We’ve talked before about the energy density of batteries used for bicycle lighting and bicycle motive power.To recap briefly, the battery in a car is a lead-acid device – big, heavy and full of nasty things, but recycleable and reasonably cheap. Batteries like these offer a theoretical energy density of 35 – 50 watt/hours per kilogram (Wh/kg). In practice, taking into account the weight of the cells, casing and wiring, the finished product rarely exceeds 20 – 40 Wh/kg. To get a decent range from such a low-powered battery, it needs to be very heavy – typically 13.4kg for the Powabyke unit.
Technology has long since moved on to the Nickel- Cadmium (NiCd) battery, with a theoretical capacity of 45 – 80Wh/kg – weight for weight, about twice as useful as a lead- acid battery, but full of nasty cadmium, so something of a hazard to the environment.The NiCd has recently been replaced by its more easily recycleable cousin, the Nickel Metal-Hydride, or NiMH battery, currently fitted to around 50% of rechargeable devices worldwide.These batteries have some odd habits, but they recharge relatively fast, and have a theoretical capacity of 60 – 120Wh/kg, which equates to around 40 – 60Wh/kg in practice. Note that although the worst NiMH performance may look similar to the best lead-acid battery, the capacity is measured in a different way, so NiMH and NiCd actually perform better than the bare figures suggest.They also have a much longer service life.
More recently, attention has switched to various kinds of Lithium-ion rechargeable cells.These promise a massive increase in energy density, with theoretical figures of 200 – 700Wh/kg being bandied about in learned papers, but the reality, for the time being at least, is more prosaic. Our first experience with Li-ion technology was the Powabyke experimental cell (A to B 45), which offered just 30Wh/kg, thanks to some ferociously complex internal wiring and a big heavy casing. In the few months since, we’ve tried some more effective technology – typically 73Wh/kg from the Panasonic WiLL battery featured in issue 46, and 69Wh/kg from the similar battery fitted to the Giant Revive in this issue.
“…the battery is 12% lighter and range is increased by nearly 10%…”
Early lithium-ion cells had a tendency to explode, particularly while charging, but a great deal of research has gone into monitoring systems, and alternative electrode chemistry has made them safer and more rugged. Until now, these monitoring systems for the individual cells (a bicycle battery needs up to ten cells) have been crammed into the charger, resulting in lots of wires and a big heavy charger, but miniaturisation has made it possible for the control systems to be fitted inside the battery itself, and the new Ezee Li-ion battery is the first we have tried of this kind.
Ezee bikes are currently supplied with a large, and quite efficient NiMH battery, with an energy density of 57Wh/kg – one of the best figures around.The new battery looks exactly the same, but inside are the electronics to keep everything running happily, and ten Li-ion cells with a capacity some 11% greater than the old battery. Despite the bigger capacity and the complex electronics, the new technology means the battery is 12% lighter than the NiMH, at 4.4kg, against 5.6kg.This results in an energy density of 82Wh/kg – the best we’ve yet tried.And with most of the electronics in the battery, the charger is lighter and easier to use. On the prototype, the charge rate has been set quite low, giving a charge time of nearly five hours, but if testing proceeds smoothly, the unit may be uprated.
For those already using an Ezee Sprint, the technology is fully retrofittable, so you’ll only need to buy the battery and charger to upgrade an older machine.
What do you get?
The lighter battery is obviously a benefit, but more importantly, range is increased by nearly 10% as well.We completed a run on our standard hilly test course of 29.3 miles at an average of no less than 16mph. Our elderly Ezee Forza has a power-hungry US spec: Keeping the assisted speed below the legal limit, we hit 34.9 miles at 14.7mph. That’s a little better than the Powabyke – which is generally considered to give the best range – but from a battery weighing less than a third as much. One slight disadvantage, hinted at by the high road speed, is that the battery runs more or less at full power until the last few hundred metres, before dying almost without warning.
Obviously the lighter battery and greater range make the technology very attractive. And despite the apparent negative aspects of carrying all the electronics around, the charging system seems relatively foolproof against the others we’ve tried.
Those with an interest in chemistry might like to hear that the first generation Li-ion batteries were mostly built around cobalt oxide cathodes, but improved manufacturing methods have made it possible to use manganese oxide, with manganese/titanium oxide on the horizon.Without getting involved with electrons and ionic transfer, all the consumer needs to know is that these are clean, recycleable technologies, and the raw materials are widely available, so prices are expected to fall by 30% in the next year or so.
Any disadvantages? Li-ion cells have been used in mobile phones and laptops for a while now, but despite plenty of lab work, no one is quite sure what will happen in high power, all-weather applications like electric bicycles. Battery life is currently a subject of debate, as is cost, and capacity improvements. Making some very rash predictions, we think performance could well double within five years, giving an electric bike range of up to 60 miles.The related Lithium Polymer battery promises to double the range again, so electric bicycle range of 100 miles, and electric car or motorcycle range of 200 miles seems realistic, but when? Will the technology arrive in time to soften the ‘peak oil’ blow? Only time will tell, but for now, welcome to the future!
A to B 49 – Aug 2005