Battery Chicken? - Bike Magic

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Battery Chicken?

Bike Light Technology.

NOT an easy problem!

The first thing to realise about the modern generation of high-powered bike lights is that it is a very demanding application. Surprisingly, the humble bike light as right at the forefront of both battery and lamp technology, due to the unexpected difficulty of the problem.

In order to see where you are going on a dark, damp moor, you’ll need in the region of 10 to 20watts of halogen lighting. If it were a free choice, I’m sure many of us would plump for 55 to 80watts, similar to a single car headlight.

Unfortunately, with a 1kg lead acid battery you’d get only 20 minutes with an 80 watt bulb, or for a 3 hour run time you’d need a small car battery!

Then you have to realise that many rechargeable batteries don’t much like being almost fully discharged in such a short period of time, or on such a regular basis – which is exactly what you do with them! Therefore the relationship between battery weight, run time, lighting power and cost is a very critical one.

Another thing you also have to realise that rechargeable batteries store far less energy per given weight than a disposable battery. A standard alkaline battery stores twice the energy as even the same sized NiMH battery, and almost 4 times as much as a NiCad cell. If you were to replace your vistalite lead acid battery pack with disposables of the same capacity, it would weigh only 170g instead of 1.3kg!! But of course you can only use them once.

All this conspires to the fact that your average 2 watt bike light runs for 8 hours on two ‘C’ cells, but your off-road 20 watt system only runs for 1 ? hours with a hulking great 1kg battery and costs three times as much.

There are three types of rechargeable batteries currently available for bike lights: lead acid, nickel cadmium (NiCad), and nickel metal hydrides (NiMH), in that order of expense.

Lead acid:
Roadies have used small 6 volt motorbike batteries to run modified dynamo lamps for quite some time. This is not as dangerous as you might think, as after all, motorbike use them! However, having a ‘sealed’ or ‘gel’ cell which is less likely to spill, and doesn’t have to be used horizontally is a better idea.
Lead acid cells are cheap and cheerful, but don’t like being fully discharged. Unfortunately by the time you notice your lights going yellowish out on the moor it’s too late – the damage will have been done, and you’ll still have to leave them switched on in order to get yourself home.

Another major problem is keeping your battery alive in the ‘off-season’. If you leave them untouched all summer, they’ll self discharge and be ruined. If you leave them on charge for too long, they’ll also be damaged. Despite trying my utmost with meticulous battery maintenance all summer, I have yet again found my battery will now only hold enough charge for half an hour.

The flip side is that replacement batteries can be had for £12 to £20 easily from an electrical supplier (Maplin, RS or Farnells – the former has retail outlets all over the country), which isn’t so bad for a whole winter’s riding. Also, during the winter, the lead acid cell has no qualms whatsoever with the usual ‘top-up’ charge after a short commuter run.
Be warned that there are also two main types of lead acid battery; the standby and cyclic types. Cyclics are what you need (if given the choice). As the name suggests they are designed to be almost run down, and fully recharged on a regular basis. Standbys are designed to be trickle charged for months, before being required to power a device (e.g. house alarm) for a few hours.

The quoted capacity of a lead acid battery (or any other cell) may well be higher than reality, as these are normally measured over a 10 hour or 20 hour slow drain. You will normally discharge your pack within 2 hours! So 5amp hours quoted size, may be nearer to 4amp hours in reality.

Lastly, to give an idea of weight v capacity, a lead acid cell of 6v 5amp hours weighs around 1.3kg.


As far as charging is concerned, NiCads are the exact opposite to lead acids. They prefer to be fully discharged as often as possible, and hate being ‘topped-up’ by a small amount. NiCad battery packs are usually produced from normal round cells soldered together into a pack, then wrap-sealed, so there is no need to worry about leakage.

Despite what you may have heard, the weight advantage of NiCads over lead acid is minimal. The same 6v 5amp hour battery pack would weigh 900g; not a great advantage. The main reason I would choose NiCads over lead acid depends on the type of riding you require them for. For short commuter runs, with a top-up charge afterwards, you’d be better off using lead acid batteries. If you stay out on the moor until the lights go out (then head home with a Petzl), you’ll be better off with NiCads.

If you continually ‘top up’ your NiCad pack it may develop a ‘memory’. This means that it will become only as big as the top up quantity. To recover the full capacity try a couple of full discharges/recharge cycles, or the old ‘deep freeze’ trick (yup – freeze it!).

The next problem is what happens if you trash your NiCad pack. Expect to pay around £40 for a new one – and you’ll probably have to at least partially build it yourself! Finding a ready made pack of the correct voltage and large enough capacity will be tricky – try looking at laptop batteries or large power tool packs. Chances are you’ll either end up contacting the lamp manufacturer for a new battery (at their prices!), or you’ll have to brush up on your soldering skills and wire together about 10 rechargeable round cells.


Wey hey, now we’re talking! NiMHs were, until recently the cutting edge for mobile phones and laptops, and are now starting to make an appearance as conventional round cells. They hold around half the energy of a disposable battery, but this is still far better than anything else mentioned previously.

This means that the same 6 volt 5amp hour pack now weighs only 500g! This allows users of NiMH battery packs to increase the capacity by around 50% and still undercut their competitors’ weights.

Currently Vistalite, Specialised, Lumicycle and Cateye all use NiMH battery packs, which allows them to have high lamp outputs and sensible run times and weights.
NiMH packs are not particularly finicky, not liking total discharge, but being pretty much resilient to everything else. I’ve seen a pack still alive after 3 years with just a standard ‘dumb’ trickle charger – not bad.

The bad news is the cost. With Joe Public paying around £2.5 for a single AA cell, or £7 for a C cell, you’ve got a whopping £60 to £100 worth of batteries inside your bag, bottle or nightstick. (Which is about the price of a similar sized NiMH laptop battery).
If you can afford them – go with them. Until of course Lithium Ion comes along!

The numbers game:

You may be a little confused by the listed capacity of batteries – usually quoted in amp hours. Forget these straight away! Use Watt hours instead. Simply multiply the capacity in amp hours by the voltage, and you have Watt hours. This allows you to directly compare the size and therefore run time of different voltage batteries.

For example: My vistalite 6volt 5Ah pack is 30Wh; Cullen’s 12volt 2.8Ah pack is 33.6Wh – very similar. A lumicycle 13.2volt 4Ah pack is significantly larger at 53Wh.

Another advantage in quoting Watt hours is that it allows you to calculate lamp burn time much more easily. If you use 20 Watts, my 30Wh pack should last for 1 ? hours. A 12 Watt lumicycle low beam with its 53Wh pack has a burn time of 4.4 hours – easy!
However, donít guarantee these capacities, due to the unusually rapid discharge rate of bike batteries, low temperatures and overvolting; all of which reduce run times slightly.

Most lamps use halogen bulbs, so much so that Watts is often quoted as if it were the light output of the bulb. In fact Watts is the power consumption of the bulb, not its light output, as anyone who has tried to buy a 100W energy saving household bulb will know!
There are in fact other types of bulbs out there, most notably the metal halide gas discharge tube, as famously seen on the monster Cateye Stadium lamp. Metal halide lamps (not to be confused with nickel metal hydride batteries) produce much more light for a given power consumption. The Cateye Stadium is not in fact an 84W lamp, but around 20W, it’s just that it produces the same amount of light as an 84W halogen bulb. This means that you can get ridiculous amounts of light with sensible burn times and battery weight (but not at a sensible price yet!)
Metal halide lamps are still very much cutting edge, and it is difficult to get them small enough for bike usage – hence the spectacularly massive output of the Stadium. In fact metal halide lamps are not even bulbs, they are gas discharge tubes (similar to fluorescent tubes) that run off 5000V, so they need a starter/transformer and a significant warming-up time. At the moment a stadium lamp will set you back £140, hence the £300 price tag on the whole system. The new specialised team fireballs, also featuring a metal halide lamp, is no cheaper.

In time metal halide bulbs will become both smaller and cheaper (the stadium has already dropped £100 in price), allowing systems with a more normal 12/20/32W equivalent light output, but with either tiny battery packs or 8-10 hour run times.

Another welcome development that should increase run times and reduce battery weights is the white power LED. Lupine are marketing an LED front lamp containing a huge cluster of white LEDs, with around the same lighting power as a 15W halogen bulb. The 300g rechargeable battery pack runs the lamp for 8 hours! Unfortunately it also comes with an astronomical price tag of around £350. Don’t worry though – the progress of time will reduce prices, as well as increase light outputs.

With both of these new lamp developments, before too long we may be able to ride around with car-like illumination levels, 8 hour burn times and sensible-sized batteries – all without breaking the bank.


Overvolting is no big deal, not rocket science, not even small missile science. If you run a bulb at higher than its quoted voltage, you get a disproportionate increase in light output. According to lumicycles, overvolting by 10% produces a 30% increase in light, for a 10% reduction in burn time. (The lamp also increases its power consumption by 10%, so a quoted 20W lamp is now 22W).

The only problem is that 12V bulbs are designed to run at 12V, not 13.2V. So if you overvolt your bulb by 10% its life is reduced by a factor of 3, which doesn’t sound good at all. However, this still leaves several hundred hours of life in a halogen bulb – normally enough for several years of use.

The last word:

Whatever you think about MTB lights, I hope you have been convinced that you are not exactly being ripped off with almost any of the systems currently on the market. Even the cheapest of bike lights contains a serious amount of technology, with large and expensive batteries.

The only real alternative to riding around on dark, wet, winter trails is to stay fit with indoors cycling. A good turbo trainer costs around the same as a good set of MTB lights, but – JUST SAY NO! From bitter experience I can assure you that the dirty, wet, cold, scary, dark outdoors is 100 times better than sending your brain into irrecoverable free-fall with 30 miles in the spare room!



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