How to replace the battery in Serfas Thunderbolt bike light


Jul 16, 2017


The Serfas Thunderbolt tail light is a great light because it is one of brightest on the market, has just enough brightness settings and a blink setting that isn't annoying, and it even has a well designed mounting system.  You can place the light just about anywhere on the bicycle, even on a rear rack or the rear triangle of the frame and it will be visible. But my one gripe with the light is the battery life is not good enough! The light only lasts about one day of commuting to work for me before I have to recharge it, and sometimes it dies in the middle of my ride leaving me with no rear light at all. My front light, on the other hand, lasts about 2-3 days of commuting before it needs to be recharged which is pretty convenient.

I wanted to keep this Thunderbolt light but make it last just as long as my front light so that I wouldn't have to recharge it everyday. With this battery hack, I have extended the life way beyond that. I now only have to charge the Thunderbolt every other week!

Here is what you will need

  • One new 18650 lithium ion battery cell
  • Thick multi-stranded copper wire, anything between 10-18 gauge should work
  • Lots of electrical tape and shoe goo or other silicon stuff for sealing the light back up 
  • Flat head screw driver and hammer to disassemble the light
  • Soldering iron, desolder wick, and solder
  • Drill

Dislcaimer: The light will no longer be as light weight or easy to take on and off the bike with a larger battery. Also, I am not responsible for anything that happens to you or your light while performing this hack. You do so at your own risk. If that is ok, then read on!


Upgrading the lithium ion battery

The battery that comes with the light is a lithium polymer cell but it only has 500mah. We are going to upgrade this to an 18650 cell that has a rating of 3200mah. That is more than 6 times as much capacity! I went with a well known Panasonic 18650 cell because it has the high capacity and I know it will last. If you don't want to read the explanation behind this, then skip to the directions.


Thunderbolt original battery
Thunderbolt original battery 500mah
New 18650 battery 3200mah
18650 battery 3200mah


I wasn't sure at first if it would be possible to perform this upgrade for a number of reasons. First, the original battery is lithium polymer, and this battery is lithium ion so would the charge circuit in the light still work? Second, would the charge circuit in the light work with a significantly larger battery? And third, the original battery has a protection circuit on it to protect against over-charging, over-discharging, and short circuits, but does the new battery have this protection circuit? Luckily, the answer to all these questions is yes. It turns out that the charging characteristics of li-poly and li-ion are very similar so the charge circuit works for both. The charging in this light is actually handled by a integrated circuit labeled 2KAX also known as BL4054 and you can check out the datasheet for it here: Standalone Linear Li-Ion Battery Charger.

As for protection, the Panasonic NCR18650b cell already has protection built-in. If you study the green shrink wrap around the battery, you will notice a wire running underneath, this is the indicator that there is a protection circuit built into the cell. This 18650 is perfectly suited to work with the Serfas Thunderbolt. Being that it has over 6x the capacity of the original cell, you will not need to charge it very often at all. Because of this, you will put the battery through a low amount of charge/discharge cycles during the life of the bike light. Also, you will notice that it takes a very long time to fully recharge the new, larger battery. If you take a look at the datasheet for the Panasonic NCR18650b, you will notice a graph of cycle life characteristics. What this is showing is that when you charge the cell to as high as you can get it(leaving it on the charger all day) then you get fewer cycles before the battery stops accepting a charge anymore. This is because charging li-ion batteries to max capacity is stressful and dimishes the life of the batteries. In our hack, though, we don't have to worry about this because we won't be putting the cell through many cycles at all, and oftentimes we won't even leave it charging long enough to stress the battery. In my case, I am only charging it every 2 weeks, and I don't even have to charge it to max. 

Characteristics of lithium ion charge cycles

Cycle life characteristics of 18650 battery. Source: data sheet Panasonic NCR18650b



In terms of battery lifespan, the new battery also benefits from the slow charging rate of this light. When you were reading the datasheet for the charging chip, you may have noticed that the chip checks how much voltage the li-ion cell reads before beginning to charge it, and the charger chip is programmed to slow the charging rate when the battery reads below a certain voltage. It does this to reduce stress on the li-ion battery. In our application, we are charging the battery at a relatively low charging rate for a battery of this capacity. This reduces the stress on the battery as it is charging and is great because it extends the cycle count of the battery. Just make sure you remember to plug it in to the charger when it needs it because it takes a while to charge.


  1. Remove the rubber cover around the light
  2. Use your flat head screwdriver and a hammer to break the bond between the front and rear halves of the light. You are going to need to "get in there"

    Thunderbolt disassembled

  3. Desolder the wires going to the original battery
  4. Using a drill or dremel tool, drill a hole in each side of the front section of the light for your wires to go through
  5. Cut some lengths of your wire. I made mine about 5 inches long
  6. Cut two small holes in the rubber cover around the light for your wires to go through
  7. Solder your wire to the light circuit board and then put the rubber cover back on the light with your wires sticking through the holes you just made
  8. Solder the wires to the new 18650 battery making sure you don't reverse the polarity
  9. Test it. If it still works proceed
  10. Put the light back together using silicon or shoe goo to glue the halves together and keep water out
  11. Wrap the 18650 battery in electrical tape to keep water out
  12. Go ride your bike!

Thunderbolt disassembled