A HID Xenon light on my bike

On this page I explain how I made a bicycle headlight on my road-race bike and how I made a charger for the battery pack of this light. I am in the process of translating this page into English.

Why a Xenon light?

I commute a few times a week and I need, especially in winter, about 2.5 hours of light then (the distance I commute is about 32 km one way). Since I had a accident when I drove with 30 km/h into a concrete bollard because I was blinded by a shovel with an array of lights, I decided to join the competition on "who has the most light".

At the moment of my crash I had a 2.5W halogen headlight. When I was blinded, I was not able to see the light spot of my light on the road. Sometimes I am forced to drive on the left side of road, then I was also blinded by oncoming car traffic. So it was time for a big leap on bike lighting (at least for my bike it was).

Xenon car headlights

At the end I bought a HID Xenon light made for cars. This is a gas discharge lamp mounted nowadays on some (expensive) cars. Hella has a fog light, Xenon DE. It consists of a Xenon bulb, a housing, a ballast and some small components: wires, a relay and a fuse. I paid 235,= for this a my local car supply store. The Hella light is 35W, which is equivalent to about 100 W halogen. The Hella lamp produces about 3200 Lumen (so 91 Lumen/watt).

For the batteries I paid 85,= (below is more info on the batteries), so 320,= altogether. This is about the same amount of money as a ready made HID bicycle headlight, but the Hella light gives 35 W instead of the 12 W of a HID bicycle light.

HID lights a about 3 times a efficient as halogen lights, see table below which is valid for big lights in buildings. Lights for cars and bikes have a shorter lifetime due to vibrations.

Lamp type



Incandescent with reflector



Standard incandescent






White LEDS



Mercury vapour



Tubular fluorescent



Compact fluorescent



Metal Halide



High pressure sodium



Low pressure sodium



Current consumption of the Hella Xenon DE

In order to investigate what capacity of batteries I needed for 2.5 hours of light, I measured the current when the light was connected to a car battery. The battery had a voltage of 11.75 V. The tested circuit is shown in figure 1. The fuse of 16 Amps, the relay and the ballast came with the Hella light. The ballast is needed to ignite the lamp with high voltage (about 25 kV) and to transform the 12 V to a higher voltage during steady state burning (about 80 V)

Figure 1. The tested circuit

The measured current v.s. time is shown in figure 2. At t=0 the switch is turned on. Initially the lamp produces a bluish light and the ballast makes a humming/peeping noise. This noise gradually decreases and the light turns in to white and the intensity increases (see animation).

Listen to the noise of the ballast: lamp.mp3

As can be seen in figure 2, in the first 4 seconds the current is about 16 Amperes. At about 30 seconds the current becomes steady-state. The steady-state current is about 3.75 Amps. This is all measured at 12 Volts.

Figure 2. Measured current vs. time through lamp at 12 V.

On the internet I read that igniting the lamps would consume a lot of energy. Therefore some people did not switch the lamp of when they paused bicycling. I wanted to know how long I had to switch the lamp of in order to compensate for the higher power consumption at ignition. The amount of charge consumed from the batteries is a good measure for this. The charge consumption is the area beneath the curve of figure 2, so the integral of the curve. The integrated curve is shown in figure 3 (black line). The red line in figure 3 is the charge consumption of the lamp is the lamp is not switched off. I shifted the black line until the red and black line coincided at the steady state condition. As can be seen this is the case after about 30 seconds. So you have to leave the lamp switched off for at least 30 seconds in order to have equal or less charge consumption than a continuous burning lamp. So it is better to turn the lamp of when a traffic light turns into red just when you arrive at the traffic light.

Figure 3. Comparison of charge consumption of a continuous burning lamp and a lamp which is turned off and on again after 30 sec.

I considered buying NiMh batteries of size D with a capacity of 9Ah. I would need then 10 cells of 1.2V to obtain 12 V. The total burning time of the light would be then about 2.5 hours (9 Ah / 3.65 A). This would be just enough to make my trip from my home to my work and back. But if in future the batteries would degrade, it wouldn't be enough anymore. I hoped the ballast (which is in fact a switching power supply) would be designed smart enough to work with a constant power consumption. So if total battery pack voltage would be higher, the current consumption would be lower. Then I could obtain a longer burning time of my lamp just by adding a few extra cells in series to the battery pack: so 12 cells instead of 10 cells in series. It is cheaper to add a few cells to obtain a higher voltage then to obtain a higher current with the same batteries. For a higher current one would need 10 extra cells in parallel. I measured the current consumption of the lamp at various voltages, all in steady state condition. This is shown in figure 4. At 6.5 volt the lamp switched of, probably due to the fact that the relay didn't hold and not because the ballast couldn't handle it. At lower voltages the ballast made more noise.

Figure 4. Steady state current consumption vs. voltage

As one can see from figure 4, indeed the current consumption decreases with increasing voltage. But is the power consumption constant? In figure 5 the consumed power is calculated from figure 4. As can be seen in figure 5, the consumed power does not increase above 12V, and the total power consumption of lamp, ballast and relay is about 43 Watt. If the light itself is indeed 35 W, then the efficiency of the ballast is about 81 %.

Figure 5. Consumed power vs. applied voltage on lamp.

With 12 cells of 1.2 V (total 14.4 V), this lamp would consume about 3 Amperes and the burning time would be 3 hours. This is more then enough for the goal of 2.5 h.

The batteries for the light

I considered four types of batteries:

Lead acid are the cheapest and heaviest type of batteries. The number of charge/dicharge cycles is limited when fully discharged (about 400 times). A 12V 10Ah lead acid batterie costs about 50,=. 6V, 12V and 24V are the most common voltages.

NiMH are slightly more expensive, but they last longer: about 1000 charge/dicharge cycles. Furthermore a cell is only 1.2 V so you can "fine tune" the voltage of your batterie pack. Finally I decided to buy 12 cells (see my motivation above) in order to have a lower current consumption. After a long search on the internet I found D size cells with a capacity of 9Ah. One of he cheapest cells I found at top-accu: 6.90 per cell, so in total 82.80 with additional shipping costs of 3.90.

NiCd have a lower capacity per unit of volume (a lower energy density) compared to NiMh. The lifetime (number of charge/discharge cycles) is the same, however, NiCd suffers from memory effect. They have to be regulary discharged completely. Also NiCd is bad for the envinoment (but if you travel by bike instead by car you partly compensate for this). NiCd and NiMH bout suffer from selfdischarge: the batteries are empty after some time (weeks or months).

Lithium Ion batteries are often used in laptops. In order to have suffient capacity I needed 2 batterypacks to obtain 9Ah at 14.4V, and this quite expensive: 90,= per pack. Furthermore I was not sure if I could make out the pin-out of these packs. The number of charge/discharge cycles is very high of these packs. Also the self-discharge is very low. In general, Lithium Ion batteries cannot deliver high (peak)currents.

Finally I choose NiMH D type cells of Camelion, capacity 9 Ah. According to the specification the cell voltage is nominal 1.2 V, fully charged 1.4 V and empty 1.0 V.

I it better not to discharge the batteries below 0.9V or 1.0 V (depending on discharge current). Therefore I made small Printed Circuit Board (PCB) with a deep discharge protection in my battery pack. The protection shuts the lamp off when cell voltage is below 0.95 V. Here is the schematic:

Combined deep discharge protection with on/off switch for the light

The relay comes with the Hella lamp.

The battery pack casing is made of 2 mm aluminium sheet which is bend and welded together with a TIG welder. Also the bottom plate in the pack is welded in with TIG. At the top I welded in a flange on which I bolted a stainless steel plate. At the stainless steel plate I mounted a connector to have a connection to the lamp or to the charger. I also made the on/off switch on the stainless steel plate. Beneath the stainless steel plate (inside the pack) I mounted a PVC tube with the fuse, the relay and the PCB with the deep discharge protection. Here you see a photograph of this:

Another posibility would be to make a pack from PVC draining pipes I guess, since rebuilding the battery pack I made is not that staightforward because a lathe and a TIG welder are not standard equipment.

On the connector I made the following connections:

The charger for the battery pack

I made a charger because I couldn't find a ready made one which could charge all 12 cells in series at once. I used a Temic IC, type U2402B. Here you find the datasheet of the Temic IC: U2402b (344kB)

I bought the IC at Conrad, price about 5.37. The rest of the components can be found on the schematic of the circuit:

Click with your right mouse button to save the schematic to the original size

I adapted the circuit slightly because the circuit from the Temic datasheet didn't initially work. Because I had to change some components, some of these new components didn't fit on my PCB:

Real dimensions 3.4 x 2.4 inch

Top view of PCB

I didn't like to redesign my PCB so some components have to soldered on an inventive way. Also some components are not placed on the PCB: the transformer, the recifier cell, the thyristors, the shunt resistor (Rsh) and the batteries of course. The shunt resistor of 50 milliOhm I made of a piece of stainless steel wire. On this page you find some usefull information on the heating of the shunt resistor.
Resistor R12 (6k8) is, as said earlier, mounted on the bottom side of the PCB since it was not forseen. Diode D9 also was not forseen so it is combined with resistor RB1 (1k) and placed as one component on the PCB on the location of RB1. Capacitors CR en C4 (10uF) are actually too big for the PCB (I changed them too).

The connections to the PCB are:

The charger works fine now. However sometimes it signals that the pack is fully charged (after a few minutes) but I know for sure the pack isn't fully charged yet. If I switch the charger off and on again the problem is solved. When the batteries were new, this problems occured more often. The first time ever I charged the pack, I charged with 0.1C (this is the charging current expressed in capacity of the battery, so for 9Ah this is 900 mA). So the first time I increased Rsh to 150 milliOhm.

The actual value of the shunt resistor is 50 milliOhm, so the ccharge current is 3.2 A (=3.2A / 9Ah = 0.35C). The charging time is 4 to 5 hours.

The IC charges with a constant current. At the point of inflection of the voltage-time curve (where the second derivate of voltage to time is zero, thats close to the top), the charges charges with a lower current (37.5% of the initial current).

At the top the charger switches to trickle charge (6% of initial charging current).

The mounting of the light on my bike

Here I want to place more text, but here are already some pictures. In the black box on top, the ballast is placed.

Some pictures of the light beam

Here some pictures of my lamp and the light of my car (normal halogen). All pictures are taken with equal aperture (F2.8), shutterspeed (1/3 sec) and ISO sensitivity (ISO 1600).

My old 2.5 W halogen light and the Hella Xenon pointing to the ground

Hella Xenon pointing nearby and far away

Normal and fog lights of my car

I normally have the light adjusted like the picture where the lamp is pointing to the ground since I don't want to blind the oncomming traffic

Run time

After the first charge the lamp burned for 2 hours. After about 5 discharging and charging cycles the lamp burned about 2.5 hours. Now (after a few dozen (dis)charging cycles) it burns about 3 hours. From the spec. of the batteries you can see the capacity of the batteries increased after a few times charging and discharging cycles:

Suggestions, questions or remarks? Mail to me: