Ebike Technologies and How to Select an Electric Bicycle

So many electric bikes are on the market now that it can be difficult to choose between different brands and different models. Finding the right electric bicycle can be a confusing task. Many of the brands have models that look identical and have very similar specifications but with widely varying prices. To add to the confusion, even when the published specifications are identical, e.g. 200W motor and 24V SLA battery, the performance of each model can be a world apart. Such specifications need to be understood and evaluated, then your own needs should guide you in selecting the right combination of technology, dealer, and price. Your ebike needs may be quite a bit different that the typical bicycle purchaser. Ask yourself some questions to get started.

Will I be using the ebike primarily for commuting and errands, or will I ride it mostly for recreation?

Why does this matter? A bike that is primarily recreational should be light, powerful and have the greatest possible range - while also easy to peddle if the battery runs low. You should consider a lightweight aluminum alloy frame, with lithium-ion batteries and with power rating of at least 12AH rated power. You can expect to pay more for this type of design than the heavier, steel frame, sealed lead-acid battery models. A primary commute bike need not be designed for streamline cruising, but should be easy to ride in street clothes, stable, and easy to maintain. It may also be important to have baskets and carriers to load books, notebook computers, or groceries for your normal use. Fenders, chain guard, and lights are also essential features.

Do I need an ebike that is fast compared to other bikes?

Electric bikes are by their very nature relatively heavy with their batteries, and intrinsically slower than conventional cruising bikes. Most are designed for simplicity, with gears to match casual biking (typically 1-8 speeds). It makes little sense to spend big dollars for technology to shave a few pounds, such as fabricating with carbon-fiber, titanium, oval spokes or other items important for racers. By federal laws, electric bikes cannot exceed 20 mph when powered only by the motor. Pedal assisting is allowed to exceed this limit for most states. We believe that most bikers will want to enjoy the health benefits of biking and use the ebike as a "hybrid" vehicle – combination electric and manual.

Unfortunately, we must make trade-offs in design, and you must prioritize your needs to consider factors such as agility versus weight, range versus speed, cost concerns, aesthetics, etc. The main components that affect performance are the frame, motor, controller, and battery.

Frame

If you are looking for as light as possible but still afforable bike, the best choice is an aluminum alloy frame. With a premier aluminum alloy such as 6061, welds can be made strong without a sacrifice in sturdiness. Bikes made of cast aluminum must be reinforced at their critical points an the frame is often as heavy as if steel were used.

The most common bike frame is steel due to its strength, flexability in design, and the material is inexpensive. However the frames are heavy and are prone to rust. Titanium frames are gaining in popularity for conventional bikes due to the high strength and rust resistance. The alloys are light and only slightly weaker than steel making titanium ideal for many bikers. Downsides include titanium's difficulty in machining and of course its expense. Another specialty frame material is carbon fiber which is increasing in popularity due to the flexibility of design, very low weight, and the relative strength of the material. Carbon fiber frames can be shaped in almost any way, unlike metals, and thus can be designed more aerodynamically than metal frames. Unfortunately, carbon fiber or graphite can be brittle and high maintenance, and are a high expense that does not receive a payoff in a cruising bike.

Motor

Electric motors used on bicycles are either external or mounted on the wheel hub. External motors are used typically on bikes converted from conventional. The recent trend in the electric bike industry is to use the much more convenient and efficient hub motors. For electric bikes, the advantages of a hub motor include:

  • The motor is in a space that is not otherwise used in the conventional designs of bicycles.
  • Hub motors are simple and self-contained, thus reducing overall cost of the vehicle by enabling the designer to use standard bicycle parts without fabricating complex drive systems.
  • The motors are sealed and mostly maintenance free.
  • The motor is directly attached to the driven wheel, improving efficiency.
  • The center of gravity is relatively low, improving balance.
  • The motor contained in the hub is not a prominent feature of the bike design.

The Drawbacks of Hub Motors

Designers and manufacturers should consider the drawbacks of using hub motors, including:

  • The cost of the motor per se is higher because the motor is more complicated than other kinds of electric motors.
  • Because the motor is sealed against water and dirt, dissipating generated motor heat can be a problem, so it is important to incorporate controllers with motor temperature monitoring.
  • The wheel is heavier with the addition of the motor.

Electric motor selection is based on operating goals, power available, and cost. The most important step in electric motor selection is determining load characteristics -- torque and speed versus time.

The Brush Motor

The standard Brush motor is the original DC electric motor, and gets its name from "brushes" (usually a graphite pad) that rests on the rotor as the motor spins. They normally operate at higher speeds than brushless motors and therefore need some gearing device to reduce these speeds to the 20 mph legally allowed for electric bicycles in the US. Brushed motor without gears have the lowest torque compared to the other motors of the same power and voltage. For 250 - 350W motors the torque ranges between 12-15 Nm which is suitable for hills up to about 4 degrees grade.

Brushed geared hub motors of 250-350W typically generate torque in the range of 15 - 25Nm. These will climb steeper hills of about of 8 -10 degree inclines. Their torque depends on motor speed (RPM) and the gear ratio of the built in planetary torque converter. Planetary gearing or epicyclic gearing is a gear system that consists of one or more outer gears, or planet gears, revolving about a central, or sun gear. Typically, the planet gears are mounted on a movable arm or carrier which itself may rotate relative to the sun gear. Epicyclic gearing systems may also incorporate the use of an outer ring gear or annulus, which meshes with the planet gears. Ideal torque can be achieved with the fastest motors and the lowest transmission ratio.

As a general rule the higher the voltage of the power supply, the higher the motor torque, and the higher the speed. Their disadvantages can be mitigated by using these motors on small wheels or powering them with an at least 36V power supply. Often such motors are use for travel over mostly flat terrain with moderate hills. The power consumption of the brushed motors is 10-30% higher than the brushless motors. The motor is coupled through the gearbox to a freewheel attached to the outside shell. The entire hub then rotates on roller bearings which are a problem for maintenance.

The Brushless Motor

These motors, sometimes referred to as "hall effect" motors, incorporate a rotor with a permanent magnet and a stator with the conducting wire coil. Energy is transferred electronically, requiring no physical contact between stationary and moving parts. By the elimination of brushes internal friction is reduced thereby increasing efficiency. Such motors also offer reduced maintenance, no spark hazard, and better speed control through higher torque. Brushless motor technology is more expensive, but most are more efficient and longer-lasting than brushed motor systems.

The direct drive hub motor is about as simple as things get. Imagine taking an electric motor, but holding the axle and letting the body of the motor spin. With a bicycle rim on top of this spinning motor body, the wheel itself is the only moving part. Brushless hub motor without gears produce higher torque and higher efficiency than brushed gearless motors. Their output is similar to that of the brushed geared motors. For a 250-350W motor, normally a 36V or 48V battery is used, and generated torque is typically 18-25Nm.

The geared hub concept takes the weight advantages of a transmission drive and packages it into the simple looking and easy to install hub motor. Brushless planetary geared hub motors distribute the load over internal gears and are well balanced. They also provide the highest torque compared to other motors and have a long life span. Their efficiency index is high and generated torque can reach 30 - 35Nm for 250 - 350W motor. These typically weigh about 50% less than an equivalently powerful direct drive machine. These type motors are excellent energy savers and they are a new trend in the electric bikes where the efficiency and saving weight is a main priority. They can provide the same torque as a motor rated twice the power in watts.

This is one reason why published specifications can be misleading, and should be understood in the context of the overall bike design and construction. Our bikes use a 250W 36V brushless geared hub motor, and have been configured for optimal performance for bikes designed for maximum ease of use and range.

Another feature of the electric motor that may contribute to overall efficiency involves running the motor backward to recharge the battery. An electric motor generates electricity when the motor spins in response to an external force. This regeneration occurs when the bike is not externally powered and resistive forces spins the motor in reverse of its normal output operation. When the bike is coasting or braking the motor may be engaged to supply power to the battery. This recharging is not very efficient and cannot be relied on for recharging the battery in normal operation, but it can help to extend the range. The regeneration efficiency of an electric bike is typically 20-40%, which means that to fully recharge a discharged battery, one would have to go downhill 3 or 4 times longer than it would take to charge the battery using the normal plugged-in charger. Realistically, if your travel route has 20% downhill coasting over an hour, you may generate enough electricity for about 5% of the whole battery capacity. This low efficiency and added complexity explains why regenerative braking is uncommon in most ebikes today. Currently, none of our models incorporate regenerative braking technology.

Geared and gearless motors

A geared motor does not have an external gear change mechanism but rather the drive parts in the hub mesh through internal gears (usually planetary gears located inside the hub) to drive the bicycle wheel. Geared motors generally have an advantage of higher torque (better ability to climb hills at low speeds), but their maximum speed sis less than the gearless counterparts. Geared motors are reliable and easy to maintain (with fewer external parts), but if they develop problems then repair is difficult and expensive. On the other hand, gearless motors are quieter to run, faster, and easy to maintain.

Battery

The battery is the crucial component of the design. The electric bike industry has been essentially experimental as battery technology has evolved to provide storage of power for transportation at reasonable costs. Consumer electronic devices such as notebook computers and cell phones have sparked the creation of efficient energy cells, and these cells have been developed into large batteries used in the electric vehicle industry.

An electric vehicle battery must have enough energy to carry a full grown man over roads and hills for 20 miles or more. This is very different from most consumer electronics batteries. A battery is not just one solid piece, but a collection of “cells". The cells are one complete unit of anode, cathode, separator, and electrolyte that produce electricity from a chemical reaction in the cell. Each cell type (also called a cell's “metallurgy”) has a nominal voltage. For example, Lithium-Ion is about 3.6 volts nominal per cell and thus we need to combine a number of cell packs for an electric motor. So 12 cells are normally used fabricate a 36 volt package. For comparison, Lead-Acid is 1.5 volts per cell.

A brief summary of battery types targeted for electric vehicles

Sealed Lead Acid (SLA)
Pros: High energy density, maintenance free, tried and tested on electric bikes, cheap.
Cons: heavy, battery cells age and die relatively quickly, no fast charge option.
Nickel-Metal-Hydride (NiMH)
Pros: High energy density, fast charge the norm, lightweight, low toxicity.
Cons: Need interval discharges, can suffer from memory effect, performance reduced in cold weather, medium expensive.
Nickel-Cadmium (NiCd)
Pros: High energy density, fast charge the norm, lightweight, low toxicity.
Cons: Need interval discharges, can suffer from memory effect, highly toxic, and difficult to recycle.
Nickel-Zinc (NiZn)
Pros: Medium energy density, fast charge, light to medium weight, relatively inexpensive, made of a high percentage of recycled material, don't require new factories because they can be made on existing nickel metal-hydride and nickel cadmium battery production lines .
Cons: Need interval discharges, can suffer from memory effect, little capacity or proven use.
Lithium-ion (Li-ion)
Pros: Very lightweight, very high energy density, durable, no maintenance, fast charge, no memory effect, reduced internal resistance keeps down heat, delivers full power until completely drained (rather than slowly dwindling).
Cons: Expensive, can be unstable, cells charge and discharge at different rates.
Lithium-Polymer (Li-Po)
Pros: Lightest battery available, highest energy density, no maintenance, fast charge, proven high level of stability under extreme laboratory tests, flexible shape, low self discharge.
Cons: Most expensive, little history of use for this application.
Zinc Air
Pros: Fuel cell produces zero-emission, and refuelable, high power density, quick charge.
Cons: Limited production capacity, prototypes still in testing.
Ultra Capacitor
Pros: Non electro-chemical storage, zero-emission, highest power density, quick charge, no heat build-up, inexpensive.
Cons: Limited production capacity, prototypes still in laboratory testing.

Which Battery Type is best today for electric bicycles?

As discussed above, it is important to determine your needs and price range, but to summarize the most common batteries in wide use:

Sealed Lead-Acid batteries are cheap and easily recycled, but unfortunately the recycling is not often efficient as so much is done off-shore in low-technology recovery plants. Also they are sensitive to maltreatment and have a limited life. Even though they are heavy, lead-acid batteries are by far the cheapest solution to current power storage. Nickel-Cadmium gives more capacity for equivalent weight, but the material is expensive and hard to recycle, although the lifetime is greater than Lead-acid. Nickel Metal Hydride (NiMh) is theoretically more efficient still, but these batteries are yet still more expensive and the advantage in range is not pronounced. Lithium-ion is about half the weight of lead-acid for equivalent power, and 75% of the NiMh. Batteries can be recharged about twice as many times as the cheaper alternatives, but the price of batteries are about four times that of SLA.

How do you measure a battery's capability?

Usually, when people ask about a battery's capability, they want to know the amount of energy stored in the battery's cells (How far can I go?) and at what rate the cells discharge electricity (How much power and speed?). The capacity of the battery in watt-hours is found by multiplying the rated voltage by the amp-hours (measure of current against time).

What are Volts?

Voltage is often described as the pressure or strength of electric power. At a consistent amperage, the higher the voltage the better for power and distribution, because high voltages pass more efficiently through wires and motors. 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 current flow in a bicycle drive system is designed from 10 to 60 amps. The higher range amperage requires heavy wiring to avoid high resistance and heat.

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. The number of watts in a system is crucial because it defines the power output. Motors are specified in watts, but it is also important to know how many amps are required to run the motor. But motors are usually specified in watts referring to absolute maximum power, and not continuous power. The legal limit defined by US statue of 750 watts refers to the maximum continuous power output. Most motors can give a high output for a few minutes, but they'd overheat if asked to do it all day - just like a cyclist would.

But batteries are usually rated in Amp-Hours?

Amp Hours (AH) is a common measure of the amount of electricity in the cells, the numbers of amps that can be provided in one hour. More amp-hours means you can go farther, at higher speeds and up bigger hills. But more amp-hours costs more, and weighs more because the battery requires additional cells.

Unfortunately, it isn't simple to predict operational capability because battery capacity varies according to the temperature, battery condition, and the rate that current is discharged. Some ebikes draw current continuously, some are controlled by a throttle, and others rely on power assist with pedaling. Basically, the more you peddle, the better mileage can be obtained on a charge, since the battery is not working as hard as with no pedaling. Obviously there are other factors in range such as overall load weight, terrain, wind (highly influenced by average speed), and even temperature.

How long does the battery last?

Lead acid batteries are familiar as the power source for starting automotive engines. But they are not designed for deep discharge. They have a large number of thin plates designed for maximum surface area, and therefore maximum current output, but which can easily be damaged by deep discharge induced mechanical stresses that arise from cycling. Specially designed deep-cycle cells are much less susceptible to degradation due to cycling, and are required for applications where the batteries are regularly discharged, such as electric vehicles. These batteries have thicker plates that can deliver less peak current, but can withstand frequent discharging. In the mid 1970s, a maintenance-free lead-acid battery that can operate in any position was developed with liquid electrolyte gelled into moistened separators and the enclosure is sealed. Safety valves allow venting during charge, discharge and atmospheric pressure changes. The low self-discharge of 1-3% per month in a Sealed Lead-Acid (SLA) allows long storage before recharging. A discharged battery may be recharged about 300 times only, so it is important to recharge often, and before it is fully discharged.

A lithium-ion battery provides 500 or more discharge/charge cycles. The battery also prefers a partial rather than a full discharge. Frequent full discharges should be avoided when possible. Instead, charge the battery more often or use a larger battery. There is no concern of memory when applying unscheduled charges and no scheduled cycling is required to prolong the battery's life. Improvements in chemistry have increased the storage performance of lithium-ion batteries, and overall efficiency will continue to improve with material research and nanotechnology fabrication.

What are the solutions?

Each battery type has different capabilities, needs, and limitations. So, very careful engineering tailored to the type of battery being used is essential. There is a trade off in terms of cost, weight, capacity, system complexity, and safety involved in all battery engineering choices. All of these factors must be brought in line with one another to create a safe, sound solution, and ultimately one that is affordable for you.

Controller

An electonic controller on an electric bike regulates the rotation speed of the motor by supplying power as required during riding. This component, likely hidden in the internal construction, functions as the brains of the bike. It is such an important component since the performance and the longevity of the electric bike depend on it. Simple controllers function as a gateway for a signal between the pedals or the throttle, and the resulting supply of power from the battery to the motor, while more complex controllers sample and assess rider and environment data to optimize performance. Malama built-in controllers sample both the cadance and torque of the pedal, to trigger motor boost for the Pedal-Assisted System. Some microprocessors within the box are programmable and software upgradable.

The quality of the controller makes a significant difference in the performance of the electric bike, and good controllers may have features such as:

  • Built-in current loop and over/under current protection
  • Current limit and torque control
  • Controller temperature measurement and protection
  • Battery voltage monitoring with cut-out circuits for insufficient voltage
  • Temperature monitoring with cut-out circuits for excessive heat
  • Extended fault detection and protection - LED flashing for fault code
  • Brake light that brightens when braking
  • Warning codes, light, horn
  • Short-circuit protection
  • Regenerative braking control

A big part of preventing catastrophic failures (such as fire or explosion) is a “battery management system". This BMS component prevents the cells from over-discharging, overheating, charging incorrectly, and other anomolies. The BMS also manages cell charge and discharge to get optimal performance and life from the battery package. The management system may either be integrated in the controller, battery, or charger. Malama electric-assisted bicycles feature integrated Battery managment Systems, and include state-of-the-art features listed above, with the exception of no regenerative braking capability is available on current models. We will continue to monitor efficiency and cost-effectiveness to determine adoption options.

 

For more information, please contact Malama at info@malamaelectric.com