Professor Pivot answers your electric bike technical questions…

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 bikes (12 – 36 volts) are quite safe. In the early days,12-volt systems were used for low-powered machines, but today the industry norm is 36 volts, with a trend towards 48 volts, especially for more powerful machines. Electric mopeds and motorcycles tend to use 48 or 60 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 bike 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 of electric bikes:

The Zap motor draws 20 Amps x 12 Volts = 240 Watts
The Giant Twist Lite draws 15 Amps x 24 Volts = 360 Watts
The Powabyke draws 20 Amps x 36 Volts = 720 Watts
The Curry Drive draws 40 Amps x 24 Volts = 960 Watts

Despite having a fairly low voltage, the Curry is the most powerful motor, followed by the Powabyke and the Twist, with the Zap coming in last. It’s impossible to calculate the power without knowing both the number of amps and volts. Large machines, like cars, trains and trucks have their power measured in the same way – usually as kilowatts, or units of 1,000 watts. The old-fashioned ‘horsepower’ unit is the equivalent of about 750 watts.

I thought the legal limit for electric bikes was 250 Watts in the UK?

Well, yes and no. The legal limit refers to the continuous power output, whereas the figures above are for absolute maximum power. Most motors can give maximum output for a minute or two, but they’d melt if asked to do it all day – just like a cyclist. Obviously, maximum power is more useful than continuous power as a guide to the way a bike will climb a hill. Look at the spec of bikes on sale and you may see 200 watts, 250 watts or (illegally) 400 watts. These figures are only a rough guide to the true maximum 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. For a moped, power will be measured in thousands of watts (kilowatts or kW) rather than watts. A continuous rating of one kilowatt will just about keep up with city traffic, but two or three are more useful, and motorcycles will obviously need a lot more to keep up with traffic out of town.

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.

Lead/acid 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 bike 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 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 on an electric bike 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 default capcity for an electric bike battery in 2015 is 360Wh (36 volts x 10 amp/hours), but capacities are gradually increasing. In the past they have varied widely, from Powabyke’s 504 watt/hour giant (36 volts x 14 amp/hours) to the tiny 84 watt/hour pack on the early 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 bike and motor efficiency, battery type, road conditions, and your weight and level of fitness.

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 and charger, but our figures are measured in exactly the same way for each test, so they should be comparable, bike against bike. The best we’ve seen is 8 watt/hours per mile, and the worst is 32… Typically, an electric bike will consume 10 – 20 watt/hours per mile. So a big battery like the Powabyke’s will give a range of between 15 miles (doing all the work in quite hilly terrain) and 50 miles (a joint effort in flat terrain). This is fine for most uses, although it’s a big, heavy battery. A typical 360Wh Li-ion battery should give a range of about 25 miles.

Do electric bikes recharge when you coast downhill?

With the exception of the Canadian BionX, 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 15% of the power absorbed climbing the hill. Regenerative systems do have their advantages though – mainly in reducing brake wear and over-heating.

Which battery type is best?

Lead-acid batteries are cheap and easily recycled, but they are sensitive to maltreatment and have a limited life. They are rare on modern bikes.
Weight for weight, nickel-cadmium (NiCd) gives more capacity, but it’s expensive and the cadmium is a nasty pollutant and difficult to recycle when the battery fails. The life is greater, which tends to compensate, but disposal problems mean that nickel-cadmium has been phased out.
Nickel-metal hydride (NiMh) is theoretically more efficient still, but these batteries are more 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 little, if any, improvement in range over NiCd. They are, however, easier and safer to dispose of when they eventually fail, and the good ones will last for a considerable time.
But NiMH is now rare, because 95% of modern electric bikes come with Lithium-ion (Li-ion) batteries. These are more weight-efficient than the other types, and are very sensitive to abuse, and have a much shorter life even if looked after with great care. Charging and discharging must be carefully controlled to prevent the cells going into terminal meltdown, so either the charger, the battery or both will be packed with electronics. Fires are now rare(!), but initial hopes that costs would tumble proved unfounded, and these batteries are still expensive. Cheaper brands abound, but their life can be very limited. Despite these problems, the Li-ion has become the default battery. Lithium-ion Polymer (usually called Li-pol) doesn’t really offer any performance advantage in terms of weight or range of Li-ion, but it’s safer and can be moulded into interesting shapes. The life of Li-ion batteries is gradually improving, but you’re still doing well to see four years.

Which charger is best?

Swings and roundabouts here. Batteries do not take kindly to fast charging, although NiCd and NiMH are more tolerant than lead-acid, which can start very fast, but prefers a long tapering charge thereafter. A fast (sub four hour) charger makes a great difference to the flexibility of an electric machine. You can, for instance, travel for the full range in the morning, recharge while visiting a friend, and run home in the afternoon. No lead-acid charger can do this, although the best NiCd or NiMH chargers will. Newer Li-ion batteries with the control circuitry on board usually have a very simple charger, but the charge rate with this type will be relatively slow for safety reasons. An advantage is that most 36-volt designs now come with a standard 3-pin battery plug, so the chargers are often interchangeable. For basic commuting, an overnight charger is safest and kindest to the battery, but if you expect to push a high daily mileage, you’ll need something faster.

Should I choose a brushless motor?

Broadly speaking, there are three types of electric motor –

Direct Current motors – simple but comparatively heavy and slightly less efficient, and
Brushless DC (BLDC) motors – smaller, lighter and more efficient over a broader speed range, but with complicated wiring
Sensorless, brushless DC (Sensorless BLDC) motors – even smaller, lighter and more efficient, with simpler wiring, but slightly tricky to start

Direct Current motors have brushes to transfer power into the rotating bit. They are simple and reasonably reliable, but now very rare, fitted to abut 5% of bikes. The vast majority (around 80%) of electric bikes now use brushless DC motors. These are a bit more efficient, because they use electronics and sensors in the motor to do the bit the mechanical brushes do, but the sensors are linked to the control box by tiny wires, so they’re vulnerable to mechanical damage. A more recent development is the brushless, sensorless DC motor, fitted to about 15% of bikes, but the number is gradually increasing. This uses clever electronics to eliminate both the brushes and the sensors, so everything is simpler except the electronics, which are fiendish. Sensorless BLDC will probably take over from BLDC, but don’t rule out Direct Curent brushed motors! They may have mechanical brushes, but they’re mercifully short of complex electronics.

What should I look for in an electric bike?

We’ve put together an electric bike specification wish-list below. At the present time, there are no machines that win in every category, but the closer 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, but some are quite incompetent:

Weight: Less than 30kg (66lb)

Price: Obviously as little as possible, but realistically, expect to pay £1,000+

Maximum assisted speed: Not less than 15mph (legal maximum), and preferably 18mph

Peak power: More than 300 watts

Power consumption: Less than 10 watt/hours per mile

Range**: More than 25-30 miles

Battery type: NiMH or Li-ion (nickel-metal hydride or lithium-ion)

Replacement battery price: As little as possible, but realistically, you’ll have to pay £300-£400 for a decent one. Whatever the price, INSIST on a two year guarantee

** You’ll need to verify this for yourself – manufacturers figures are universally dubious

A few other pointers: If you are expecting to tackle very steep hills (in excess of 17%, or 1 in 6), we’d recommend a Crank Drive motor. This type puts power through the rear gear system and can be fine-tuned to suit almost any environment. It’s the best system if you can afford it. The more common Hub Motor effectively has only one gear, and although some are very powerful, it will prove less efficient in a really hilly area. For most other purposes a hub motor is fine, but avoid Friction Drive unless you intend to make light use of the bike. The roller and/or the tyre tend to wear out in a few hundred miles.