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 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.
How many watts do I need?
As a general rule, a cyclist can produce several hundred watts briefly, and one hundred watts for a reasonable length of time. To be really useful, a motor needs to produce another 100 Watts on a continuous basis, with peak power of at least 400 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 usually measured as the amount of current it can supply over time (defined as amp/hours). However, this is useless on its own, because you’ll need to know the voltage too. By multiplying the two figures together, we get watt/hours – a measure of the energy content of the battery. Unfortunately, it isn’t that simple… but you didn’t think it would be, did you? In practice, 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.
Sealed Lead Aacid (SLA) batteries are tested 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 look overly optimistic.
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.
Do electric bicycles recharge when you go downhill?
The answer is generally NO. Taking into account wind-resistance, road friction and so on, there’s surprisingly little energy left over for recharging the battery, even before generator and battery losses are taken into account. In most systems the motor coasts when you ride downhill, but those that don’t (mainly electric scooters) are capable of putting back only about 5% of the power absorbed climbing the hill. Regenerative systems are typically over-hyped and not worth the cost.