Originally Posted by
mrbubbles
Most lights are not charged through a usb port. I have 3 bike lights plus a box full of blinkies, none of them are charged through usb ports. mini usb has a limited life cycle before it conks out. (I also go out of my way not to ever buy lights using usb charging only, because that's just dumb). Also, usb does deliver more than 2.5w, just not through usb 2.0, usb 3.0 delivers more than 2.5w, usb dedicated wall chargers for tablets and newer smartphones deliver around 10w or more.
LED differs a lot. The newer leds are 150 lumens per watt. My newest light can be powered at 600 lumens for 5 hours on a 200 gram battery, which weights not a whole lot. That battery takes 4 hours to charge because it's not usb.
If you need 300+ lumen for more than 5 hours, you should be looking at dynamo lights, not battery lights. For folks who want to avoid charging their bike lights everyday or every second day, dynamo lights is the way to go.
USB is de facto standard for midrange lights. Cheap lights use batteries because they are weak and they can last a while on standard batteries without burning a hole in your pocket (one AA battery is 3 Wh, 2 AA batteries will power a 60 lumen light for 6 hours.) High end lights require custom chargers. In the middle, it's all USB.
Dedicated wall chargers can play fast and loose with wattage, but ultimately the problem is that you can't send much power over a standard USB cable without overheating. You need extra thick wires to handle the current. USB 3.0 is rated up to 4.5 W, which is better but still leaves much to be desired.
150 lumens per watt is a spherical cow, you don't see those outside labs or theoretical papers. 60 lumens per watt is what you see in the real world.
Yes, if the goal is to have 300+ lumen for 5+ hours, some kind of external power needs to be considered, like a dyno hub.