All the head/tail lights on the market appear to have their own separate battery and separate switches, so you basically need to manage the charge state of two batteries, and you better not forget to turn on your tail light because you're not going to see that it's off, including when it runs out of juice. Which made me look for a central battery that can power both head and tail lights, a single switch that turns on/off both and ideally a charge indicator of that battery. To my surprise, not finding anything off the shelf.

I could go with a hub dynamo, but I am not particularly impressed with the lighting options there, I need to go over steep hills where I think the dynamo may not produce enough power for the rear, and I do need a backup as my commute entails traffic lights, unfortunately.

So I am thinking I need to DIY that, basically hacking "normal" lights to not run off their own battery, solder in a waterproof connector and pick a battery/switch/charge indicator that fits the bill. Before I embark on that project - does anybody have a better idea ?

  • ride.lezyne.com/pages/smart-connect might be a solution. Rear light is controlled wirelessly by front.
    – mattnz
    Jul 17 at 0:08
  • 1
    The better idea is to live with managing the charge on two lights. However, if you really need two lights wired into one battery, there's an almost off-the-shelf option already: most e-bike lights are designed to run off the e-bike's main battery. So get an e-bike headlight and taillight, and wire them into a central battery. If you're really looking for a project to keep yourself busy, then go nuts with the DIY.
    – Adam Rice
    Jul 17 at 2:04
  • In the same line as @mattnz proposed, Bontrager has also a range of bluetooth controlled lamps (and you can also purchase buttons to fit on the handlebar).
    – Renaud
    Jul 17 at 5:47
  • 3
    ... I hadn't noticed that it was the rear you were worried about on hills. That's simply not a concern. Even if the standlight capacitor drains on a long hill, the light will flicker (it does so at certain low speeds anyway, when it can't choose whether to drop into standlight mode). The front is more of an issue.
    – Chris H
    Jul 17 at 9:00
  • 1
    My opinion is that if it's not in market, then most possibly it might not be such a good idea after all. Separate batteries mean separate points of failure. I have frequent long consecutive night rides (brevets) and I always carry at least 2-3 rear lights and at least 2-3 front lights, with at least 1+1 on AA batteries that can be replaced commercially anywhere. I personally despise dynamos for their weight.
    – Wtower
    Jul 17 at 10:40

5 Answers 5


Supernova do a set of lights under the name 'Airstream 2' where the rear light is powered by the battery in the front light.

The front lights come in regular and upside-down configurations - the light beam is shaped like a dipped car headlight to avoid blinding oncoming traffic, so they don't work the wrong way up.

The rear lights come in seatpost and rack mount versions.

  • Awesome - precisely what I was looking for. But dang... expensive.
    – Rawbert
    Aug 13 at 22:26

All the head/tail lights on the market appear to have their own separate battery and separate switches

Not so.

There are battery-less lights that you can run from dynamo. Generally the rear light is wired to the front light, and the front light has a single switch that controls the rear light too. Also some front light models have automatic light sensors, that mean you don't need to turn them manually on and off based on light conditions.

Also, typically these days the dynamo-powered lights are also available as e-bike versions. The e-bike supplies a certain voltage, usually lower than the e-bike system voltage, and the lights run from the e-bike battery. Typically the e-bike system voltage is 36V and light voltage is either 6V or 12V, with some systems allowing selecting the light voltage. Out of 6V and 12V, the most recent standard is 12V so most recent e-bike systems give 12V.

Also many quality lights run from practically any voltage. For example, Busch&Mueller Lumotec IQ Cyo Premium (e-bike version) runs from anything between 6V and 42V.

Of course on non-ebikes, you will have to invent some way of mounting a battery. This is in practice so difficult that you may find it more useful to just go with a hub dynamo.

As for dynamo not producing enough power for rear lights, that's completely false. Dynamos produce traditionally 3 watts out of which traditionally 2.4 watts was used for front and 0.6 watts for rear -- at the time the rear was an incandescent bulb. Today the rear is LED, so you may expect it to consume anything between 0.1 and 0.2 watts, i.e. practically nothing.

If riding up a very steep hill, the front light brighness dims but the rear light does not all that much. You will be visible from the rear in practically all conditions. Besides, most quality rear lights have a standlight function which produces light even if stopped at traffic lights. Today all quality front lights also have a standlight thanks to a supercapacitor, but it's mainly useful for you being visible from the front, because the front standlight is not as bright as the beam when riding at very high speeds.

Just make it simple and use a hub dynamo!

Or if you prefer the DIY approach, buy the e-bike specific lights and add a battery of compatible voltage. For example a holder for 6 or 8 Sanyo Eneloop batteries wired in series would give the required voltage (usually 6V min), but I suspect riding at very high speeds might make the Eneloops drop from a battery holder unless you add electrical tape around them, which makes charging them more difficult since you need to remove the electrical tape to put them into a charger and add the tape back again after charging.

  • 2
    To add to this: the modern supercapacitor powered standlight is impressive. Don't underestimate it. I've got B&M front and rear dynamo driven lights. I've done many trips where I've locked my bike up, done whatever I came to do, and found my rear light was still going 20minutes or so later when I came to unlock my bike. The front standlight is underwhelming, but good enough for standing still at traffic lights!
    – MrBunsy
    Jul 17 at 8:56
  • The NiMH AA batteries like the non-self-discharging ones you refered to (there are several brands, no need to advertise one brabd as you always do) were top some 20-15 years ago. I have a lot of them still. But the world is elsewhere now. Li-Ion batteries have much better charge density and can deliver much higher currents. Jul 18 at 17:52
  • They are still top. I tested a 15-year old Eneloop. It had more than 90% of its capacity left. Show me a lithium ion battery that's 15-year old that still holds more than 90% of its initial charge. Well maybe some exceptionally high quality LiFePO4 battery could do that. Well maybe some exceptionally high quality NMC battery could do that also, provided that you never ever charge to 100% but limit your charge to always 80%.
    – juhist
    Jul 19 at 17:58

I have ended up with a solution for this - I converted a front light that no longer held charge to run off a USB battery pack. At the same time I adapted a rear light to also run off 5V, and plug them in together with a splitter cable. The switch is the one on my battery pack. To convert the front light I bought a new circuit board on eBay (in the end I got a single-mode one, i.e. full-power only, but you can get boards with variable brightness and flashing). The back was simply a matter of soldering in a resistor. I will probably convert a better rear light at some point, and when I do I'll combine the cables in a better way, to save bulk and weight, and reduce tangling.

The rear light in that setup isn't much, because I normally run a better one as well. That's easy to keep an eye on as it's on my seatstay, on the side I have to look behind me regularly.

I have also run with extra bar-end plug lights (on drop bars, so rear-facing and red). They're nice because you can turn them on while riding, and see them. But they can't be very bright as they sit in your field of view. They also don't work with a bar-end mirror.


I had something like this on my M5 recumbent. I had a bag behind the seat, where a ryobi 18V drill battery sat. I bought a clip-on USB adapter thinking it would be handy, but in the end never used that.

I did extend the 18V wires out and in to a harness. I obtained some LED arrays which were rated for 12-24 V DC A front and a rear light were good for around 4 hours of riding off a charge, not spectacular but it was a 2 Ah battery.

The wires themselves were another challenge. They never looked right with zip ties, but even when I made proper mounting points they were still "ugly" Internal routing would have been my next step, but that has other side effects.

The choice of light fittings was quite limited - and nothing was "good" at illuminating especially on the black night winter's roads. So I treated this as a backup and used a main light, as well as a separate head-mounted light. On the rear I had four different lights.

Overkill? Potentially, but I had one occasion where most of the batteries went flat on the same ride, so the last ~5 km was done with a single red rear blinky and my pocket torch as a front. Fortunately that was late at night and traffic was minimal. Now I carry 2x 18650 spare batteries, which fit a couple of my lights.


I did that with a maintenance free lead battery like this one. It fits neatly under the bottom bracket where I secured it with an old inner tube which I pulled really tight and double-knotted. You use the same lights you'd use with a dynamo but simply run the positive from both lights to one terminal. The other pole is connected with a short wire to the frame, as with a dynamo. I used the water bottle holder screw for that. I had to insert a switch into the wire, right. I forgot how I attached the wires to the terminals; perhaps with large screw terminals? The setup was simple and robust.

The charging has to be done without detaching the battery, implying that you need to be able to take the bike inside or close enough to the house that you can run a wire to it with two alligator clips.

But I find it much nicer to have today's regular setup: AAs in two detachable lights. My personal advice is to have two independent back lights, a steady one and another one which blinks. This both increases your visibility and the attention you attract, as opposed to a single light, and serves as a backup in case one of the lights fails silently, falls off or runs out of juice. Back lights don't need the batteries changed often. Make it a habit to carry a 4 set of spare AAs.

  • Thanks - yeah, I have a redundant AA powered light on the back of my helmet. Can't say I like throwaway batteries much for the main lights.
    – Rawbert
    Aug 13 at 22:36

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.