Serfas USL-6 Thunderbolt
  • 90 claimed lumens
  • 100 measured lumens
  • 1h 30min reported battery life
  • 1h 30min measured battery life
  • Be-seen
  • 50 g
  • flashing mode
  • no pulse mode
  • 2 brightness levels
  • internal Lithium batteries
  • low battery indicator
  • micro usb rechargeable
  • Included mounts: Strap

The Serfas Thunder series lights originated with the Thunderbolt headlight and taillight; among the first bike lights to feature a large array of micro LEDs. Variations on the original Thunderbolt are now available in different performance and output ranges.

Review: A bright, wide-angle be-seen light with a novel mounting system

Review by , Sept. 12, 2013, midnight


  • USB rechargeable.
  • Unique design: very wide angle visibility makes it a great be-seen light.
  • Versatile mounting (could also be a con depending on where you want to put it, but I think you could find a spot on almost any bike).
  • Easy to get on and off the bike quickly.


  • Not great for seeing the road – mostly just designed for being seen.
  • Short battery life at full brightness.

Despite the short-lived battery, the Thunderbolt was one of the lights I was most excited to test out, and it exceeded my expectations: bright, mountable almost anywhere, and excellent off-axis visibility. This light is definitely a very unique design, and I can see many applications of it, especially for biking in an urban environment where side-lighting can help with safety: you could strap one of these on each side of your bike to help cars see you from off-angles. There is also a red taillight version available, which makes for a nice set.