I'd like to increase my visibility on my Tern GSD, particularly in the winter months, by adding a second rear light. Rather than using a light that requires charging, I was hoping to connect another integrated light to run off the bike's battery.

This is the light I'm talking about. https://www.ebike24.com/tern-gsd-gen2-rear-light-brake-light

Has anyone had experience with fitting an additional rear light like this, or know anyone that has? Any thoughts on whether this would be possible or if connecting another light to the battery wouldn't power two lights?

Many thanks in advance.

  • Just another train of thought: What about upgrading the original light with something of better quality? In this context, that means higher light strength and additional safety functions such as a braking function. Brands and products like the Supernova TL3 Pro come to mind. Just as an example but look for a high-quality light with good LEDs.
    – DoNuT
    Commented Dec 31, 2023 at 11:36
  • At least I think, fitting another (better) light next to the stock one will just create a lump of light in the same position that's not a lot more visible than the stronger of the two alone. So, you'd have to fit an additional light somewhere else (which probably complicates cable routing), e.g. on the seattube to increase visibility - that would traditionally be an easy-to-mount battery light or helmet with integrated lights, etc. Bikepackers often fit additional lights on "pocket rocket" bags because of the higher/exposed postion. Just "more lights" might not do the trick.
    – DoNuT
    Commented Dec 31, 2023 at 12:25
  • Probably technically doable, but would require to intervene in the drive unit. Tern uses Bosch motors, that usually require to be done by certified mechanics (and a complication can be that the GSD has a brake light). Personally, I'm not 100% convinced that adding lights increases visibility: if the light is just among many red lights, adding another one among the others might not be of great help (although helmet lights are distinctive but can't be powered by the bike). Reflectors on the child seat (saying that because 90% of the GSDs have one or two child seats ;)) can be better.
    – Rеnаud
    Commented Dec 31, 2023 at 12:40
  • I've edited to be about all ebikes with Bosch drive units. What's really in question is what the drive unit will allow and what the consequences may be of the various imaginable DIY ways of wiring in a second light. Those will be the same for all the many bikes with the same drive unit. Commented Jan 1 at 7:58
  • 1
    @juhist Shimano seems to be much more open to allow users to modify their setup than Bosch. On the EP800 (I can't speak for other Shimano units), there are only two terminals (+ and -) on which you can plug the light cables, and adding a rear light wouldn't be an issue at all (mine has 3 lights for instance, a front light, a front day light and a rear one). Bosch has a different approach, with proprietary connectors (and they require different cables for rear and front), and they are generally speaking much more closed. So I wouldn't generalize a Shimano answer to Bosch.
    – Rеnаud
    Commented Jan 2 at 8:17

2 Answers 2


Most likely it will work, but you need to somehow tap into the existing voltage source.

The easiest way would be probably to take the existing light, and look at its markings to see which wire is plus and which is minus. Remove the wires and if they aren't marked mark the plus and minus before removing.

Then put new wiring from the old light to the junction, from the new light to the junction, and connect the old wires to the voltage source to the junction as well, observing the polarity. Solder them and cover with insulating self-amalgamating tape (you stretch the tape and after stretching wrap it, which causes it to fuse to itself).

The reason this works is that the motor unit has light connection which is fed by a switched mode power supply. In some cases, the voltage is fixed, in other cases you can control it by software with bike shop tooling that connects to the motor unit. A constant voltage source allows easily connecting as many loads as you want, the only restriction is that the voltage source has a maximum current and if the maximum current is exceeded, things may not work. But a rear light has always very small current, only front lights may reach the current limitations of the supply.

So you can connect practically as many rear lights as you want, provided that each rear light is compatible with the current light voltage of the system. I recommend though first measuring the voltage of the rear light when lights are on by using a multimeter, and ensuring that this voltage you measured is compatible with the rear light you're planning to buy. Many ebike rear lights have a very wide voltage compatibility, but obviously this can vary from light to light.

Actually, chances are both the front and the rear light are fed with the same voltage source, so you already may have multiple lights on the voltage source. You would just need to increase the light count from 2 to 3.

I don't recommend Criggie's way of a battery light. The tinkering with wiring is a one-time job, and if you insulate it well with self-amalgamating tape, it will be water-proof and if this fails, the only risk of water-proofness is limited to the lights and not to the motor unit. A second additional rear light won't practically reduce range at all since rear lights have small current. Only a very bright front light would reduce range markedly. Besides, the main battery is so big it won't become dead, since there is usually by legislation a two hour reserve capacity: once you use it to the reserve capacity, the pedal assist power cuts and only lights are fed. Also as long as you have enough wiring, placement of the second light is free, no need to restrict it to be near the first rear light.

  • How would you "just need to increase the light count from 2 to 3"? If it requires the proprietary cable to connect the computer to the drive unit and the "professional" software, it may not be something that anyone can do...
    – Rеnаud
    Commented Jan 2 at 9:10
  • @Renaud the tooling is needed only if you change the voltage of the system. If the existing voltage is okay, you can have as many rear lights as you want at that voltage. I don't believe Bosch has any current monitoring that would brick the lights if current exceeds the configured value.
    – juhist
    Commented Jan 3 at 19:22

ANSWER: Rather than mess with the existing wiring, consider fitting a second battery powered light instead. This gets you the requested second light.


  • Doesn't require tinkering with existing wiring
  • No risk to the water-proofness of the existing bike
  • Will not reduce main battery life/range
  • Redundancy, if the main battery is dead you can still have a working rear light while you pedal home unassisted
  • Placement - you can mount the second light higher or lower, or even up on your helmet for increased visibility


  • Second thing to remember to charge, or replace batteries inside
  • Remember to turn it on separately.
  • No brake-light function
  • 1
    Battery lights may have a braking function too, so it's not really a con.
    – DoNuT
    Commented Jan 1 at 0:38
  • 1
    @DoNuT the braking function of battery lights seems to be very random (personal experience, too sensitive). Which may cause other users to ignore this "feature" if it gives too much false positive because of the battery powered ones (in the case of the Tern GSD, the brake light is trigger by the brake levers)
    – Rеnаud
    Commented Jan 2 at 8:20

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