head and tail light connected to a single battery?

Question

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 ?

Answer 1

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.

Answer 2

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.

Answer 3

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 initially just a matter of soldering a resistor into a cheap light. When I reworked a better light I combined the cables in a better way (soldering instead of a splitter cable), to save bulk and weight, and reduce tangling.

The rear light in that setup is now also rebuilt from a rechargeable one, but I normally run another one as well to cover all angles even with luggage. 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.

Answer 4

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: Rechargeable 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.

Answer 5

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.