The Serfas UTL-200 USB Tail Light has a respectable 30 lumens of output and is able to give you 10 hour of flashing runtime per charge. It runs off 2 rechargeable AAA NiMh (included) that can be charged in place with the included USB cable or removed and charged externally. The light can alternately be run off 2 AAA alkaline batteries.
Quick release rubber strap mount with angle adjustment can mounted on your seat tube or frame.  Integrated clip allows mounting on a seat bag or pack.

Specs:

• Modes: High Solid (30 Lumens): 5.5 hrs, Flashing (30 Lumens): 10 hrs & Alt Flash (15 Lumens): 14 hrs
• Weight: 92 Grams (3.25oz) With Bracket
• Battery: NiMh
• Provides 180 Degree Visibility