Our guide to bicycle lights
As soon as you go beyond low-power commuter lights, the sort that are more to be seen by than with, the choices and options rapidly become bewildering. So here's our guide to what's what in high-power bike lights. It's split into two parts as it was in danger of turning into a novella, albeit a badly-plotted one with unconvincing characters. Here's the first bit, dealing with batteries and chargers - the lamp-based denouement can be found here.
While small commuting lights just run on regular alkaline cells, disposable batteries would make the running costs of most high-power lights quite hilarious. So it's rechargeables all the way. There are a number of different battery types available, all with mysterious abbreviations - SLA, NiCd, NiMH... To make sense of it all, the first cut we can make is between sealed lead-acid (SLA) batteries and all the rest. SLA batteries are essentially miniature car batteries. They're usually filled with an acidic gel rather than liquid, so they work upside down and don't leak. The chief benefit of SLAs is low cost and a reasonably user-friendly charging regime, but they're very heavy.
In the pursuit of lighter weight, light manufacturers first turned to nickel-cadmium (aka Ni-Cad or NiCd) cells, as used in power tools, radio-controlled cars and the like. These are a load lighter than SLAs for their capacity, a property known as "energy density". As the chart on the right shows, NiCds pack nearly twice the capacity into the same weight as lead-acid cells. They're not the energy density champions any more, though. Nickel-metal-hydride (NiMH) cells upped the ante by another 50% or so, opening up potential for seriously lightweight lighting systems. And most current high-end systems use Li-ion batteries, giving another nudge up the energy density scale.
You may have noticed that bike light battery technology has generally been a step behind that used in laptops and mobile phones. And as such you'd probably expect Li-ion polymer batteries to be next in line. They're kind of the joker in the battery pack, though. The main advantage of polymer cells is higher capacity per unit volume, rather than weight - in terms of energy density they're not much better than NiMH cells. If you're designing a phone or a portable computer, low volume is a huge boon. For a bike light, it's not such a big deal - most riders are more concerned with low weight than low bulk.
The other key difference between SLA and all the other batteries is the discharge curve. SLAs just gradually run down like normal batteries. But the others have a very 'flat' discharge curve - you switch the light on, the voltage across the battery drops a bit initially and then stays steady until the battery's nearly flat, at which point it rapidly drops away to almost nothing. In practical terms this means that your lights remain at the same brightness until the battery's exhausted, at which point they go out.
Obviously the downside of the flat discharge curve is that you get little warning of imminent flat batterydom. Several manufacturers build some sort of gauge or warning system into the lights, with varying degrees of sophistication and effectiveness. High-end lights often have a "reserve" feature that automatically switches the light to a minimum-power setting with half and hour or so to go, but it's always a good idea to have an additional light on board as a get-you-home backup - even a 2W commuter light is better than nothing.
The other reason why more sophisticated batteries tend to be limited to high-end light systems is that you need cleverer chargers to get the best out of them. SLAs don't really care much - just use a charger with a low current output and leave it a long time. As the cell charges its internal resistance increases and the charge current drops. With a suitably low-current "trickle" charger you can leave them on for days without harming the battery.
The same approach works for NiCds and other batteries too, but it's slow - up to 14 hours for a full charge. It's entirely possible to charge them a lot faster than that, but you need some method of avoiding overcharging. This is particularly important with Li-ion batteries, which are very intolerant of overcharging - try and get too much in there and you're quite likely to wreck them. So as well as high-tech batteries being more expensive in themselves, they also need more sophisticated chargers that measure the voltage across the cells and stop charging (or switch to a trickle charge) when they're full.
Usefully, though, you don't really have to think too much about all of this - if your lights need a posh charger they'll have come with a posh charger, so just follow the instructions. And since for most clever chargers the instructions are essentially "plug in, switch on, leave for a bit" that's not too onerous.
On the subject of chargers, one big difference between the different types of battery that we haven't touched upon is the number of times you can charge them - their "cycle life". Good old NiCds score highly here, with Li-ions not bad (see chart on left). The interesting thing here is that Li-ion polymer batteries don't do terribly well on cycle life. Taking that and their only middling energy density into account, we reckon that they're not a great option for bike lights - there simply aren't the size constraints that would make them worthwhile.