Electric Bike Tech
Electric Bike Tech
Electric Bike Tech
Torque is a measure of rotational or twisting force around an axis or pivot. Torque is often cited in E-bike literature to denote the amount of force the motor can apply to the rear wheel of the bicycle. Generally a higher wattage motor will have more torque potential than a smaller one. Higher torque motors are useful when climbing steep hills or carrying heavy loads, but most e-bikes have enough torque to perform quite well under average circumstances.
Motor StylesDirect drive hub motors – These are the simplest type of motor with the axle being directly attached to the inner copper windings and the magnets on the outer hub shell. These are also the only motor type that is capable of regenerative braking. Direct-drive motors tend to be somewhat quieter motors than geared or mid-drive motors.
Geared hub motors – This style has a motor encased inside the hub shell that is connected to three planetary gears, which drive the hub shell and thus the wheel. Due to the added gearing these motors allow the motor to work at a higher rpm than the direct drive style and consequently are able to develop higher torque values than direct drive hub motors at the same wattage rating.
Mid drive motors – This motor style is mounted to the bicycle cranks and drives the rear wheel through the chain and gear system, applying power in the same way the rider applies power. Because the power of the motor is multiplied by the bike’s gears, mid-drive motors generally have up to 50% higher torque values than either direct-drive or geared hub motors. This motor style has several advantages over hub motors. First, the motor weight is located low and centered, making for a well balanced bicycle. Second, because the motor is not in either wheel the wheels can be easily removed for flat repairs, transport in a car, or general service. Mid-drive bikes also the same rolling resistance as a traditional bike because neither wheel has a motor at the hub.
Regenerative Braking (recharging the battery while riding)
This is a very popular question and rightly so. It seems like a perfect combination of using power when needed and then recharging the battery while pedaling the bicycle or going downhill. Unfortunately this is where the “perfect world” bumps into physics and does not perform as efficiently as we might imagine. At this time there are only a handful of bikes on the market capable of regenerative braking, and the resulting increase in range is minimal.
Of the three motor styles described above, only the direct drive hub motor can be used to charge the battery while riding. With a direct drive motor the axle is affixed to the copper windings of the motor, which permits it to be switched to generator mode. This is also the motor type with the most drag when pedaling alone or coasting. From the studies that have been completed the findings show there is usually not much gain from regenerative braking due to the limited mass of an e-bike and rider. Traveling on level terrain one might expect to get a maximum return of 1%. On hilly terrain you could get more power back due to the longer braking periods. The catch is the hilly terrain also uses significantly more battery power to get up the hills and returns a small percentage coming down.
Here is a great, although technical, article on the value of regenerative braking on electric bikes written by Brent Bolton at Ecospeed LLC.
This question actually has two parts. Part one refers to the overall life of the battery while the second part is “how long will a charge last”?
Part 1 – Most of the batteries on the bikes we sell are rated for a lifespan of 800 to 1,000 full charge cycles. This means fully discharged and then completely recharged. So if we look at the range per charge of 20 to 40 miles for the bikes with hub motors and assume we get the lower of the range at 20 miles x 1000 charge cycles that equals 20,000 miles for the life of the battery. If you know how much you ride in a year you can divide that number into 20,000 to get the number of years you can expect the battery to last. It is also important to understand that the batteries do not simply stop working at the end of their life. There is a reduction in storage capacity when the batteries get old and that translates to a shorter range with each charge. This acts as a warning that the battery is getting near the end of its useful life.
Of course there are a few other factors that go into battery life that can be easily controlled.
- If you completely drain the battery, or drain it very low, be sure to charge it as soon as possible. LIthium-Ion batteries left in a discharged state for long periods of time will have a reduced useful life.
- Research has shown that doing partial charges can also improve the life of the battery. For example, if a battery is rated at 1000 full charge cycles you would likely get more than 2000 half charges.
- It is best to charge the battery in above freezing temperatures because the charging cycle is a chemical reaction and slows down in cold temperatures. This will reduce the capability of the battery to fully charge. Cold temps also increase the resistance for electrons to move through wires which also reduces charging capacity.
Part 2 – This is the part of the battery question that influences how far one can go on a single battery charge.
Batteries can have very different capacities depending on their amp/hour ratings. Typically a battery with a higher amp/hour (ah) rating will have a larger capacity and thus enable a longer range per charge. One important factor to remember is the battery is part of a system, which includes the motor and controller. Each component in the system needs to be considered to determine whether a specific bike will work for your needs.
This can be the confusing part of electronics for many people. I will try to explain using water as a substitute for electrons. Understanding where the different numbers come from can be explained as follows.
Volts x Amps = Watts
You can also rearrange the equation to calculate amps or volts using the known values.
Volts – This is the pressure or force pushing the electrons from the battery into the motor, similar to a hose with high water pressure compared to one with low pressure. The higher-pressure hose can do more work over the same period of time. E-bike batteries will commonly be rated at 36 or 48 V with older models being 24V or less.
Amps – Amperage represents the flow of electrons similar to the flow of water coming out of a hose over a fixed amount of time. The actual flow of electrons in 1 amp = 6.24 x 10^18 electrons/second. The amp hour (ah) rating of a battery represents the amount of electrons it can store at the rated voltage and has a large impact on the mileage range per charge of a battery. Modern e-bikes have somewhere between 6 and 12 ah with a few going higher. For a battery with a 10 ah rating it should nominally be able to put out 10amps for 1 hour.
Watts – Wattage is a measure of the power used by the motor over a period of time.
Sometimes battery information will have the watt/hours listed instead of the amp/hours. A quick example would be if a battery has a 500wh rating and the bike has a 500 watt motor the battery could power the motor at 500 watts for 1 hour or 250 watts for 2 hours.
Another important factor to consider is the weight of the battery that you will be carrying around. One could build a huge battery with 72 volts and 20 ah but it may not be practical for powering an electric bike. For a preferred weight or size there is a limit to number of volts and amps. Batteries are wired in both series and parallel to increase the volts and amps to get to the optimum performance for that system. Increasing the voltage in a fixed size battery will require the amps to be decreased.