Buy a DMM (digital multi-meter) and test your batteries.
measure twice cut once.
use thread lube on your lights...keeps 'em from feeling el' cheapo (ref)
on your dive lights use silicon grease regularly to keep 'em waterproof & preserve the o-rings.
in general smaller lights get more use
If your P60 drop-in normally runs on one 18650 make sure it can accept 2 x CR123A cells (6 volts) before attempting
Cheap chargers may continue to charge after indicator light is turned off, thus overcharging battery.
Don't believe the lumen/lux numbers unless you see something like ANSI
realize that you're in one of these stages of flashaholism. you may want to just jump to the end stage and call it good...
Progression:
Newbie: I’d like to get a brighter AA flashlight
Intrigue: I can use another light
More Intrigue: and another
Power hungry: Hummm these Li-ions give me power… want more… more POWER!
delusion: I have the ultimate in PoWer… nothing can beat this light!
Tint snob: I notice these lights have a shade of purple or green tint
Elitism: These lights disappoint me. CRI. What is CRI? (inspired by CPF post)
Ok scaru (post below) I added some more