(51 customer reviews)

UTL-6 Thunderbolt USB Tail Light


Durable Silicone Rubber Encased Light Body

Very Bright For Its Size – 35 Lumen Output

Highly Versatile Mounting Possibilities

4 Lighting Modes

Built-In USB Charging

Water Resistant Construction

SKU: UTL-6 / Categories: , , Tags: ,

UTL-6 User Manual & Runtimes


Limited Lifetime Warranty
Serfas® warrants to the original purchaser that the product is free of defects in material, construction, and/or workmanship for the lifetime of the product.

Battery Warranty – One Year Coverage
Serfas® batteries are covered by a 1 year warranty from the original date of purchase.

The Serfas® UTL-6 Thunderbolt Tail Light gives you style and safety in one high-powered & affordable package.

Being seen on your bike at night never looked so good with Serfas’ Thunderbolt (UTL-6) Taillight. USB rechargeable convenience and 35 lumen output while giving riders versatile mounting options.


  • 35 Lumen High/High Flash
  • USB Rechargeable
  • Charge Indicator Light
  • Removable Straps Allow Endless Mounting Options
  • LED Strip Creates a Non-Directional Flood Of Light


  • Modes:
    • High – 35 Lumens, 1.5 hrs
    • Low – 10 Lumens, 4 hrs
    • High Flash – 35 Lumens, 2.5 hrs
    • Low Flash – 10 Lumens, 8.5 hrs
  • Weight: 48 Grams With Bracket
  • Battery: 500 mAh Lithium Polymer
  • Color: Black

51 reviews for UTL-6 Thunderbolt USB Tail Light

  1. A Martz

    Very bright, I noticed cars slowing down to pass. On the first charge I got an hour on bright and another hour with bright blink. I didn’t really read the time’s it would last, so going forward, I’ll likely set it to dim blink. Mounting is simple and it stayed where I put it despite several jarring bumps. Disconnected from the bike, the straps come completely off and would be very easy to lose if I walked away with it and wasn’t paying attention to securing them. Charging time from completely dead took 2-3 hours.

  2. Razer

    Very Nice light, I live in Seattle & it works really good, I am certainly impressed and so are others who see it. The package had no Instruction Pamphlet, but there was one Caution printed on the box. It states as follows >’CAUTION: Do not look at or shine this lamp directly into eyes.’< Well as usual I ignored the caution print & was seeing red lines for sometime. This maybe a little over kill for a tail light so when you see it in the flesh do take the precaution, I wish I had done so. Worth every penny, 5 Stars from me.

  3. bob

    great light

  4. Doug

    Very Bright!!

    Easy to take from one bike to a other.

    Great product!!

  5. Jason

    Great light. Bright, versatile, well designed. My only complaint would be the use of the mini USB rather than a more common micro USB. I would really like to be able to charge it with one of the many cell phone chargers I have lying around at any given time.

  6. stefan k

    I love it, I am told I can be seen very well. The only thing is that I can’t tell when the battery is low.

  7. Ron

    Purchased both front and rear lights. These lights are amazingly bright. They are easy to mount anywhere on the bike. USB rechargable (2 to 3 hrs charging gives 9 hrs of light) and lightweight. Easily removed for recharging or theft prevention. Highly recommend these lights to everyone.

  8. James Parsons

    Very impressed with the brightness and viewable angles of the light.

    The only downside, those straps. I didn’t realize at first that the small round hole was to hold the straps (semi-)permanently to the light. I lost one right off the bat (fortunately was found and returned to me a couple of days later).

  9. chris

    AWESOME LIGHT !!!! I ride to work early in the morning its dark. Riding through the hills people come around turns and actually see me its super bright. you would have to be blind to not see me from a good distance. i can actually see it lighting the ground on my sides while im riding. AWESOME!!!!! well worth the price..thanks SERFAS” and thanks ernie @G’s cyclery in whittier”

  10. Mark

    Disclaimer: I work at Bicycle Garage Indy, and these are -so- much better than any other rear ‘blinky’ that they are flying off the shelf.

    Even the low-output setting is brighter than most other lights. The viewing angle is very close to 180 degrees– whereas most lights have a focused beam, and are really dim when viewed from the side. Also, most seatposts and seatstays are not vertical, so this is even more important for a light that mounts w/ straps like this one. I love that the black one pretty much disappears on a black seatpost– very stealthy.

    I wish for three improvements:

    1) Micro USB

    2) USB port on the end, so it could be charged on the bike.

    3) An extra strap included w/ the light, because I’m pretty sure I’ll lose one…

    The verdict: I even bought one for myself, and I’m jaded! Also, I think I’ll be buying a front one when the Monon Trail (our local bike/hike/jog/stroll/dog/rollerblade/birding/ice-creaming hangout) gets crowded this spring. Nothing parts the waters” like a decent front flasher!”””

  11. Kevin

    Its a nice light, the battery life could be better, thats why 4 stars. On low solid I have only gotten about 3 hours of life off a fresh charge, definitely no where near the 7 hours they advertise. I like the mount options, I agree with others about improving and attaching the straps and changing the charging receptacle to one of the ends. For the money its a good buy.

  12. MDubois

    WOW – Totally love this light. Easy to mount. I put this light on my seat stays. It holds in place (even over some less friendly pavement). This light is BRIGHT (with a capital B)! I wanted a rear light that is light and powerful. Magic! As a guy that rides to work and back, it’s nice to know that I’m seen from at least a half a mile (probably more) with this light. The flash mode gets attention (in a good way). I’ve had other lights, but this is by far the most powerful and bright. I would strongly recommend. My safety is worth the price.

    P.S. the usb charging option is sweet (no more batteries hurray).

  13. S.C.

    I can’t really add anything more to what has already been stated. It’s the best rear light ever!!!

  14. Jason Bailey

    This thing is awesome!!! I don’t have to use batteries all the time and it charges while I’m on the computer. Very bright and visible. I like the options for settings. I was caught in a long rain storm and 0 issues

  15. Nawaaz Ismail

    I haven’t owned too many rear taillights for my bicycles, but when I went into my LBS and asked what would be their best option for my safety and to compliment my new road bicycle, they clearly pointed towards the THUNDERBOLT. I am very satisfied with my purchase… Although the price was higher than expected, but I’m not going to argue with that too much since my safety is priceless. I know everyone reading this is more interested in the faults of this system, so here are a few: On the brightest flash setting, the battery goes out too fast. It lasts much longer in the low-power flash setting, and that mode is still plenty bright in catching everyone’s attention from behind. Also, there isn’t a very solid warning to let me know when the battery is running low. I realize that a warning light may flash from the mini side display, but I fail to see how that’s helpful when I’m always looking forward when riding (maybe a nice and clear beeping would be a good idea?). At first I was worried about not charging this system after every usage because I didn’t want the battery charge life to falter… But that’s not a good idea… It’s absolutely a must to charge after every reasonable use just to be on the safe side. As others have mentioned, the two plastic straps are prone to getting loose and lost, therefore I try to be extra careful in making sure I don’t misplace them. It would be nice if Serfas could’ve provided an extra strap or two. Other than all this nitpicking, the THUNDERBOLT is well-suited to being strapped to a nice performing road bike. It’s not bulky at all, and it has high-quality lighting which I am thoroughly impressed with. I definitely am looking forward to buying a second one in the future because of my long and consistent daily riding. Thank you, Serfas!

  16. Jim Henry

    Waiting to see how it performs in the 7 month raining season with usb port. Same concerns as others about straps (now have small hole around one of the cleats) and battery life (I’ve ridden with high blinking, lasted about an hour. have been using at low-blinking and was dead at the end of 4 hour ride this morning.)

    Recharging – recharging light fades in and out while charging, and stays solid when fully charged?

  17. Rheaah

    The brightness is AWESOME — guaranteed to be seen. Mounts easily, however, it slips off if the jarring is hard enough or rains heavily. It sucks because I just lost mine that way :(. It would be nice to have a better re-design of the casing to include a secure, quick-release clip or extra anchoring.

  18. David B.

    Light works great. I strongly agree with the other reviewers suggestions for improvements. An audible beep to signal that the battery is getting low is an extremely good idea. I’ve also found that this light can be easily attached to the back of a variety of helmet designs by simply using large size #64 RUBBER BANDS !!! Try it, it’s awesome. I put this thing on the back of my helmet up nice and high then I put their 60 lumen rechargeable taillight on my seat post. Two taillights is definitely better than one. CAUTION: If you are thinking about running a flashing red taillight on the FRONT of your bike, think again. The COPS have a legal right to pull you over and ticket you if you do this on the street.

  19. Diego Alves

    Well Above Average.

  20. Brandon S

    I sincerely feel safer as a result of this taillight. It is very, very bright. I like the fact it has multiple brightness levels as well as two different speeds for blinking. This is the first and only taillight I’ve ever purchased so I’m not sure how it compares to other products out there.

    I purchased this along with two TSL-350 headlights and I can say this much: I am so happy thus far, barring catastrophic failures of the devices, Serfas has a life long customer in me.

  21. Robin

    I thought that I’d give this light 4 stars due to the battery life on high beam. However I should have fully read this when I bought the light!

    I have noticed whilst cycling down pitch black country roads here in Warwickshire that the back of me is lit up red. You literally can’t be missed. I’m currently using one of these with a fibre flare on the back stay. I want to buy another to cover the fork too.

    Charging would be fine if you were using a computer at work, just keep the charger on you. Takes around 3 hours from dead.

    Definitely worth the price.

  22. Thomas Jondall

    The Thunderbolt Taillight is the best taillight I’ve owned, With four different modes to choose from it makes it impossible for cars not to see you while riding at night.

  23. Fabrizio Von Nacher

    Very bright, cars can see you perfect at night and in foggy days. Perfect to avoid cars pass you fast in days with no so many light.

  24. Cory Williams

    When I use the thunderbolt it looks like a motorcycle tale light. The thunderbolt stands out to the cars that are coming up behind you making you visible to traffic. The cars give you space and go by you slower this light is awesome!!

  25. Steven Davis

    Personally I love this little gadget. I say little, in a Great way, for it installs so easily, is super small, really light, and nearly unbreakable; yet, puts out a tremendous amount of light. Do not be mislead by its size because it puts out all the light, with multiple settings, that one could ever hope for.

  26. Mark

    Would love to give this 5 stars because it’s the most intense light I’ve seen that is usb rechargeable. However, I had to return the first one to the dealer because it stopped working and I’m having the same problem with the second one, although I seem to have found an after the fact fix. The problem is that if I ride in the rain or if the street is wet from rain the light stops working. If I plug it in to recharge it starts working again. But that’s only after I get home or to the office. So I have to have use an older taillight simultaneously in case the Thunderbolt stops working. Not that it’s bad to have two taillights but it’s disappointing that this one will not work consistently. Prior to this Serfas I had the TL-STN which I also liked a light, especially for the price, but it stopped working the first time I rode in the rain and I just had to toss it because it was not rechargeable. I see that it now has a more water resistant Silicon casing” so maybe you will also find a way to make the Thunderbolt waterproof (my guess is water getting into the usb port). Hope you figure it out because otherwise it’s a fantastic light.”””

  27. Matt

    Slim, sleek, and stylish! The rubber mounts work great to keep the light secured to the seatpost. The included USB cable makes charging at your computer during the day a breeze. The light intensity is impressive for the compact design. Great job Serfas!

  28. Tom

    Very bright and easy to install. what I like the most is that you can install it anywhere on the back of your bike and knowing that everybody can see you no matter the angle of viewing

  29. Daniel

    I love the brightness of this light!! Would recommend it even though I find the following problems. You really need to press the on/off button hard to get it on or off. If you work in a place that has secure computers you can’t plug it into the usb for charging so an optional wall charger would be great!!! Also it needs a more visible charge indicator. Hard to see the one on it. That said, this thing is a blast of light and you can’t be missed with it. So BUY it.

  30. Tim

    This is one bright ass light! Easy to mount with lots of battery life for most any type of riding.

  31. Eric

    Very bright and easy to recharge.

    However, it recharges at the fairly slow rate of 2-2.5 watts. Lithium batteries should take a charge faster than that say 5-10watts.

    Since it has a USB plug, could you also give it an app that would allow us to program our own light behavior. Like flash rate, dimming amount, fade in/out, flash pattern, behavior on restart (last used mode vs selected default), double click mode option. At least reprogram from the factory to start in low power flash mode.

    As other have said, I’d also like to see the USB connection be on one of the ends so it can be powered while on bike. Use the main LED as a battery meter on start up activating each led individually.

    Including an accelerometer to use it as a brake light would be very cool.

  32. Charles Van Kempen

    Great light. It doesn’t need to be on a perfectly vertical angle to be seen. It puts out a huge amount of light in every direction (high or low beam). I’m impressed with the amount of run time. I commute daily and this lasts 3 days for me on a 15 mile route. (and for the complaints about straps, no worries, just keep track of them). Ride on!

  33. Mikhail

    Awesome! Ridiculously bright. I can see light reflecting off of all the surfaces in front of and behind me. Tail light is just as great. Battery life can be a little short but it’s convenient to charge up the light with a USB (though Micro USB, common for smart phones now, rather than mini, common for digital cameras, would have been better.) I actually didn’t mean to drop $90 on lights, but I am really glad I did, and wouldn’t be surprised if they pay for themselves in cost of batteries, or replacing other cheapo options when they get wet.

  34. Alex

    Just Excellent! I have a head and a tail light for over a year. They are very bright and very easy to set up and remove from the bike. I’ve noticed cars slowing down and giving me enough space when passing by. A good set of lights such as this one can prevent accidents and save your life. Safety first folks!

  35. Brad

    The best and brightest! I put 2 on my bike and i feel so much safer.

  36. Doug Gerard

    Brightness and mounting convenience are top notch. Several riders in our cycle club have purchased these and use them regularly. Almost too bright at night on the high setting. I reserve that for bright summer daytime rides and use the lower settings for night time rides.

  37. John

    This thing is very bright and noticeable. Simple, but quite effective as far as I can tell.

  38. George G

    Love this light. I have several that I use for ultra distance rides. My only issue, and a frustrating one is the charging port. It’s a challenge working with an asymmetrical connector in the dark. Can’t wait for these lights to use a symmetrical connector like the USB 3.1 type C

  39. Mark

    Great light! Bright and fits nicely on seat tubes above brakes on my road bike. I bought 3 of these, but did have a battery problem with one after a few months. I sent it back to Serfas and they replaced the light with a new one. I have found the flashing modes great for both daytime and night riding, although riders behind may complain that it’s too bright….

  40. Josh

    Excellent light in most respects.

    Pros: It is small, very bright, and highly weather resistant (I’ve used it in the rain and snow with zero problems). It is easy to put on and off the bike, and stays put. I have the model with the clear” cover”

  41. Joseph Alejandro

    Thunderbolt of lighting very very brighten, this tail light is all that in a small package. It packs a powerful punch with 4 modes you’re made to be seen. I’ve use on my bike and my kids stroller. It make adventures fun day or night. Charging it don’t take forever either, it worked right out the box.

  42. Anthony Haas

    This light is ok, I use it when i commute and on shorter rides as the battery doesn’t last very long on the higher settings. I need a daytime light that is bright and has long battery life.

  43. lcsherman

    Ive been riding w/ this light for almost 3 years . I commute to work in the middle of the night & also enjoy pleasure riding in the early
    hours & I have never been more satisfied w/ a rear flasher . I run it on low blinking for maybe an hour a day & its more than bright
    enough & I recharge once every 7 or 8 days . Admittedly I ride in mostly urban environments where theres a certain amount of ambient
    light but still …. this is a great comforting addition to your Dark Riding . I also use a Serfas headlight (E-Lume 450) & give it the same
    recommendation —- different specs but the same quality & performance . This is good stuff …. just buy it .

  44. Marcelo

    My one after a 2 years usage got the battery out of order. I am trying to replace it but I cannot open the box/case. Does anyone know how to do it?

    • Serfas

      Hello Marcelo. Unfortunately, the thunderbolt battery can not be replaced.

  45. Mcarroll

    This si a great produce. I have recommended it to all my bike riding friends. I’ve had mine for over 5 years and it has performed flawlessly. It is so bright that it is visible at extreme distances even on a bright sunny day.

    • Serfas

      Mcarroll, Awesome to hear how much you enjoy your Thunderbolt! Thank you for the sharing with us 🙂 – The Serfas Team

  46. Ryan

    Fantastic product, I have had mine for going on four years. As a commuter, it takes a beating and gets wet occasionally. It looks as good as the day I bought it. Customer service is excellent; they serviced the light under warranty and sent me some replacement rubber straps. I will always recommend this brand to my customers.

    • Serfas

      Thanks for the kind remarks, Ryan! We’re always pleased to hear from a long time satisfied customer, and we of course hope you continue to choose Serfas for all your cycling needs. Thanks so much again! – The Serfas Team

  47. Ed Patrick

    The run time is a big downside to this unit. It is only lasting for a couple of short rides and then you have to recharge it for several hours. Nice design for fastening on the bike rack and the light brightness is very good. But the brightness is what kills the battery. Better to have a low light flashing than a dead bright one. Unless you are just a casual rider that only needs a little bit of run time avoid this tail light.

    • Serfas

      Hi Ed, we’re sorry to hear about your dissatisfaction with the battery life of your Thunderbolt tail light. Its max runtime is only 8 1/2 hours on the Low Flash (10 Lumens) setting. If you are in need of a light with longer battery life between charges and overall better performance we recommend our Vulcan series lights, (Models: TSV-130, TSV-350). The TSV-130 can achieve up to 22 hours runtime on Low Flash (20 Lumens) and the TSV-350 up to 20 hours on Low Flash (50 Lumens). Thank you for your time and we appreciate your feedback. Whether it be a Serfas light or a competitor’s we hope you find a light best suited to your needs. – The Serfas Team

  48. David Jozefkowicz

    I bought this light this spring , I’ve logged 3000 km with it and must say it’s a great light , easy to attach to the bike and easy to recharge , and very bright , I feel safe with it ! But the run time is disappointing , I bought it because right on the front of the box it said ” Up to 9.5 hours of run time ” . Nowhere in the box is there an explanation of run times so I was very disappointed to find that for rides over about 80 km the light would die before I got home , I was using high flash because also on the front of the box it said ” Can be seen up to 1 mile away ” and that was another selling point that attracted me to your product .
    Now that I have visited your site and see that I can get 8 hours on low flash I will try that for my long rides . But I’d like to know why on the box it said up to 9.5 hours of run time were by viewing your chart here that is not true . And is the claim “can be seen up to 1 mile away” on high flash only ? , what distance can it be seen on low flash ?
    Like I said I love the light , will keep using it and will recommend it , just wish there was better information in the box and hope that for my next century ride can get 4.5 to 5 + hours on low flash . My rating for the light it self is 5 , but for how its presented to the buyer , well 4

  49. Larry

    Positives … Very bright. Easy to attach. Small and low weight. Negatives… On/off button difficult to find and difficult to press. Attachment straps can fall off too easily when the light is off the bike. Recharge port is tiny, difficult to access and possibly susceptible to breaking – I broke mine without excessive force. I’ll have to replace the light.

    • Serfas

      Oh no! Larry, we’re sorry to hear about the charge port breaking on your Thunderbolt, it could possibly be a defective unit. You might be eligible for replacement under warranty, if you haven’t already done so please feel free to reach out to us at your convenience via our Warranty Department by email at warranty@serfas.com or by phone 623.888.5841 and we’ll see what we can do for you. – The Serfas Team

  50. Robert Asterino

    Visibility is good. Operating duration is fair. Won’t get you through the day or night however and the goddamn on/off button is frustrating in that it is difficult to turn on and off on a couple that I bought but work well on the other 3. I own 5 of these and because of the poor on off buttons and the lack of battery run time I am finding that I’m using a cheap Walmart 2-AAA battery operated flashers. They beat the bricks off your light because they will operate with acceptable brightness for several days and have a dependable push button with fast and slow blinking modes. Not as sleek as yours but one hell of a lot more dependable and less then half the cost.

  51. Vaughn

    I have owned one of these lights since 2013. I am now finally at the point where I have to replace it because the battery no longer holds a long enough charge. Absolutely one of the best tail lights out there. Kinda sad it’s time to let go of the original, but considering it’s lasted me 9 1/2 years, and over 17k miles I think I got my money’s worth.

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