In reply to HardenClimber:
> Obv 18650 batteries are not as 'universal' as AA's
Agree but look at the watt hours (Wh). mAh/1000 (to become Ah) then multiply by voltage
e.g. rechargeable AA enloop (standard not pro) 1900 mAh has a nominal 1.2v so is rated as 2.28 Wh. A decent 18500 battery is up to 2000 mAh or so and nominal 3.7v so is rated at 7.4Wh
You can see right away just how much more you will get out of using 18650 batteries. IMHO they're definitely worth investing in, even if that means people having to buy a charger
A word of warning about rechargeable AA batteries. Some torches do not work correctly with rechargeables and some do. Several Fenix models that take AA and AAA do not "like" rechargeable batteries. Read the instructions!
Rechargeable AA and AAA run at 1.2v, compare that to Alkaline at 1.5v or lithium AA/AAA at 1.6v. When you have them in series that difference is multiplied. I have a little Fenix wf05e on the side of my helmet (as an emergency light in case main light problem). Timed this the other day... If I put in 2 x AAA rechargeables (Bonai 1110 mAh), on full power setting the torch starts to warning flash at 44 minutes. If I use 2 x AAA Alkali (Energizer Max) it starts to warning flash at 3hrs 24 mins (at 18 deg C). The instructions say to use Alkaline batteries.