It depends on the mod mostly, mods with built in batteries have basically the same batteries as the mods with replaceable batteries only they are soldered in, with my single battery mods I use a external charger and replace the battery when it gets low, my dual battery mods I charge in the mod. I guess its a matter of personal preference. The only mod I can think of that batteries should be charged outside the mod is the RX 200, because people have had problems with the batteries charging unevenly due to the battery configuration. Charging with a external charger makes it easier to just switch batteries instead of waiting for the battery to charge, Just my opinion.