Lately I’ve been thinking about building the most efficient flashlight, instead of the most powerful (my last build was a DD single 18650 triple XP-L)
From what I can find, the XP-L V6 bin is the most efficient LED available.
I want to have the following modes (±): .2, 3, 20, 130 and if it doesn’t effect the efficiency of the driver a ~600 lumen turbo mode.
What is the most efficient driver (based on an 18650) to do this?
Any of you guys got experience building an efficient light? Please share 
You need buck driver with contact current regulation (without pwm). Not sure it exists in 17mm size.
Looking at HKJ’s driver tests, buck drivers are 65-95% efficient, depending on the voltage overhead and quality of the driver. So if you get a good driver this would be an efficient option. Mtnelectronics has a selection of buck drivers.
Linear drivers are going to be around 75% efficient at currents around .4A with a fresh battery. The efficiency gets better as the battery voltage drops.
For currents around .4A, using lower voltage LiFePO cells would be more efficient since the voltage difference between the cell and LED is smaller.