I plan on having a couple of my flashlight “ready to go” meaning having a charged unprotected 18650 stored in the flashlight so its ready to go if I need it in an emergency.
Is it bad to keep 18650’s in a flashlight when not in use or should I store them externally?
If the flashlight has a proper forward or reverse clicky switch, the circuit to the battery is open when the light is off, so storing the battery inside the light is the same as storing it outside the light.
If the flashlight has an electronic switch, you will have some parasitic drain, but most well-engineered lights control the parasitic drain pretty well (to the point where it is comparable to the battery’s self-discharge rate). If you recharge the battery of a stored light with a clicky switch annually, you might want to recharge the battery of a stored light with an e-switch every six months. Or, on lights with anodized threads, you can just unscrew the cap a tiny bit to open the circuit and eliminate the parasitic drain.
Anyway, Li-ion batteries have no chance of leaking like alkalines do, so no worries there.
I agree with all the above, so I’ll just add that if you’re storing the flashlight for long periods of time, you might want to charge the battery to about 4.0v, rather than a full 4.2v.
The slightly lower charge will still give you 80% of its capacity, but will be much easier on the battery over the long-term, and retain most of its capacity for years. If you store it at a full 4.2v, your battery capacity will permanently drop by about 20% each year.