Halogen-Bulb High-Powered Bike Lights

This somewhat technical article discusses the common higher powered lighting systems. These consist of a lighting head and separate battery. It's not about specific systems, although it does use specific lights as examples. It's intended as a discussion of the ingredients in a typical lighting system, and it should help you make up your mind about what you might like to look for in a system.

Table of Contents

How Bright are Your Lights?

Most of the better bike headlight systems nowadays are based on halogen bulbs. The only ones that aren't are systems like the Cateye Stadium Light which are based on Metal Halide Arc bulbs. These very exotic and expensive systems won't be covered here. For information on this technology, see Marty Goodman's article.

So assuming we're sticking to halogen bulbs, what determines how bright your light is? Or, perhaps a better question is: what determines how useful your lights are?


The most obvious way to quantify how much useful light you get from a light is the amount of power consumed by the bulb, which is measured in watts (abbreviated W). By and large, the more power consumed, the brighter the light. Very generally speaking, 2W to 3W is good for commuting in lit areas, maybe 5W to 6W is needed for riding on the road in unlit areas (although people typically use anywhere between 2W and 15W depending on their needs), and 10W or more is needed for off-road riding at night (although perhaps you can get by with as little as 6W). However, there are many variables that need to be taken into consideration.

Beam shape/pattern

If the light is put in the wrong place (has the wrong shape), it's not any good. For example, the old Specialized Preview Plus (5W) lights put out a very wide light that had very little depth to it. For road use, it allowed a good view of the edge of the road, but wasn't very good at showing you what was on the road itself. It was useless for riding off-road, since it illuminated very little of the trail. I find that I prefer a round patch of light, such as is provided by MR-11 bulbs.

Does the beam have a good pattern? Is the light distributed smoothly, or are there bars or bands of lighter/darker patches in the beam? For example, the old VistaLite 400 lighting head has dark rings in its beam when used with normal (clear) bulbs. (To compensate for this, VistaLite frosted the 10W bulbs provided with its 420 and 430 lighting systems, but this tended to scatter the light in all directions, putting less light where you want it. Backyard side-by-side tests with a 6W clear bulb and a 10W frosted demonstrate that the 6W bulb provided about the same amount of usable light as the 10W bulb.) Some people find that these dark/light bands hide irregularities in the ground and don't like them. I don't mind them, but this is a matter of personal preference.

Many bike lighting systems (for example, VistaLite Nightstick series, all NiteRider bike lighting systems, and Cateye Daylite II) are constructed out of MR-11 bulbs. An MR-11 "bulb" consists of the actual bulb (light-generating part) and a very good reflector, put together into a very small (about the same diameter as a 35mm film can, but shorter) and neat package. Since the bulbs are self-contained units, the lighting heads that house them only need to provide some protection and a way of getting power to the bulb; thus they tend to be small and light. The bulbs provide a beautifully smooth, round beam. Typically, there is a brighter patch in the interior (the angle of this bright patch, be it 10 or 15 degrees or whatever, is what is meant by the the angle of the bulb, if it is specified) and a much wider (about 120 degrees) round patch of dimmer light surrounding the bright patch.

Over or under volting

This one is more subtle. This has to do with how much voltage is applied to the bulb, vs how much voltage the manufacturer expected to be applied to the bulb. To sum up this angle briefly, if you apply more voltage than the manufacturer specified (overvolting), then you consume more power and the bulb burns more brightly, but you get more extra light than extra power consumption. This is a win, but the downside is that the bulb burns out more quickly. Conversely, if you apply less voltage than the manufacturer specified (undervolting), then you consume less power and the bulb burns dimmer, but you get more of a reduction in light than reduction in power consumption. The upside is that the bulb life is lengthened.

For the technically inclined, here is a graph (generously provided by William Burrow) that shows how the relationship between these quantities. (For the non-technically inclined, skip to ***.) It shows how bulb life (Life), the amount of light put out by the bulb (Luminous flux), and power consumed by the bulb (Power) vary when different voltages are applied to the bulb.

The horizontal axis is the voltage applied to the bulb, where the middle of the graph is the voltage recommended by the manufacturer, the "nominal voltage" (shown as 100%). The vertical axis represents the variation in the plotted quantities when different voltages are applied. All the curves cross at the center of the graph (at the 100% mark on both scales) since the plotted quantities all represent variations from the normal operating mode (when you apply the nominal voltage), not actual numbers. So, for example, when you apply 110% of the nominal voltage, the light output goes up by nearly 40%, but the power consumption goes up by only about 15%. Thus you're getting more light for your power. However, the bulb life is down to about 33% (1/3) of what it would be if run at the nominal voltage. Since for some bulbs (notably MR-11s) the bulb life is on the order of thousands of hours (around 2000) when operated at nominal voltage, this may very well be a reasonable compromise. At the far right of the scale, at 120% of nominal voltage, we get an even better light/power ratio, but the bulb life is now 1/10 of what it would be at nominal voltage. This seems like a good upper limit to the amount you should overvolt a long-lived bulb.

To see the effect of undervolting, look to the left of 100%. We see that Luminous flux decreases faster than Power, so you get less efficient light, but much longer bulb life.

*** Both overvolting and undervolting are used by today's systems. NiteRider usually overvolts the bulbs in its current 6V systems; however older NiteRider bulbs weren't overvolted and are therefore dimmer than current ones. VistaLite offers overvolted bulbs for its Nightstick series lighting systems: their H.O.T. (Halogen Overdrive Technology) is simply overvolting.

Since you get less light for the power consumed, undervolting seems like a bad idea. That is, unless you want to get several lighting levels out of one bulb. NiteRider does this with its Digital systems. For example, the Digital Pro-6 has three power settings with one bulb. The 15W setting is done by slightly overvolting the bulb. The 10W setting is done by providing less voltage, probably slightly undervolting the bulb. The 6W setting results from undervolting the bulb even more. It's terribly inefficient, but if you need that low setting to get you home after a long ride, it can be a good thing.

Ability to aim lights independently

If you have two bulbs of differing power (a high and low beam), it's useful to be able to point them in different places. Since you'll use the low beam when you're going slower, it's best to have it pointed somewhat close to the bike to illuminate the details near you. You'll use the high beam (or both beams) when you're going faster, so you'll want to point the high beam further from the bike, to illuminate the road/trail further ahead. If you have a dual-beam system where the lights are stuck together with no independent movement (as in NiteRider dual-beam systems), you don't have this option. Having completely separate lighting heads (as in the VistaLite Nightstick systems) or separately aimable bulbs in one mount (as in the Specialized Fireball systems) does give you this option.


Next we consider the source of power: the battery. Most batteries used for bike lighting systems are sealed lead-acid or nickel cadmium (NiCad) batteries. Some newer systems use Nickel Metal Hydride (NiMH) batteries, which offer increased energy density and lack cadmium, a poisonous heavy metal.

Why use rechargables at all?

Most cheap, low-powered lights are designed to be operated with disposable (e.g. alkaline) batteries, while the better, higher-powered systems use rechargable batteries. One reason for this is obvious: since the lights consume alot of power, you'd be spending alot of money replacing disposable batteries if you used them. There's another aspect too, which is just as important: the most common kinds of disposable batteries (alkalines and zinc-carbon batteries) have a high internal resistance, which means basically that they have difficulty supplying the high currents that bright lights need. Alkalines can be used in higher-powered systems, but are best combined with a voltage regulator (such as the LVR2) and wired up in such a way that no one cell provides too much current. Zinc-carbon batteries have even more internal resistance than alkalines, and thus are nearly useless for lighting applications.

Lithium cells, however, have a very low internal resistance, but they are very expensive. Randonneurs (cyclists who ride very long distances at a stretch, going on well into the night and sometimes overnight) can find disposable battery packs made of these cells useful.

6 volt vs. 12 volt

Most systems either use 6V or 12V (or 13.2V) batteries. I usually refer to batteries with 13.2V as "12V systems", since there isn't that much difference (the extra cell in the 13.2V system is put there to overvolt a nominally 12V bulb).

Usually, 12V batteries have higher capacity than 6V batteries. A 6V NiCad battery will probably be 4.5 to 5 amp-hours, giving you 27 to 30 watt-hours of power. (The capacity in watt-hours allows you to estimate how long your light will run. Take the wattage of your bulb, divide the capacity in watt-hours by the wattage, and you have a theoretical runtime. To make it more accurate multiply by a 0.9 fudge factor.) A 12V NiCad battery will typically be 2.5 to 3 amp-hours, giving you 30 to 36 watt-hours of capacity. Commercial NiMH systems seem to all use 12V batteries, and these have even higher capacities, going from 44 to 52 watt-hours.

Bulbs for 6V systems are usually available in lower powers (5W to 15W for most systems) than bulbs for 12V systems (8W to 35W, the most common being 12W and 20W). However, one company, Lumicycle, offers 5W bulbs for its 12V system.

Lead-acid vs NiCad batteries vs NiMH

Lead acid batteries come with the cheapest systems, NiCad batteries come with more expensive systems, and NiMH batteries come with the most expensive systems. There is a simple reason for this: good quality NiCad cells are cost a fair bit of money, and NiMH cells are even more expensive. The cost of a NiCad or NiMH powered lighting system is usually dominated by the cost of the battery!

NiCad and NiMH batteries have many desirable characteristics. If they are treated well, both will outlast lead-acid batteries, providing more charge/discharge cycles, with NiCad being even better than NiMH. Lead acid batteries typically last for 200-400 cycles, NiMH for 300-1000 cycles, and NiCad 300-2000 cycles. For a given capacity, a NiCad battery will weigh less than a lead-acid battery, and NiMH will weigh less yet. A 12V 4Ah lead-acid battery weighs just under 2 kilograms, while a 12V 4Ah NiCad battery would weigh about 1200 grams, while a 12V 4Ah NiMH battery weighs about 720 grams. The main disadvantages of NiCad and NiMH batteries are their high price and the fact that they can slowly self-discharge, losing about 1-3% of their capacity per day. Lead-acid batteries retain very nearly their full charge over a long period of time (months).

As you are powering your light, the batteries lead-acid batteries discharge in slightly difference ways. The voltage of a lead-acid battery drops off with time, gradually dimming your lights. However, a NiCad or NiMH battery keeps almost the same voltage until it's nearly exhausted. This aspect of NiCads is both a benefit and a drawback. It can help you to get more useful energy out of your battery, as your lights will burn at full power until the battery is close to empty. However, once your lights start to dim or go yellow, you must turn them off immediately to avoid damaging the battery. Thus you get little to no warning when your battery is about to run out of power.

A lead-acid battery has somewhat higher internal resistance than a NiCad or NiMH (although much less than an alkaline), which means that it isn't as good as a NiCad or NiMH at supplying large amounts of current. However, there is some variation within the brands of batteries. The Hawker Energy Cyclon batteries supplied with VistaLite 420 and 430 systems have less internal resistance than other types of lead-acid batteries (such as those provided with the NiteRider Trail Rat). The 5 amp-hour Cyclon battery that came with my VistaLite 430 does about the same in powering a 10W bulb as the Trail Rat battery: they will both allow a 2.5 hour burn time. However, the Cyclon does much better at supplying higher currents. According to a chart given by Hawker, the battery will supply 15W of power for about 1.5 hours, while NiteRider clearly states (in their Charging Information page) that if you put a 15W bulb in the Trail Rat, you will get only about 20 to 30 minutes of run time.

The temperature at which a battery is used affects the available capacity. If it's cold, the internal resistance of the battery increases and the chemical reaction slows down, so you get less usable capacity. This effect is more pronounced with high currents. This effect is worst with lead-acid batteries, although NiCad and NiMh batteries suffer from it a bit too. However, if you are using your batteries in Artic conditions (-20C or below) you'll need to take try to protect even NiCad or NiMH batteries from the cold.


The best way to charge a lead-acid battery is with a constant voltage, while NiCads and NiMH prefer a constant current. Sadly, very few systems come with good chargers. They come with simple AC/DC converters that will damage your battery if you leave them plugged in longer than the instructions say to. The faster the battery is being charged, the more important it is to unplug it when the charge is finished.

The best solution is to have a specialty charger for the type of battery you have. One option is a smart charger. A smart charger charges at a high rate (via a high constant voltage for lead-acid batteries, and a high constant current for NiCad and NiMH batteries), and then when it detects that your battery is full, it switches to trickle mode, a much lower charge rate that maintains a full battery (via a lower constant voltage for lead-acid batteries, and a lower constant current for NiCads). Power Sonic makes a variety of smart chargers for lead acid batteries, and they are highly recommended. For example, PSC-12500-A is a charger for 12V 2-10 amp hour lead acid batteries. Mouser Electronics in the US sells them, search for "Power Sonic charger". In the UK, you can get good smart lead-acid chargers from Maplin.

For lead-acid batteries you can either buy a smart charger that will charge your battery quickly, or you can make (pretty cheaply) an overnight charger. The latter sort of charger basically charges your battery at the trickle rate at all times. It will take maybe 8 to 10 hours to charge your battery, but it is absolutely safe for the battery, and much easier to build if you want to go the D-I-Y route. See here for details on how to make your own.

For NiCads and NiMH, the situation is more complex, as a true trickle charger will take far too long to charge the battery for it to be practical. A smart charger is the best and safest. However, they are expensive, so manufacturers almost never include them with their lighting systems. Instead, they include a simple AC/DC converter that charges at a modest rate, typically a rate that will charge the battery in about 14 hours. The problem here is that they don't shut off when the battery is full, and if you leave the battery plugged in, you will overcharge it. In NiCad batteries this results in voltage depression (see the section on Charge memory and discharging batteries below). In NiMH batteries, this damages the battery, leading to fewer charge/discharge cycles. You can make a charger that will charge for a certain period of time and then switch off. This is somewhat complicated, and if you have only partly discharged your battery, this can still result in overcharging. The best way to care for your batteries is to get a smart charger to use with them. Unfortunately, there are no off-the-shelf chargers immediately usable for bike lighting batteries, and so you must either make one from scratch yourself or modify a charger meant for something else. See here to learn how to construct a NiCad bike battery charger from a power tool charger. See here for info on NiMH smart chargers.

A compromise between stupid AC/DC converter and specialty chargers are provided by some of the NiteRider systems. The best of these (for example on the Digital Pro-12) have circuitry that monitors how much of the battery has been drained while the lights are on, and then when charging, switches over to trickle mode when just that capacity has been replaced. Some of the NiteRider systems (such as the Digital Premium Systems) just charge for a set period of time, then switch to trickle mode. Both of these chargers are better than plain AC/DC converters and are worth having, but not all NiteRider systems come with them. Read the charger information carefully to find out what kind of charger it comes with before buying a NiteRider system.

Charge memory and discharging batteries

All rechargable batteries are composed of several cells wired in series. Lead-acid cells are 2V each, so a 6V battery has 3 cells. NiCad and NiMH cells are 1.2V each, so a 6V battery has 5 cells. You should never completely discharge a rechargable battery of any kind. This can lead to cell reversal. This happens when one cell has slightly less capacity than the others (a common occurrence due to small manufacturing variations). When the capacity of the weaker cell is exhausted, the other cells continue to happily pump current into it in the reverse direction, thus destroying the cell. This in turn destroys your battery, unless you can take it apart to replace the offending cell.

This is worth repeating: Do not ever completely discharge your NiCad, NiMH, or lead-acid battery. Once the lights go yellow or dim, turn them off.

Some people think that NiCad batteries should be completely discharged now and then to prevent loss of capacity via the "memory" effect. This is not the case. To explain why, I'll discuss "memory", and what you can do to avoid it.

Most people think NiCad "memory" comes from recharging your batteries after only partial discharge. The folk lore is that the battery somehow "remembers" the smaller capacity that was used, and only allows you to use this capacity in the future. This is rubbish. This effect has never been documented in any consumer system, in certainly not in bike lights.

However people in the battery industry use the term "memory" to apply to another phenomena that NiCad batteries do exhibit, which is more properly called voltage depression. Voltage depression is caused by overcharging NiCad cells at a slow rate (typically, the rate given by cheap AC/DC converters provided as chargers by light manufacturers). Once the battery is charged, the additional energy being added to the battery is converted into heat, and the heat changes the crystal structure of nickle and cadmium alloys, producing a different kind of crystal that produces less voltage than the desired crystal structure. When the battery is then discharged, the presence of the bad crystals means that they voltage of the battery is lower than it ought to be. The result is a somewhat dimmer, more yellow light, where before they were whiter and brighter.

Please note that voltage depression only results from overcharging your battery at a low rate. If you overcharging your battery at a high rate, you will do irreparable damage to the battery.

There are two ways to avoid "memory". (I must admit that I hate the term memory as applied to voltage depression, because it has nothing to do with what most people think of a NiCad "memory", and because the battery isn't "remembering" anything, it has just been overcharged.) The best way to avoid it is to get a charger that doesn't overcharge the battery, namely a smart charger that switches to a very low current when it detects that the battery is full. See here for more info on smart chargers.

An alternative, if you really don't want to invest in a smart charger, is simply not to worry about it. The main symptom of a battery that is suffering from voltage depression is that after using your lights for awhile they start to go slightly dimmer and yellowish, but then remain this way for quite some time rather than continuing to get dimmer and dimmer. The dimmer light is due to the battery discharging its energy at a slightly lower voltage (about 0.1V/cell) than it should. If you continue to use your battery through this slightly-dimmer phase, the problem goes away, as the less desirable crystals give up their energy. When the battery is recharged, the better crystals are re-formed. If you do this, please keep in mind that when the light starts to get dimmer and dimmer as the battery capacity reaches its end, you must turn off your lights. If you discharge the battery to lower than about 1 V/cell, you run the risk of damaging the battery through cell reversal.

The fact that doing a deep discharge (down to about 1 V/cell) of your battery cures the problem with voltage depression is probably what lead to the myth that you should always discharge the battery before recharging it. However, somehow the concept of "deep discharge" has gotten mixed up with "full discharge" (down to 0V/cell), which is quite likely to damage your battery. In this case, the cure (a full discharge which can result in cell reversal) is far warse than the disease (voltage depression, which only gives you slightly dimmer lights the first time you use the battery after it developes the problem).

This may be not what you are used to hearing about NiCad memory. The problem is that misinformation (the idea that NiCad "memory" is a loss of capacity caused by recharging after only a partial discharge) is so prevalent that many bike light manufacturers believe in it and propagate it. However, NiteRider at least some has some pretty good info about battery care. (Even here there is conflicting information. They suggest that "To prevent OVERCHARGING: Do NOT charge your battery until it has been fully drained (or charge it only enough to replenish it)" although they make it clear just below that you should not drain the battery beyond the point where the light turns yellow/dim. I presume they're hoping that the customer will figure out that "fully drained" means "until the light turns yellow/dim".)

Battery shape

Batteries basically come in two shapes: more or less rectangular ones in padded nylon bags that you strap to your frame, and in water bottles. By and large, only NiCads come in water bottle shapes, while both NiCads and lead-acids come in rectangles.

With my VistaLite 430 I got a rectangular lead-acid battery. I struggled to find a place to hang it. I couldn't attach it to my top tube because it had three unexposed cables going along the top of it, separated from the frame with little rubber donuts. I couldn't put the straps on top of the cable, and it wouldn't fit between the cables and the frame. I tried mounting the battery under the seat, and at the intersection of the seat tube and top tube (where the cables were in housing) but finally gave up.

I unhooked the cables, threw away the little rubber donuts, and put the cables through thin (about 4mm diameter) plastic tubes, available from the local bike shop. This protected the frame much better, since the rubber donuts always collected themselves at one end of the cables and let the cables scratch the frame anyway. It also allowed me to put the straps on top of the cables. So I did. There's an additional stabilizing strap that goes around your down tube to stop the battery swinging around. I did that up and after a few weeks of riding noticed that the paint was being rubbed off my frame where the corner of the bag met the downtube. Arrrgggg...

So I Hammerited the bare spot and put a piece of electrical tape over it. Then I noticed that a hole had been worn in my bag at that place. I dismantled an old bike glove and sewed a piece of the palm (padded leather) to the bag at the corner. That worked, but it was still a bit of a pain to take on and off with all those velcro attachments.

Then I got a water bottle NiCad battery, and my worries were over. The moral of this story is that water bottle-shaped batteries are a much better approach.


Most of this information I learned by subscribing to the bikecurrent mailing list. I would like to thank subscribers to this list, especially Marty Goodman, for answering my many questions.

More information on bike lights, batteries, charging, and NiCad "memory" and voltage depression are available from:

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