Review: Serfas Thunderbolt (UTL-6) Taillight Rear LED Rechargeable Red Bike Light – Best and Brightest Ever?

Serfas Thunderbolt USB rechargeable super bright tail light blinker review


As someone whose ride time is primarily before work at 6 a.m., I consistently use a rear red blinking taillight on my bikes. I’ve gone through lots of different rear bike blinkers, including models by Planet Bike, Portland Bike Works, and even a different model by Serfas that is a clip on unit that uses AA batteries. The Serfas TL 200 1 watt bike light. I have bought this model at full retail twice, because I forgot to clip the first one on correctly and lost it during a group ride.

Serfas was kind enough to send me the USB rechargeable Thunderbolt tail light for review, and I’ve been using it for about a week.

Unlike the usual style of rear bike blinker that I usually use, which is the style that either clips onto your rear bicycle bag or your back jersey pocket, the Thunderbolt is designed to strap onto your bike. It has these thick rubbery, silicone like straps that will stretch to go around different sized objects, so you can put the light on your seatpost, or somewhere on your rear seat stay or wherever it most makes sense for your situation.

On my mountain bike and my cyclocross bike, both with standard round seatposts, I typically strap it onto the seatpost. But on my road bike, which has an aero seatpost, I mount the Thunderbolt onto this spot on my rear seat stays that you see in the photo. I am able to stretch it around my aero seatpost, but it’s easier to put on the seat stay.

Serfas Thunderbolt USB rechargeable tail light strap detail review

Here is a side view of how the straps on the Thunderbolt work. If you look at the strap that isn’t attached to the light, you can see that one side of the strap has a small hole that is intended to more or less “permanently” attach to one side of the light. And then you have a series of bigger holes on the other side that allows you to strap the light around just about anything.

Thunderbolt USB bicycle rear light charging review

The Thunderbolt has a built in rechargeable battery that recharges with a mini USB cord. They include a small cord with the light. The spot where you plug in the USB cord has a rubber cover that you pull off, so that you can stick in the USB cord. Serfas describes the whole system as “highly water resistant,” but I have not yet ridden it in the rain. The USB plug is hidden so that it will be strapped firmly against the bike, and is covered with a rubber plug. And the rest of the unit looks like a solid, rubberized kind of silicone material. So it definitely appears to be water resistant.

When you are charging the light, you’ll see a blinking orange light on the side of the device. When the light is fully charged, the light stops blinking and goes solid, so you know that you can unplug it. It takes anywhere from 3 to 6 hours to give you a full charge, depending on how depleted the battery is, and how much current it is pulling from your computer.

I charge my Thunderbolt with a Garmin charger that plugs into the wall, because Garmin uses mini USB to charge many of their products like the Garmin 800 bike computer that I use and love. I use that same Garmin charger to recharge my Serfas headlight that I also use daily. I’ll typically charge the headlight first as soon as I get back from a ride and leave it charging. If I notice it is finished before I go to work, I’ll then charge the taillight. Otherwise, I’ll swap them when I get home from work and let it charge until bedtime, leaving them both fully charged and ready to go the next morning.

With my Serfas headlight, I have to charge it daily, because most of the year I ride at least the first hour of my ride in the dark, with the headlight on one of the brightest settings. But with the taillight, I can go several days between charges. On the lowest blinking setting, you can run it for about 9 and a half hours. And let me tell you that the lowest blinking setting is still very, very bright.

Let’s talk about how bright this light really is.

In terms of lumens, you’ll get 35 lumens of red light when you have the light set on high. That might not mean anything to do, but I can tell you that it is substantially brighter than my Portland Bike Works rear light, or my Serfas 1 watt rear blinker light. On the package, they say that you can see it at least 1,000 yards away, and up to 1 mile away.

The extreme brightness of the light might be the only actual “downside” of this rear blinker. I typically ride with a small group in the mornings. I don’t know if you’ve ever ridden in the dark in a paceline behind someone with a really bright blinker pointed at your face, but it can be blinding and unpleasant. I definitely set this light to the low blinker setting when it is dark outside. And I have to make sure that I have it positioned where it doesn’t point into other riders’ faces.

If you use a tail light during the day to make yourself more visibile to cars, then the Thunderbolt might be the very best rear light that you could ever buy.

Since I ride on the city streets of Dallas, which isn’t the most cyclist friendly city, I use my rear tail light blinker even during the day. One of the problem with a standard rear blinker is that although it might be really, really bright at dusk and in the night, it will still be difficult to see during the day.

Up until now, my favorite option was the Serfas TL 200 rear bike light, a powerful 1 watt LED that you can see from a really long way away in the dark, and that does fairly well during the day.

But the Thunderbolt is significantly brighter than that model, and it blows away anything else I have ever used. For daytime riding, I have found nothing that compares to it.

The Thunderbolt has four different modes, and you cycle through them by clicking the single button on the side of the unit. The first mode is a super bright solid light, which will run approximately 1.75 hours until the battery runs down. Next is the low beam solid red, which is not as bright, but lasts up to 7 hours. And then there is the high beam blinking mode, which I use during the day to alert cars. It will last up to 3 hours. Finally, there is the low beam blinking mode, which isn’t as bright, but lasts up to 9 and a half hours. If you are riding in a group, you’ll probably want to use one of the low beam settings.

If you want to turn off the blinker, you don’t have to cycle through every mode. You can click the button and hold it down, and it will shut off.

The Serfas web site say that the Thunderbolt comes in several different color options, but I have only seen red in the bike shops where I have seen it stocked, and the unit they sent me for review was also red.

Serfas Thunderbolt (UTL-6) Taillight Features:

  • 35 lumens
  • USB rechargeable
  • Features a light silicone body
  • Features 30 micro-LED strip
  • Modes: 1.75 hours (high beam); 7 hours (low beam); 3 hours (high blink); 9.5 hours (low blink)
  • Unique mounting system that allows for endless mounting possibilities
  • Highly Water resistant
  • Weight: 50 grams

Did I leave anything out? Do you have questions about this blinker, or want to add something from your own experience using it? Leave a comment!


One thought on “Review: Serfas Thunderbolt (UTL-6) Taillight Rear LED Rechargeable Red Bike Light – Best and Brightest Ever?”

  1. Thanks for the review. The first one (orange) I bought wouldn’t turn on and I returned it to my local bike shop. The next one I bought from Amazon and they had a choice of colors; I chose black to match my bike. Charging as I type. 🙂

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