8000 lumens is bonkers. You don't have windshield glare and you're not going 100mph. 300 is adequate and 800-1000 is great. 8000 is probably a lie anyhow... Don't believe too much the mAh of that pack either. 2200 mAh is typical for 18650 cells and 3000 mAh is about the top end... if you don't wind up with counterfeits. 9000 isn't real. So 90,000 mAh for real would be a hell of a brick, and it's probably more like 22000 if it's ten cells.

USB is indeed pretty low voltage. It's supposed to be 5v regulated, so if you are charging a battery from that it needs to be even lower. USB-recharging lights mostly use a single cell, my 6yo Cygolite Metro is a single 18650 style.

My new light's battery is a 4-cell 18650 pack which is rated 8.4v / 4400 mAh so that's 2x2. It uses a wall charger and the Magicshine style sealed connectors, not USB.
http://kaidomain.com/bike-lights-and...for-Bike-Light
http://kaidomain.com/bike-lights-and...ens-Bike-Light
The above is a reliable seller who will send you what you pay for and does not traffic in fiction, but they are located in HK and there was a few weeks delay on my kit, probably because of the riots.

Despite this all sounding cautious, LED lights, brushless motors, and lithium batteries are all a hell of a lot better than anything we had twenty years ago. Rejoice!