative…
The Olight Universal Charger.. It has the convenience of USB, small size, works on both Eneloop and Any Size LiIon.
Any updates on when the FW1AA will be out?
Worth the wait…I’m diggin’ it. NM1. Needed a little center tweaking, and isn’t perfect, but in line with the rest of my NM1 lights.
Some cursory wall shots…. Camera makes it look better than in person, but I think it will be fine.
(Oh…Bunny on the switch button 
Camefromaol on Reddit posted pictures of the production model, it now has a completely flat shelf and a larger (15-16mm?) MCPCB, very good news for modding.
Now only the SMO reflector can be problematic with some LEDs, hopefully we can find a good OP reflector or TIR replacement.
Edit : it’s even available with SST-20 and 219C.