What batteries can be charged inside USB rechargeable flashlights?
Each flashlight can only charge the primary battery that they use. For a flashlight that uses and comes with an 18650 battery, it will be able to charge the 18650 battery. Many of these lights can also use CR123A batteries but CR123A batteries cannot be charged in them.
Similarly, if the light uses a 14500 battery, like the Fenix RC05, it can only charge the 14500 battery. Even though the RC05 can also run off of rechargeable AA batteries, the charging circuits are not designed to charge AA batteries.
It is also important to note that some units that use proprietary batteries can only charge those batteries. For example, the RC20 or the discontinued RC15 and UC40UE come with the ARB-L1 proprietary battery but they can also be powered by a standard 18650. However they are not able to charge standard 18650 batteries and can only charge the proprietary ARB-L1 series batteries.