So people throw out disposable vapes all the time. I cracked one open to take a look and it turns out they have a 18500 battery in it. So I was thinking of 3D printing a spacer to make it the same size as a 18650 and putting some shrink wrap on them. I just wanted to see what people who know more than me think. Is this a bad idea that is going to burn my house down? a waste of time perhaps?
I did a trial run in my wurkkos fc11 and they seem to work, it charges and looks just as bright.
So if I charge them using the flashlight would that have the protection built in?
No,
Lights designed for charging removable batteries will (usually) have charging circuit designed for protected batteries as they expect the battery to provide feedback on voltage, temps etc.
Random unprotected cells should be charged in an external charger in a fire resistant area and used extremely carefully in lights you don’t mind loosing.
The only thing I would recommend for you to do is send them in for recycling.