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Battery technology

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 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.

Panasonic Quick Charger – an RCUK favourite

Most commuter lights accept 4 x AA batteries. This means that you can easily adopt a Ni-Cad charger and have two sets of batteries and never really have to buy any standard batteries. Commuter lights are best served with an AA battery because if you really get stuck most garages and newsagents sell AAs. We currently use a rechargeable LED light (for vision) and a Cateye (for British Standard
“insurance”) and as a back up, loaded with a set of Panasonic 2600m/h rechargeables.

Why NiCads then? 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. They also come in individual cells, allowing manufacturers to make up packs of whatever voltage and capacity they require, rather than being stuck with off-the-shelf SLAs. Another benefit for bike lights is that NiCd batteries 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. SLAs behave like disposable batteries in that the voltage gradually declines as they discharge so your lights get gradually dimmer.

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. 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 main disadvantage of NiCds is that they can be a bit finicky about charging. SLAs are perfectly happy being partially discharged, topped up, left half-charged for weeks on end and so on. But NiCds aren’t so tolerant. Debate rages about how much of an issue the so-called “memory effect” is – the tendency of NiCds that are regularly partially discharged and topped up to start to refuse to take a full charge – but in most people’s experience NiCds work a lot better if they’re run until they go flat and then fully recharged, and stored flat if they’re left for a long time.

The Night stick – one of the first lightweight NiMH bike batteries

This is were nickel-metal-hydride batteries come in. NiMHs are lighter again, and perhaps more importantly don’t mind partial discharge/recharge cycles. They’re a product of the portable electronics revolution, being developed for camcorders, laptops and the like. NiMHs are now pretty much the standard battery for bike lights, with NiCds still being found in some budget models.

High-end bike lights, though, are increasingly using lithium-ion (Li-Ion) batteries, as found in the current crop of laptops and mobile phones. These are another leap up the energy-density ladder, packing yet more capacity into even less weight and bulk. They’re inevitably pricier than NiMH batteries, but costs are coming down. USE have used this technology in their latest tiny headlamp the Joystick. The next step will probably be lithium-polymer batteries, and beyond that we might start getting fuel cells. That’s probably a good few years away yet, though.

The Joystick – tiny yet powerful

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. The simplest way to do this is to have a timer that shuts the charger off, but that’s a bit hit-and-miss. More sophisticated “smart” chargers measure the voltage across the cells and stop charging (or switch to a trickle charge) when they’re full.

Generally you won’t have to think about this, though. Just use the charger supplied with the system and follow the instructions…


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