The PLANET-7 International

Professor Pivot answers your Technical Questions
ELECTRIC bikes
What is VOLTAGE and which Voltage is best?
Voltage can be thought of as the pressure or strength of electric power. All things being equal (see AMPS below), the higher the voltage the better, because high voltages pass more efficiently through wires and motors. Very high voltages (100+ volts) can give you a nasty shock because they also travel through people rather well, but the sort of voltages found on electric bicycles (12 - 36 volts) are quite safe. As a rule, a 12 volt system is fine for low-powered motors, but more powerful machines work better with 24 or 36 volts.
What are AMPS?
Amps can be thought of as the volume or quantity of electric power. To aid this analogy, the flow of amps is called the current, as in the flow of a river. Unlike a river, though, the speed of the current is fixed - only the volume varies.
The maximum flow of amps in a bicycle drive system can vary from 10 to 60 or more. A current of 60 amps requires thick wiring and quite substantial switchgear.
What are WATTS?
Once we know the voltage (or pressure) and current (or volume), we can calculate the power , or wattage by multiplying the two figures together. The number of watts in a system is the most important figure of all, because it defines the power output. A few examples:
The Zap motor draws 20 Amps x 12 Volts = 240 Watts
The Giant Lafree Lite draws 15 Amps x 24 Volts = 360 Watts
The Powabyke draws 20 Amps x 36 Volts = 720 Watts
The Curry Drive draws 80 Amps x 12 Volts = 960 Watts
Not surprisingly, the Curry is the fastest motor, followed by the Powabyke and the Lafree, with the Zap coming in last. Note that the Giant Lafree draws the smallest number of amps, but the highest voltage. Note also, that the Curry Drive is the fastest, but has the lowest voltage. It's impossible to calculate the power without knowing both the number of amps and volts.
I thought the legal limit was 200 Watts in the INDIA
Well, yes and no. The legal limit refers to the maximum continuous power output, whereas the figures above are for absolute maximum power. Most motors can give a high output for a minute or two, but they'd melt if asked to do it all day - just like a cyclist.
How many watts do I need?
As a general rule, a cyclist can produce 100 watts for a reasonable length of time. To be really useful, a motor needs to produce another 100 Watts on a continuous basis, with a peak power of at least 300 watts. Just to confuse things, our measurements are of power consumption - losses in the motor and drive system mean that the power output to the wheel can be much lower.
If you expect the motor to do most of the work, especially in a hilly area, you'll want a peak consumption of 600 watts or more. On the other hand, if you prefer gentle assistance, a peak of 200 watts may be enough.
How big a battery do I need?
The capacity of the battery is found by bringing together volts, and the measure of current against time - amp/hours. By multiplying the two figures together, we get watt/hours - a measure of the energy content of a battery. Unfortunately, it isn't that simple... but you didn't think it would be, did you? In practise, you're unlikely to get results that match the stated capacity of a battery because battery capacity varies according to the temperature, battery condition, and the rate that current is taken from it.
Lead/acid batteries are rated at the '20-Hour' rate. This is the number of amps that can be continuously drawn from the battery over a period of 20 hours. However, an electric bicycle will usually exhaust its battery in an hour or two, and at this higher load, the battery will be much less efficient. So the figures for lead/acid batteries tend to be optimistic.
On the other hand, Nickel-Cadmium (NiCd) batteries are rated at a 1-Hour discharge rate, so although the stated capacity of a NiCd battery might only be half that of a lead/acid battery, performance will be much the same. Nickel-Metal-Hydride batteries (NiMH) are measured at the 5-Hour rate, so their performance tends to be somewhere between the two.
The capacities of typical bicycle batteries vary from Powabyke's 504 watt/hour giant (36 volts x 14 amp/hours) to the tiny 84 watt/hour pack on the SRAM Sparc kit. It's best to choose a package that will provide twice your normal daily mileage. It's difficult to guess the mileage from the watt/hour capacity, because actual performance depends on the bicycle and motor efficiency, battery type, road conditions, and your weight and level of fitness. As a general rule, a big battery like the Powabyke's will give a range of between 10 miles (doing in all the work in quite hilly terrain) and 25 miles ( a joint effort in hilly terrain). This is enough for most uses, although it's a very big and heavy battery. Small units, such as the SRAM Sparc, give a maximum range of 5 - 10 miles.
How can I measure the efficiency of an electric bike?
We measure overall efficiency by dividing the watt/hours used by the battery charger by the mileage achieved, giving a figure of watt/hours per mile. This varies according to the terrain, the weight and riding style of the rider and the type of battery, but our figures are measured in exactly the same way, so they should be comparable, bike against bike. The best we've seen is 8 watt/hours per mile, and the worst is 32...
Which battery type is best?
Lead/acid batteries are cheap and easily recycled, but they are sensitive to maltreatment and have a limited life. Weight for weight, Nickel/Cadmium gives more capacity, but it's expensive and hard to recycle, although the life is greater, which tends to compensate. NiMh is theoretically more efficient still, but these batteries can be expensive and because the capacity is measured at the more generous 5-Hour rate, the advantage is not what it appears to be. Our experience is that NiMH offers very little, if any, improvement in range.
What should I look for?
We've put together a specification wish-list below. At the present time, there are no machines that win in every category, but the more yours gets the better. If the salesman is unable to provide all the answers, or starts blustering or attempting to blind you with science, we'd recommend looking elsewhere. A good shop should be able to provide most of the figures in a straightforward and honest manner:
Weight Less than 30kg (66lb)
Price Less than ￡600
Maximum assisted speed Not less than 15mph (legal maximum)
Peak power More than 300 watts
Power consumption Less than 10 watt/hours per mile
Range** More than 20 miles
Battery type NiMH or NiCd
Replacement battery price Less than ￡100

** You'll need to verify this for yourself - manufacturers figures are universally dubious

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