What is worth understanding?
At least a basic understanding the two main motor types used in EVs is useful. So many articles available are confusing, often because the author is outside their field, that a sound knowledge base is required to avoid being misled. Surprisingly, motoring websites and motoring videos are in fact a frequent source of misleading information. EV specialists, who own EVs, rarely get things wrong, however motoring journalists who are not EV specialists, often have not experience with owing an EV, and repeatedly themselves make false assumptions, such as assuming a high regen setting will regen will improve efficiency!
The biggest impact of ICE vehicle thinking is to assume the motor of an EV is the equivalent to the internal combustion engine and determines the KW or horsepower of the vehicle.
While I feel some general background very useful, the section on what is changed by having electric motor is more relevant to everyday driving EVs, as opposed sections designed to assist with gaining deeper understanding.
It is also worth looking at how the distinction between AC and DC can become blurred, and while induction is a more advanced topic, some understanding may also be interesting, and
I will provide links more and content for more and more in-depth information in future.
Of course, knowledge on batteries, charging and range all also help understanding EVs.
While I believe it is easier to understand EVs than internal combustion engine vehicles (ICEVs), most people went through various steps over many years in gaining their understanding of internal combustion engines vehicles, and there has been less time to build equivalent knowledge. The sections here reflect my steps in coming to grips with EVs, and as many took me some time, my suggestion is to pause at any step that feels new to allow it to sink in.
Motors as an accessory to the real factor determining power: the EV battery.
In the old days of ICE vehicles, an aftermarket carburetor could unlock more power from an engine as it could feeding more energy supply by better delivering the fuel to the engine.
With an EV, the battery is the carburetor or the supplier of energy, and with an EV the battery is the hard part, and the motor has become a far easier part which can be done with a much smaller, lighter simpler, and yet more powerful unit.
The battery determines the maximum possible power, and all else being equal, the greater the battery capacity, the more power is possible. Greater battery capacity will result in a less stressed powertrain that will last longer.
Amps, Magnets, DC and AC electricity.
Amps and Electrical Current.
A common misconception is that a 10Amp circuit is a circuity that will limit the supply of current to 10Amps, so that if 15Amp appliance is connected to a 10Amp socket, then the 15A amp appliance would only receive 10 Amp and no more. This is NOT how it works. The best analogy I can think of is fishing line, which has a strength. Try to lift something too heavy with fishing line and it doesn’t just lift a little bit or very slowly, the line breaks. The current draw of the appliance is like the weight put onto the power line. A car AC charger can’t just attach its full weight to the power line and see what happens, it needs to know what weight it can put on the power line.
From high school electronics, even though with AC it is technically impedance, not resistance, the formulae V = IR, or Voltage = Current (I despite being Amps) x Resistance still basically applies. Although technically an appliance that draws AC power has impedance, not resistance, the principle is still the same. Divide both sides by resistance, and:
Current (Amps) = Voltage ÷ Resistance.
At 220 volts, if the appliance to be connected has a ‘resistance’ 22, then the current will be 10Amps.
Connect an appliance with resistance of 11, and the current will be 20 Amps.
You can’t make an electrical circuit that will limit the supply 10Amps no matter how low the resistance. The formula always applies. A circuit can either deliver a lower voltage as resistance falls and the current drain increases, or detect too much current is flowing, and shut of the power. Home power does the latter…. request too much power, and the circuit breaker trips.
However, most home power circuit breakers are at 20Amps, not 10Amps. This is because there are multiple 10A sockets on the one circuit from the switchboard, so the circuit may deal with more than 10Amps. So, if you have only one appliance on and it is alone in drawing power on an individual power circuit from the switchboard, the circuit breaker at the switchboard will not trip even if that one appliance is a percentage over 10Amps. In fact, a 15A socket usually uses the same type of wiring from the switchboard but has only one socket on its own dedicated connection to the switchboard.
The individual socket is still only rated for 10Amps, but as long as the contacts do not become worn, it should be ok even with 12 Amps. That said, there are rules, and breaking them can void insurance or be breaking the law. Best to avoid even 10Amp load with a 10A socket that is plugged in and out often, and to be sure the socket is of good quality. It is better to a have a ‘buffer’ than be pushing the limits.
Being rated at 10Amps means, if 10Amps or less flows, nothing should be overloaded or ‘break’. Not that the maximum current that will flow is limited to never be more 10Amps.
From a battery, when the resistance is low and too much current is requested, the voltage drops. The power available is limited by the capability of the battery to provide power.
For home power that is connected to a very powerful grid, you always get whatever power is requested until either something breaks, like the wiring in the house, or the circuit breaker shuts off the power. Hopefully the circuit breaker shut off the power before something else breaks.
Electricity and magnetism.
Electrons always move, and when they do, the create a magnetic field. In any material, even at absolute zero, electrons have motion, so there are magnetic fields. Normally, as these fields are randomly arranged, magnetic fields mostly cancel each other out, so when looking beyond individual atoms, there are only extremely weak magnetic fields. However, weak magnetic fields are everywhere, and MRI machines (magnetic resonance imaging) work by imaging the magnetic fields inside the human body.
However, instead of the magnetic fields mostly cancelling out resulting in weak magnetic fields, getting electrons mostly moving in the same manner generates a far more useful magnetic field. Then the magnetic fields can reinforce each other instead of cancelling out. Generally there are two ways to get electrons moving in the same manner:
- A crystal structure that resulting in electron movement being aligned: a ‘permanent’ magnet.
- Make a flow of electrons as ‘electric current’, creating an ‘electro-magnet’.
Every wire with electric current flowing through it has a magnetic field around that wire. Every wire. This gives some idea of just how prevalent magnetic fields are.
This means magnetic fields are everywhere, but not all magnets fields result in what we recognise as magnet. A magnet requires the magnetic field to result in magnetic poles.
In reality, it is not the electrons moving though the wire that carries the energy from one point to another, but rather, it is the electrical and magnetic fields that transfer the energy from point to point. The role of the movement of electrons, it to generate the magnetic field that carries the energy, as explained in the Veritasium video.
But to focus the magnetic field to create a magnet, with north and south poles, requires making a coil with the wire generating the magnetic field.
Magnetic fields from electromagnets have the advantage that they can be switched off and on, and even reversed by reversing direction of the flow of electrons. In practice, there are enhancements to just making a coil such as adding a core to concentrate the magnetic field, but principle of using a coil to generate a magnetic field is used over and over within electric vehicles.
DC and AC.
Most of us have some idea of DC and AC as ‘direct current’ and ‘alternating current’. The constant voltage from a battery or DC plug pack is clearly DC, and the power from a main socket is clearly AC.
The DC most of us are familiar with is low voltage, and low power. The first hint that those old rules do not always apply is that batteries in EVs can produce even 900v of DC, and for EV charging, the AC is lower power than the DC charging.
The graph here shows a DC voltage that is slowly decreasing in the manner of a battery losing charge, and AC waveform and ‘Square wave’ which takes the form of a series of DC voltage pulses. Note that a real ‘square’ wave is not quite square, as it always takes time for the voltage to rise or fall, so the sides of the pulses are never quite vertical.
But is the ‘square’ wave AC or DC? The square wave ‘alternates’ in amplitude, but relative to the zero volts line, it is always a positive voltage, which could suggest this is still a DC signal. However, relative to the grey line, the ‘square’ wave is no longer always positive, so is the square wave DC relative to zero volts, and AC relative to the grey line? If you ask a large group of electrical engineers, you will get some answers that a square wave can be DC if it never passes zero volts, and other answers that say in practice, as zero volts is always relative to something else, and there is nothing unique about any voltage, a square wave can never be considered as only DC.
Following this logic, an AC voltage is any voltage that alternates over the time relative to any fixed voltage and can be seen to have a frequency over the time being considered, and thus can be AC depending on which voltage becomes “zero”.
On the graph above, the ‘square’ wave and the since wave can be seen as AC or DC depending on the reference voltage, but the green ‘DC’ voltage has to be DC because it has no repeating charges or ‘frequency’.
However, whether a voltage has a frequency depends on the period of time being considered. Within the time of the high voltage of the first pulse of the square wave, the square wave is DC as during that pulse the voltage is constant, which makes the purple signal DC for a short time. The red sine wave is also during that time, as while the voltage rises during that time, it does not fall, and therefore has no frequency. Over a period of the graph, the green signal is DC. But if that green signal was the state of charge of an EV battery, for a car driven each day and charged each night over a week, the voltage the battery would even have a frequency. While there are applications where such a long frequency could be considered an AC signal, in the context of an EV, anything with a frequency below 1 cycle per second is going to be considered DC, which highlights how what can be considered AC or DC can depend not only on the reference voltage, but the interval being considered.
The difference between AC and DC can in some cases be very clear, but there are also times when the difference is no so clear. It turns out that when considering motors, this ‘it depends on the perspective’ difference between AC and DC can result in motors which are the same, can sometimes be described as AC, and other times be described as DC.
Mains AC: Transmitting AC vs DC.
There is a common misconception that the grid is AC because AC is the best way to transmit power, when in fact DC is the best way to send power.
Regardless of AC or DC, the higher the voltage, the lower the current for the same amount of power:
- power = volts x amps.
So why is the grid AC? Because AC was the easy solution to another problem at the time the grids were established. The more amps, the thicker and heavier the cable that is required, while volts only increase the need for insulation, but do not increase the wire gauge required. By increasing he voltage by 100x, then 100x more power can be sent over the same wire. The problem to solve was how to increase voltage for transmission, and then decrease voltage again at the other end of the wire.
With AC all that is required to increase the voltage is a transformer, and AC won the war of the currents not because transmitting AC was more efficient, but because it was easier to increase the voltage of AC power, and then decrease the voltage again at the other end of the wire.
The longer the wire and the more power than must be sent, the more important it becomes to find a way to DC power over the wire. The higher the frequency, the greater the losses, with DC being zero frequency keeping losses to the minimum, as explained in this more technical video.
AC enabled using technology the late 1880s, transformers, to increase the voltage, and transmitting at high voltages results in efficiency gains far outweighing the losses from transmitting as a relatively low frequency of 50 or 60 Hz. However, from the 1930s, methods to increase DC voltages emerged, and became more practical in the 1950s though high voltage valves and far more cost effective in the 1970s with the development of semiconductors.
With the technology available from the 1970s and that has continually improved since, transmission of HVDC (High voltage direct current) DC wins the war of the currents in the 21st century for long-distance high-power transmission.
How electricity actually is transmitted is sufficiently complex that even experts can be confused, as they were from the initial video by Veritasium explaining how electricity does not even flow inside the wires.
Electric Motors: and you could say they are all AC!
Principles of all electric motors.
If you get a rotor and attach some magnets to it, it becomes obvious how another magnet could be used to cause the rotor to rotate to a new position. But to make the rotor keep moving and spin, it becomes clear that the magnet used to first move the rotor would need to either be moved, or reversed to move the rotor further. Keep moving the magnet around the rotor and you then you can make the rotor spin, but that is suing something already spinning, to make something spin.
The alternative is to switch to polarity of either the magnets on the rotor, or on the housing of the rotor at the right time so that the rotation of the rotor never ‘catches up’ and becomes aligned with magnets on the housing. As you can’t switch the polarity of permanent magnet, either the rotor magnets, or the casing magnets, or both, will need to be electromagnets. To summarise:
- A set of magnets on the rotor, and another on the casing, where at least one set is electromagnets.
- A way of reversing the polarity of electromagnets at the right time to keep the rotor spinning.
Reversing the polarity of electromagnets periodically requires reversing the current periodically, which requires AC power for the electromagnets. This means all electric motors require AC power for their electromagnets, and the frequency of the AC power will determine the speed of the motor.
With two types of motors, and two places to put each type, that makes 4 ways to build a motor:
- Permanent magnet rotor, permanent magnet case: Won’t keep spinning, doesn’t work.
- Electromagnet rotor, permanent magnet case: Brushed DC Motors.
- Permanent magnet rotor, electromagnet case: Permanent magnet / Synchronous motor.
- Electromagnet rotor, electromagnet case: Induction / Asynchronous motor.
With only 3 of them really possible, and only the last 2 used in EVs.
Brushed DC Motors.
How can a motor use DC, if the the electromagnets will require AC?
The solutions is the make the rotor magnets the electromagnets, and connect these magnets to the DC power though ‘brushes’ which make contact with connectors which rotate with the rotor. The result is the current through the electromagnets reverses, in this case with two brushes, twice every revolution. If the power to the motor increases, it spins faster, which means the electromagnets get reverse current at an increased rate.
The electromagnets get AC current, that automatically varies in frequency to match the speed of the motor. Brilliant!
The setup is:
- fixed permanent magnets on the casing.
- reversible electromagnets on the rotor.
However, the brushes themselves are a problem. The nature of electric circuits is that the flow of electricity though coils is ‘reluctant’ to change, which makes the brushes and contacts highly likely to spark, increasing the wear even beyond that already present from the friction.
The AC for the electromagnets is made in a clever way, but the life of the brushes can be a problem.
BLDC / Brushless DC / Permanent Magnet AC / Synchronous / PMSM.
Overall, any of these labels above can be applied to these motors that are basically the reverse of the brushed motor:
- permanent magnets on the rotor.
- fixed reversible electromagnets on the casing.
Adjusting the speed of the motor requires changing the frequency of the AC power to the electromagnets, which means a motor powered by mains frequency AC, would have a fixed speed.
Since variable frequency AC requires starting from DC, this type of motor either is a fixed speed if starting from AC power, or requires DC power, which it will then be converted to either a square wave, or smoother waves like sine wave(s), at variable frequency to enable varying motor speed.
For variable speed motors, as used in an EV, it is necessary to start with DC and then created the variable frequency ‘AC’ that is needed for the motor.
The simplest way to get a variable form of ‘AC’ for this type of motor from DC power, is to switch the DC power off and on at the required speed so as to create a square wave, as shown in blue on the graph in ‘DC and AC‘ above. To a solenoid, such a square wave will act as very rough edged AC signal.
Changing the frequency of the switching off and on will, provided there is enough power, set the speed of the motor, and the rotor will rotate synchronously with the magnetic field created by ‘AC’ frequency.
The all ‘on’ and then all ‘off’ of a square wave is less efficient then using more complex circuits to generate waves more like sine waves, and most EVs will have such circuits which are sophisticated inverters with the ability to produce variable frequency, and often also variable voltage.
As you can see there are different names for the one overall motor type. Plus, there is confusion on DC vs AC, with both used within the same type of motor. Many articles written about motors where the author is trying to learn while writing, and getting confused on the way. Here is an example where BLDC and PMSM are listed as two motor types:
Similar to a BLDC, but the PMSM has – as you’d guess from the name – permanent magnets embedded in the rotor to create a constant magnetic field.Typical mistake: Everything you need to know about EV motors
Of course, every BLDC also fits that same description, which makes sense as BLDC and PMSM are the same type of motor. There are many, many articles with the same type of mistakes, as most are written by people trying to come to grips with the new topic.
One confusing area is this same type of motor can be used directly with 50 Hz or 60Hz AC mains power. When this motor type is used that way, the motor will have one fixed speed, and will be inefficient, but technically it is the same type of motor, and what has changed is how it is used, and how it is powered.
It can be seen that synchronous AC motors spin at the frequency of the AC, which can be taken to mean 50Hz or 60Hz AC, but in EVs, that is not what it means. When used in EVs these motors are driven by their own AC power, generated from DC and at a variable frequency in order to control motor speed, which is the type of use that gave rise to the label BLDC.
Sometimes, in some resources, the labels are considered reflect differences in the control circuits used to control the motor, but there is nothing universal. Some people consider small variations better fit using slightly different naming, while many others use the labels as interchangeable. This results in statements like “EVs use BLDC motors” on one site, and another site saying “EVs don’t use BLDC, motors” simply because one site is using the label ‘BLDC’ for the family of motors, and another is using BLDC only for motors within the the BLDC family that use a specific controller configuration, or do not smooth the square wave into a sine wave, even though the motors are all basically the same.
Variations: Axial, Radial, & Transverse.
There can be mentions of axial and radial flux, as in this discussion on the KoKoenigsegg Quark Motor, but these are all just different ways of arranging the windings withing permanent magnet motors.
Induction / Asynchronous Motors.
All motors have both spinning and fixed magnets, either of which can be an electromagnet or permanent magnet.
Brushed DC motors have electromagnets on the rotor(spinning), and Brushless permanent magnet motors have the electromagnets in fixed position on the casing, which leaves the other options as electromagnets for spinning and fixed, or no electromagnets. That fourth option would have of permanent magnets for both rotor and case, and as neither magnet could be controlled or reversed, it does not seem logical there would be a way to make that work.
But option 3, both spinning and fixed magnets being electromagnets is practical.
Induction motors used the magnetic flux of electromagnets in the casing, to power the magnets in the rotor by induction, using the same induction principles as a wireless charger. This means no brushes, and no need for permanent magnets on the rotor.
The same general principle of needing to vary the frequency of the power supplied to the casing electromagnetic coils applies as with brushless DC/ permanent magnet motors.
However, they are also called asynchronous motors, as the rotor must rotate slightly slower magnetic field created by fixed electromagnets to induce current in the electromagnets in the rotor. The difference in speed provides the power for the rotor coils.
A key differentiating characteristic of induction motors is that they can spin relatively freely without power. Spinning either of the permanent magnet motor types will always generate electricity, as moving magnet relative to a coil will generate electricity in the coil. With the induction motor, when there is no power, there is no magnet, so the ‘generator’ is off. This enables a mode where no significant is power required to spin the motor, allowing a passive mode of operation. Induction motors when passive provide no regeneration power, but once activated with the right control logic can still provide regeneration.
What is changed by having electric motors?
No reverse or other gears ratios, no clutch, no starter.
Normally, the electric motor connected via a fixed gear ratio directly to the driveshafts to drive the vehicle. In some cases, the electric motors are in the wheels. Unlike ICEV where the motor always rotates the same way and a special gear is used for reverse, in an EV the motor simply spins backwards for reverse. The motor can be always connected, so no clutch is required.
Having a motor always connected to drive wheels means that unlike even an automatic ICEV, there can be no way to roll the vehicle without the motor rotating.
There are some EVs with a clutch, including the very rare EVs with two gearbox speeds, and some that such as the Hyundai Ioniq 5 AWD models which use a clutch disengage the second motor and save fuel when AWD is not needed and one motor is enough.
Of course there is no starter, nor alternator, nor associated belts, as electric motor(s) to drive the vehicle perform all these roles. With maximum torque often available at zero revs, the whole ‘launch’ problem is non-existent.
AC vs DC motors in EVs.
There is a lot of confusing content about whether motors are AC or DC, even with EVs. As covered previously, inside all electric motors, the electromagnets are powered by AC.
However in all but fixed speed motors, where the speed is set by incoming AC mains power and are not suitable for EVs, the variable frequency AC for the motor(s) must being as DC, not AC.
So the motors use AC internally, even when described as BLDC (brushless DC motors), but the power for the motor always starts as DC.
So there is always both DC and AC in use within the motor, making AC or DC labels ambiguous and best avoided. Just stick the fact that EVs use only either:
- Permanent magnet motors (which can be labelled as DC or AC depending on the source).
- Induction motors (which use the same mix of DC and AC as permanent magnet motors).
The Uses of Different EV Motor Types: Permanent magnet vs induction.
Despite the use of different names for the one motor type, almost all EVs use a permanent magnet motor for their main power, and use an inverter to produce smooth 3 phase sine wave AC power for the electromagnets, as this is normally the most efficient arrangement.
Different sources list these same motors using different names, and sometimes really stumble over AC vs DC, thinking AC and DC are motor types.
EVs do also use induction motors. The main use of induction motors is not for the main motor, but for additional motors, such as the second motor in a 2 motor AWD vehicle. The advantage for the second motor being induction is that when the second motor is not needed, simply cutting power to the induction motor, can allow that motor to spin freely. This allows a simple solution for part time AWD, with power for part time drive axle being provided by an induction motor.
Regenerative Braking and One Pedal Driving: the myths and facts.
(note this section being updated: see this one pedal driving and regen page for more)
Introduction and regen myths.
Regenerative braking is one of the most misunderstood topics, and while online reviewers who have specialised in ICE cars often make statements when covering EVs that are completely incorrect, fortunately some specialist electric vehicle reviewers understand regen so well that it is second nature to them and manage to get things right.
The main misunderstanding centres around ‘regen settings’, with many falsely assuming that all regen is determined by the regen setting, which while true with some vehicles and those with older technology, in modern vehicles with brake-by-wire and “blended braking”, is now completely false.
The confusion can occur due to two very different systems being used with different EVs:
- The original Tesla style “engine braking based blended braking”.
- Blended braking and most regen via the brake pedal.
With any internal combustion engine vehicle, when you ‘back off’ and stop pressing the ‘throttle’ or ‘go pedal’, the compression of engine that then is only supplied with a minimum of fuel, or even no fuel at all, will create and “engine braking” effect, and the vehicle will not just coast, but instead slow significantly even without use of the brake pedal.
Electric motors are also capable of “engine braking”, but unlike ICE where this is behaviour is an engine characteristic, with an electric motor, the amount of this engine “regenerative braking” is a software characteristic and not inherent in the design. The bonus is that if this regenerative braking is used, there is energy recovery!
In order to make driving EVs a seamless transition from driving ICE vehicles, EVs at first added a level of regenerative braking when lifting off the accelerator to produce a similar experience to that a driver would feel with an ICE vehicle, and the idea of active “regenerative braking” linked to the accelerator pedal began.
Regen history: Enter Tesla, and “engine braking based blended braking”.
Regenerative braking by an electric motor can be far stronger then ICE “engine braking”, which means limiting the use of “engine braking” in an EV to a level that provides a familiar experience, results in using less than the full potential ability of an EV to apply regenerative braking.
One solution is to allow multiple setting for the level of regenerative braking in an EV., with a ‘low’ setting that mimics the experience of driving an ICE vehicle, and a ‘high’ setting that allows the full regenerative braking of an EV to be available as ‘engine braking’ from lifting off the accelerator. The high setting does have one problem, which is that the amount of regenerative braking can vary in ways that are difficult for a driver to predict, such as there being less braking available when the battery can consume less charge due to either the temperature or the battery being at near full capacity.
Tesla provided a solution to the unpredictability regenerative braking, by automatically compensating with the friction brake when less regenerative braking is available from the release of the Model S back in 2012. The same term ‘blended braking’ is used to describe the use of both regenerative and friction braking together whether activated by backing of the accelerator pedal to provide strong braking analogous to “engine braking”, or activated in many brands by use of the brake pedal.
With this powerful regenerative braking far stronger than traditional engine braking, many drivers found they rarely needed the brake pedal, and by avoiding using the brake pedal, could drive with greater efficiency.
Then, in 2017, Tesla introduced the next step, “one pedal driving”, with the blended braking not only using friction brakes to compensate for reduced times when regenerative braking is diminished, but also for situations when regenerative braking is always ineffective: at low speeds and for coming to a complete stop.
The now most common regen approach: blended braking via the brake pedal.
Meanwhile, other brands had a different paradigm, and rather than considering regenerate braking as “engine braking”, then considered regenerative braking as “brake pedal braking”.
With almost any modern EV, the full regen the vehicle is capable of is now accessible through the brake pedal, and pushing the brake pedal only results in application of friction braking when regen braking alone will not be sufficient, as the system is brake-by-wire.
With this system, the “regen” setting only changes how the accelerator pedal works, and NOT the amount of regen braking available, as all “regen” and maximum efficiency is always available via the brake pedal. Changing the regen setting does not technically extend range, and in practice with this system, the longest range is easier to achieve on the lowest possible regen setting for the accelerator pedal.
Yes, vehicles with older style braking systems can only benefit from ‘automatic’ regen and thus require a high regen setting, but newer vehicles with brake by wire can be even more efficient on the lowest setting than any vehicle on a high setting. Even some motoring journalists get this wrong, while specialist EV journalists, can provide and correct explanations.
Here some key points:
- depending on regen settings, even the accelerator pedal can, and will, activate traditional friction brakes.
- a higher regen setting does not increase the amount of regenerative braking available, only which pedal must be pushed to access the maximum regenerative braking.
- the brake pedal of a modern EV will activate regen braking first, and only apply traditional friction brakes when regenerative braking will not be sufficient.
- only friction brakes can efficiently bring a car to a complete stop, as the lower the speed, the less effective regenerative braking becomes, and the more effective traditional friction brakes become.
Another misunderstanding centres around the unrealistic suggestion that when the battery has been fully charged, regenerative braking cannot operate. While this is technically true, this happening in reality is unrealistic. Regenerative braking is putting back a percentage of energy that came from the battery.
You only need to apply the brakes, after having first sped up. The energy from the battery used when speeding up, will result in enough ‘room’ in the battery to store the lesser amount of energy that could be reclaimed from then braking. Since efficiency is not perfect, there will be more than enough room.
The only way to ever have more energy available to put back than was used to make the car reach speed, would be if the car started out with a full battery and at the top of mountain, so the car could gain speed without using electrical power from the battery. While there can be limits on the amount of regen available when the battery is near full, the completely full battery scenario is quite unrealistic.
There is a principle of physics: Energy is never created or destroyed.
Energy from the battery or fuel tank is converted into kinetic energy: the energy of motion.
To stop, the kinetic energy of motion must be converted into another form of energy, as it cannot be ‘destroyed’. Disc brakes and drum brakes are both types of friction brakes. Friction brakes convert the kinetic energy into heat energy. The hotter friction brakes become, the less effective they become as it becomes more difficult for them to store even more energy as heat. To not lose effectiveness, friction brakes need to be able to dissipate heat, which gives rise to ventilated disc brakes.
Regenerative brakes use an electric motor, operating as a generator, to convert kinetic energy into electrical energy. The effectiveness of the regenerative braking is reliant on the ability to ‘dissipate’ electrical energy by storing that energy it in the battery. Without keeping the voltage at the motor below the voltage generated by spinning coils in a magnetic field, the braking effect would become minimal far sooner than a friction brake becomes too hot, but as long as the electric power can be transfer to the battery as electrical charge, regenerative brakes can prove an effective and almost wear free brake system that generates electricity. The latest Formula-E race cars, have no other rear brakes!
What really limits regenerative braking?
In a vehicle without brake-by-wire, regen can be limited by the amount available from lifting off the accelerator, but this is mostly a constraint of the past, for todays’ vehicles, the limits are mostly determined by:
- The power of the motor(s).
- Front-wheel-drive vs rear-wheel-drive vs all-wheel-drive.
- The ability of the battery to accept charge.
The power of the motor(s): the more powerful a motor, the more deceleration it can provide, and hopefully this needs no further explaining.
Front-wheel-drive vs rear-wheel-drive vs all-wheel-drive: While the main wheels for handling around curves and for acceleration are the rear wheels, the main wheels for braking are the front wheels. The best possible result for regen braking is from most regen braking power from the front motor and less regen braking power from rear motor. Ultimate regen braking power, as with Formula-E is determined mostly by the regen braking of the front motor, but the gentler the braking, the less critical it becomes whether the braking is from the front or rear, but the best balance will aways be when the front wheels are doing most of the braking.
The result is the friction brakes are needed most in a rear-wheel-drive vehicles, less in front-wheel drive-vehicle, and even less in all-wheel-drive vehicles. Rear wheel drive better for ‘go’, front wheel drive better for ‘stop’.
The ability of the battery to accept charge: Braking is about converting the energy of movement into another form of energy and are limited by the amount of energy they can deal with. Just as friction brakes are limited by the amount of heat they can dissipate, regen brakes are limited by the amount or electric charge they can send to the battery at the current state of charge. A vehicle with a flat charging curve can handle a similar amount of max regen at all levels of charge, while a vehicle with a peaky charge curve will likely vary more in terms of regen braking relative to state of charge. Many vehicles have regen-charging limits higher than the regular charging limits, due to the fact that the duration of regen is much shorter than that of charging.
ICEV brake pedal and accelerator pedal vs EV brake pedal and accelerator pedal.
The pedals are not the direct control of engine and brakes they once were.
On an ICE vehicle, at least traditionally, the accelerator controls the engine, and the brake pedal controls the brakes. Simple.
Originally the accelerator was called the throttle, because it controlled a flap that closed off the pipe of fuel-air mix (video here) to be delivered to the engine. Accelerator ‘flat to the floor’ being the throttle ‘fully open’ to allow maximum fuel air mix into the engine. How far the pedal is pressed mechanically controlled the flap allow fuel air mix into the engine.
Originally the brake pedal pulled on a cable that activated brakes similar to the way a bicycle brake lever works, but became foot operated to allow maximum strength pushing on the pedal, because a lot of force is required to get brakes to be able to stop a car. Modern cars all now use hydraulics to give assistance and make the pedal easy to push, but try the brake pedal with engine off, in order to feel the force that is required!
Accelerator and brakes ‘by wire’.
With controls, ‘by wire’ means instead of a cable operated by the control physically operating what is being controlled, an electrical signal is sent by electrical ‘wire’ to a control unit which manipulates the equipment being controlled.
With the accelerator in a modern ICE vehicle, the move to electric fuel injection means that there is no longer a ‘throttle’, and now a computer controls the fuel air mix injected into the engine, and the accelerator pedal is just an input to the computer. This makes the accelerator on an EV similar to that on a modern ICEV: in both cases it is just an input to a computer.
Brakes are a little more complicated, since they need to still work if the computer fails.
As already mentioned, brakes use a hydraulic system and the force required to push the brake otherwise is too great. However, the hydraulic system still directly sends hydraulic pressure to apply force an activate the brakes, so even if the power assistance fails, a person can push as hard as they can, and the car will still stop, even if the driver cannot brake as forcibly as they can with the assistance working.
For ‘brake by wire’, there needs to be a system that allows the physical pedal to still operate the frictions brakes if all else fails. The problem is, with an EV, mostly the friction brakes are not wanted even when the brake pedal is pressed. The solution is that the hydraulics have valve, kept open by the computer, that automatically closes unless instructed by the computer to stay open. When the value is closed, the brakes are in manual mode as with an ICEV, but normally, the computer handles all braking, and only uses friction brakes when necessary.
On EVs, both ‘by wire’ pedals can control friction brakes and the motor.
Traditional ICVs: Each pedal has only one job.
On modern ICE cars, the accelerator or ‘throttle’ usually no longer directly controls the fuel intake and now simply provides input to a computer, but it still controls only the engine. Similarly, on cars without brake-by-wire, the brake pedal only controls the brakes.
Yes, the computer also controls both acceleration and brakes without the use of the pedals at all, but traditionally, each pedal on an ICEV is used by the driver for only one system. This no longer applies with an EV.
On an EV the accelerator often also controls the brakes, and the brake pedal always also controls the engine. This is because braking with any EV can always include regenerative braking, and only sometimes include friction braking. So, pressing the brake pedal will always activate regenerative braking by the engine in an EV.
For an EV to have competitive level of efficiency on an WLTP test cycle, given how it is calculated, (or EPA or NEDC test), it is essential that regenerative braking is the predominant braking system during normal driving. Due to the much lower energy density of batteries compared to gasoline/petrol, prioritising energy recovery is essential to useful range, especially on urban cycles.
So, the brake pedal must control both regenerative and friction brakes, or more accurately, must result in the vehicle’s control systems using regenerative and friction brakes as required. Generally, friction braking will only be used when regenerative braking is insufficient, which will be either in emergencies, or at very low speeds when regenerative braking becomes increasingly ineffective. The goal is usually to try and simulate the behaviour of friction brakes alone, as that is experience most familiar to drivers. With friction brakes, it is normal to ease off the brakes as the vehicles reaches very low speeds, to avoid aggressively bring the vehicle to a halt as the friction breaking become highly effective as speed falls. In contrast, if braking with regeneration alone, the driver would need to increase the amount of braking as speed dropped, as regenerative braking becomes less effective. The challenger for control systems is to smoothly manage the introduction of friction braking as speed drops towards zero.
When accelerating, input from the accelerator is used to control both amount of power from the battery to the motor, and the frequency of that power. These two factors cannot be directly controlled by a pedal, as they do not retain the same ratios, the only solution is for the accelerator pedal to be an input to the vehicle’s computer, which can then do whatever software has been programmed to do, including anything controllable by software. What is also normally controlled by the accelerator pedal is regenerative braking, and sometimes even friction braking.
All EVs offer regenerative braking. In fact, as most EVs have permanent magnet motors, just trying to push the EV even when power is off, will result in some regenerative braking. The natural behaviour of an electric motor is to produce regenerative braking when the motor is forcibly rotated.
To put the physics in the simplest form, regenerative braking generates energy, and to obtain maximum regeneration, that energy must be consumed. Friction braking also generates energy, which is consumed as heat energy, but with regeneration, the energy is normally consumed by charging the battery.
If the battery cannot accept charge, the regeneration effect will be minimal unless some other way can be found to consume the energy generated by the motor when it is rotated.
How much braking power? Why friction and regenerative brakes combine so well.
The more energy produced and consumed by regenerative braking, the more ‘stopping power’.
For friction brakes, the energy is consumed as heat, so the more heat that can be generated and dissipated by the friction brakes, the more stopping power. To generate the heat, the callipers need sufficient force, and the faster the vehicle is moving, the more force is required. Friction brakes normally increase in effectiveness as vehicle speed decreases. Friction brakes are very effective at bringing a slow-moving vehicle to a complete stop.
For regenerative braking, the more power from braking that can be applied to charge the EV battery, the more ‘stopping power’. To generate the electrical energy, the motor needs to be a powerful electric motor, and this time the slower the vehicle is moving, and thus the slower the motor is rotation, the more powerful the motor needs to be to generate the maximum power that can be applied to charging the battery from the rotation. Regenerative brakes normally decrease in effectiveness as vehicle speed decreases.
Regenerative brakes are almost useless, at bringing a slow-moving vehicle to a complete stop.
With any vehicle, ICE or electric, lifting off the accelerator produces deceleration. With an ICE vehicle, the driver can select different rates of deceleration by their choice of gear ratio, but with an EV, the deceleration from ‘lifting off’ is determined by how much power is the motor is set to recover. Most EVs allow the driver to select between different rates of deceleration in response to lifting off the accelerator. Making this deceleration in response to lift-off smooth is not easy, as the regenerative braking effect is initially quite strong when speed is highest, but then falls away as speed drops. To avoid drivers being caught out by the loss of declaration as speed drops, most vehicles will increase the level of regen attempted as speed falls. So as maximum power available from regeneration also falls, the percentage of theat maxium requested will increase until eventually friction braes will also be required..
One pedal driving.
The myths of one-pedal-driving.
With the strong initial deceleration available from lifting off the accelerator, it seems logical to continue that deceleration until the vehicle comes to a halt. This behaviour of continuing even strong deceleration from lifting off, until the vehicle comes to a halt, is known as ‘one pedal driving’, since it allows stop start motoring using just the accelerator pedal.
However, there are a few myths around one pedal driving:
- Some people think one pedal driving is a result of powerful regenerative braking, it is not.
- Reality: Regeneration settings only change the amount of regeneration before using the brake pedal.
- Some vehicles with very low regen capability offer one pedal driving, while others with more regen capability do not.
- There is a myth that the vehicle can be stopped by regenerative braking alone, while it can’t.
- Reality: Regenerative braking is very ineffective and almost non-existent at very low speeds, and far too weak to be used for coming to a full stop. Actual stopping can only result from either consuming energy and using the motor in reverse to stop the vehicle, or more efficiently by applying friction braking.
- There is a myth that the maximum regeneration will result in the maximum efficiency, while the opposite is usually true.
- Reality: Driving for maximum can be achieved equally well with any regeneration setting, and the settings just change how the pedals work, not how the vehicle works or its efficiency.
- The is also a myth that EVs get better economy in the city than on the highway because they can’t use regen braking on the highway.
- The reality is all braking, even regen braking, result in efficiency losses, not in any gains, and the greater efficiency of EVs in urban environments is explained elsewhere.
The most efficient driving is achieved by braking as little as possible. When braking is needed, it ideally would be regenerative braking as often as possible, but his can be achieved with by appropriate use of the brake pedal and does not require ‘one pedal driving’. In the end, one pedal driving can feel better, and that is the real benefit.
A danger of one pedal driving: a diminished emergency stop reflex.
In reality, there is no true ‘one-pedal-driving’, but rather only mostly-one-pedal-driving. This is because the brake pedal is still required and essential to bring the vehicle to an emergency stop.
- Chevrolet: Use your brake pedal in an emergency or as needed to stop or slow the vehicle.
- Tesla: In an emergency, fully press the brake pedal and maintain firm pressure, even on low traction surfaces.
- Audi: You never need to use the brake pedal (you can if it is an emergency)
The goal of one-pedal driving is that when lifting off the accelerator fully, the vehicle will come to a stop. But how quickly to a stop? Realistically, no one wants the maximum possible braking for an emergency stop just from lifting off the accelerator!
This means the reflex to use that still rather essential second pedal, the brake pedal, in an emergency must remain. Regular driving using the brake pedal refreshes that reflex, as every stop using the brake pedal follows the same pattern: press the brake pedal to slow down, and press harder to slow faster.
Most drivers have that reflex for an emergency stop so well trained already, that the reflex would take years to be lost, but for newer drivers, or over a longer time, there is a danger the reflex could become diminished.
Why high regen settings and one-pedal-driving usually result in worse efficiency.
To quote Audi:
From an efficiency perspective, you should not use one-pedal driving. You only get approximately 80% of the kinetic energy the moving car has back to the battery when doing recuperation. Using one-pedal driving will, for most drivers, mean you will end up doing more recuperation compared to using the brake pedal or letting the car automatically regenerate only when needed.How to use one-pedal driving on electric Audis
There are still cars on the road without a modern drive-by-wire blended braking system, and for these cars, the higher the regen setting for the accelerator, the more regen braking, as there is no access to regen braking through the brake pedal.
Optium application of regen braking is possible through the accelerator, but it is far more challenging to perfect than accessing regen braking though the brake pedal.
The trap is that ALL braking, even regen braking, results in energy loss. With friction-braking, the result is 100% loss, while with regen braking, the loss is only around 20%, but it is still a loss. The goal is to avoid all unnecessary braking, and it is much more challenging to avoid unnecessary braking when braking comes as a result of lifting-off the accelerator.
The best efficiency can be achieved by:
- For cars without, brake by wire blended braking:
- use the highest possible regen setting.
- For cars with brake by wire blended braking:
- use the lowest possible regen setting for even better results.
EV Tech and ‘quirks’.
Why do EVs still have 12v Battery? (12 volt battery)
The 12v battery is for when the main battery is ‘off’
It can seem crazy that EVs need a 12v battery, when they already have a battery with several hundreds of volts. Why not just produce 12v from the main battery? It turns out that almost all EVs do generate 12v from the main battery, but only when the car is ‘on’, and only use the 12v battery when power from the main battery is switched off.
Switch off the main battery for safety.
The main battery has hundreds of volts. When in the car, or during normal operation, this presents no safety risk as the high voltage comes nowhere near the passenger compartment. But what about when doing repairs or when the car has been damaged? There are occasions when the main battery should be disconnected or ‘isolated’ from the rest of the cars, and at these times the 12V battery is needed to keep systems alive, and to operate the electronics needed to reconnect the main battery to the car and switch things back on.
When parked, EVs normally have the main battery electrically isolated from the car. If an EV needs to be towed, or is hit by a falling tree, is attacked by an axe murderer or any other strange event, and the owner is not around, safety requires that there is no high voltage power active.
So, every EV has two modes, main battery disconnected from all wiring in the car, and main battery operations. When the main battery is operational, the 12v can be charged, but is otherwise no in use. When the main battery is disconnected, then only systems power by the 12v battery can be used, and one of those systems is that of the relays that ‘wake up’ and connect the main battery.
Most EVs wake up the main battery at least every 2 or 3 days even in ‘deep sleep’ mode, and usually far more frequently, just to ensure the 12v battery is charged, and can even charge that 12v battery if necessary. As an example, many BYD vehicles ‘wake up’ hourly.
Gears: Drive, Reverse, Neutral and Park.
EVs do not normally have a gearbox in the way ICE vehicles have a gearbox, as mentioned above.
Yes, the Porsche Taycan has a two-speed gearbox that is supposed to assist in acceleration and provide efficiency. At time of writing, the Taycan is the fastest accelerating EV from a traditional ICE vehicle company, but EV specialists make faster accelerating EVs using a single speed gearbox, such as the Tesla Model S Plaid, Lucid Air Sapphire, BYD U9, and even the exotic Rimac Nevara. All use a one speed gearbox, and despite claims the Porsche 2-speed improves acceleration and efficiency, these faster accelerating EV are also all more efficient, and have a longer range than the Porsche.
So, highly unusual cases aside, all EVs use a one speed gear system. This means no gear changes. Not even for reverse or neutral. The motor is always connected to the drive wheels. To go in reverse, the motor turns in reverse. In neutral, the motor still turns.
This makes a far simpler system, with no clutch mechanism or gear change mechanism, and as an electric motor scan spin just as fast in reverse, a potentially very fast ‘reverse’ mode, although software normally limits speed in reverse.
The limitation is ‘neutral’. Even when in neutral, rotating the drive wheels will cause the motor to rotate. Almost all EVs have the main drive wheels driven by a permanent magnet motor, which means turning the motor will generate electricity. Neutral mode still allows an EV to roll and be pushed or even towed, but only at low speeds. For ‘towing’ an EV over any distance or at normal traffic speeds, a flatbed ‘tow’ vehicle is normally required and stipulated by manufacturers, and the very minimum would be a requirement to lift the drive wheels off the ground.
So, what about park?
Unlike conventional cars that use a physical parking pawl, which is a metal pin that engages a notched wheel to lock the transmission, almost all electric vehicles use an electronic brake system to apply and release the brakes. The electronic brake system works in conjunction with the car’s computer to ensure that the brakes are applied and released smoothly and safely.
Prior to 2018 the Telsa Model S and Model X did have a parking pawl, however this this removed in 2018 as part of a redesign of the vehicles’ drivetrains aimed at simplifying the drivetrain and reducing production costs. I am not aware current EVs using a parking pawl, or, for example, BYD ever using a parking pawl with an EV.
The result is that some EVs will ignore selecting park when moving and assume it was unintended, while the other option is to active the parking brake, as tested in this video “pulling e-brake”.
Frunks, Froots and Hoots.
Many EVs have storage in the front, which is mostly labelled as a ‘Frunk’, even in countries where the term ‘trunk’ is not normally used. Labels are:
- Frunk: Front trunk.
- With the main cargo area being labelled the ‘trunk’, from a time there was trunk strapped to the rear or a vehicle.
- Froot: Froot boot.
- Front boot.
- Hoot: Hood boot.
- Hood boot, mixing the US ‘hood’ as opposed to bonnet with the non-US boot as opposed to the US ‘trunk’
The term ‘trunk’ came into use because many early vehicles had not internal space for luggage and an actual trunk mounted at the rear of the vehicle.
Although the label ‘frunk’ seems to have become popular only recently, front storage is far from new, and has long been present in mid-engine or rear-engine vehicles such as the VW Beetle. There have even been some vehicles with both front and rear storage areas before EVs, but with electric motors being so much smaller than internal combustion engines, it is expected that EVs, should have both front frunk/hoot rear and trunk/boot cargo spaces.
With pickups/Utes, this can add the convenience of a sedan style cargo area in addition to the ‘tray’ area of pickup/ute, as well demonstrated with the Ford F150 lightning.
When best executed, as with the Ford F150 and also the Lucid Air, the luggage space is covered only by ‘hood’ or ‘bonnet’, with no second internal cover to open, and the hood or bonnet can be remotely power opened and closed. This then not only gives all the convenience of the rear luggage area, but can be closer to the driver, which can mean for shopping or small items, the front storage area can be more convenient than conventional rear storage areas.
In EVs with less sophisticated trunk/hoot spaces, the space is quite small and can require opening a second area after lifting the hood/bonnet, marking the fort storage more appropriate for charging cables, allowing the cables to be accessed even when the rear storage area has the luggage for the family holiday.
There is no one ‘range’ number for any EV.
Just as an ICE vehicle does not deliver the same fuel economy under all conditions, every EV delivers a different range depending on how it is being driven. This means no EV has single ‘range’ which would be achieved in all conditions. A WLTP range, as explained in full here with the test procedure and conditions also fully described.
Most people really want highway range, which is seen as ‘real range’ and comes from other sources.
EV Batteries Cannot Yet Come Close to The Energy of a Full Tank.
While battery technology has substantially improved, it is a long way from enabling an EV from having the energy available from a tank of gas (petrol) or diesel fuel.
Did you know the fuel capacity of a Toyota Corolla provides the energy equivalent of a 438 kWh battery?
Or that the full tank of a Mercedes GLS holds the energy equivalent of a 788 kWh battery?
This means, without being more efficient, an EV Corolla with a 60kWh battery would only have 1/7th the range of the current Corolla!
Fortunately, electric drive trains are around 4x more efficient, but 4x would still mean a Toyota Corolla EV would need a battery with over 100 kWh to match the gasoline equivalent, unless even more efficiency gains can be found. Well, there is regeneration on braking, also not possible without electric drive, but while regeneration brings things closer, but an “Electric Corolla” still need a close to 100kWh battery to have the same highway range as the current Corolla, and such a battery still costs too much, and weighs too much, so most typical EVs today do not have quite the range to match fossil fuelled cars.
But do we need the same range from recharging, as we needed from refuelling? It turns out, mostly no, but on occasions, yes.EV Range.
Efficiency and RPM: Why drag and increased speed is less critical for ICEVs than EVs.
Efficiency of an engine will improve until peak torque but for a gasoline engine is also affected by turbulence of the fuel air mixture and low RPM and other factors, that all combine to result in efficiency of a gasoline engine improving until around the point of peak torque even more than with other engines.
The graph of the power vs torque shows that for a gasoline engine, peak torque is only available when the engine is producing a significant proportion of its maximum power, while electric motors are at peak torque and peak efficiency from very low power.
Note that the energy, and thus fuel, consumed by an engine/more is equal to the power produced, multiplied by the efficiency percentage of the engine.
So, an ICE producing 100kW at peak efficiency of 25%, is consuming 400kW of energy, and will only have that peak efficiency and producing 100kW, however, an electric motor could have peak efficiency for the entire band from 0 to 100kW.
The result is the engine of an ICEV gains efficiency as the work to be done increases, and thus power produced increases. From low speed up to an optimum engine speed, efficiency increases as power required increases.
If it was not for the fact that air resistance increases with square of speed, an ICEV would get maximum efficiency near its top speed. If only ICEVs could bedriven in a vacuum!
This has the following effects on the differences between EVs and ICEV:
- With an ICEV, a more powerful engine will increase fuel consumption significantly, while with an EV a more powerful motor need not increase battery drain.
- The most efficient speed to drive an EV will be much lower than the most efficient speed to drive an ICEV.
- Increased aerodynamics and other efficiency gains are less effective on ICEVs as engine efficiency rises when there is more work to be done.
Some simple terminology.
Power vs energy.
I plan to add to this section over time, but first the difference between power and energy is that energy is power multiplied by time. Power is instant, and a 100W light bulb will consume 100W at any instant when in operation, but will requite 100W hours of energy to operate for an hour.
- *2023 January 7 : More on one pedal driving.
- *2023 January 6 : Added Gears.
- 2022 October 29: More on regen, plus why 12v battery?
- 2022 September 17: Added regeneration information.
- 2022 Aug 29: First version.
Future Updates: Induction and other content to follow….