Hi all thanks for the replies.
Really appreciate it.
I’m a real newbie when it comes to flashlights and headlamps so most likely i am too excited with my expectations to get about 500 lumen for 40 hours.
I’ve seen flashlights with 13000 lumens that does 3 hours on a battery so i thought if i would not be too greedy with my lumen requirements that might save a huge load on the 26650 battery 
I already thought of using multiple batteries and load with a powerbank but the issue (besides the load of the powerbank) might be the backpack shaking all around, seen the micro USB ports on the batteries itself that might break while running and charging.