How do Electric Cars Work?
Having worked at EVBox for 6 months, I decided it was time to deepen my industry knowledge, starting with the fundamental question: how do electric cars work?
Well I knew the basics. You charge the battery with electricity which powers the motor and makes the wheels turn.
To satisfy my curiosity (and avoid embarrassment in case I need to strike up a conversation with a hardware engineer), I decided to take a course on electric cars from TU Delft.
I have now finished the first part on technology and I'm happy to say that I have the answer to the question. Here is what I learned.
How do Electric Cars Work?
History of electric cars
Advantages of electric cars
Challenges of electric cars
Parameters of electric cars
Types of vehicles
The History of Electric Cars
To understand electric cars, we first need to go back 200 years in the past. The invention of the electric motor in the 1830s made electric transportation conceivable. However, it was only in 1890 that the first practical electric car was created.
Electric cars rapidly gained popularity. They were quiet, easy to drive and didn't pollute. At the start of the 20th century, around a third of cars on US roads were electric!
However, the mass production of the Ford Model T starting in 1908 made gasoline cars affordable and widely available. The major flaws of electric cars became apparent: high cost, low top speed, and short range.
The discovery of cheap crude oil helped contribute to the decline of electric cars. By 1935, gas-powered cars had officially won the first round of the competition for mass adoption.
Cheap and abundant oil combined with improvements of the internal combustion engine (ICE) fueled the growth of the automotive industry over the next 30 years.
When oil prices soared in the 1970s, automakers started to explore electric cars but nothing significant happened until 1997 when Toyota introduced the Prius. This was the first mass-produced hybrid car. It became an instant success and marked the start of the second round.
In 2006, Tesla introduced the Roadster and in 2010, GM released the Chevy Volt which was the first commercially available plug-in hybrid. Due to technological advancements, battery cost decreases and sustainability pressure, automakers have progressively shifted to the production of electric cars, buses, and trucks.
Key Advantages of Electric Cars
Sustainability. Electric cars have no tailpipe emissions and zero emissions when charged with electricity generated from renewable energy sources. The net emissions of an electric car depends on the energy mix used for electricity power production. However, electric cars are always better than gasoline cars when it comes to net emissions irrespective of the energy mix for electricity production. In addition, the electric car battery can be used as a storage for renewable energy and can support the grid via vehicle-to-grid (V2G) technology.
Efficiency. Electric cars have higher efficiency and lower emissions than ICE cars. This holds true from a well-to-tank, tank-to-wheel and well-to-wheel perspective. 72-89% of the energy is wasted by driving an ICE car.
Convenience. Electric cars have no gears because the electric motor and drivetrain can provide peak torque at nearly all speeds. They have fewer components and a simpler drivetrain than ICE cars (meaning lower maintenance) as well as larger internal space. Electric cars are also quieter, especially at low speeds. You can charge an electric car whenever you want at home (like your phone) and they will have complete self-driving capabilities in the future.
Economy. Electric cars have a higher purchase price but a lower total cost of ownership due to lower maintenance costs, lower taxes, cheaper fuel, and substantial government subsidies.
Key Challenges of Electric Cars
Long charging times
Limited charging infrastructure
High battery cost
Battery energy density is much smaller than that of gasoline
E-mobility – A New Mindset
To achieve mass adoption of electric cars, technological advancements are not enough. We need to think differently as well.
Charging times are being reduced progressively but charging an electric car will always remain fundamentally different from refueling an ICE car.
We now charge our smartphones once or twice per day instead of once per week because we see the technological advantages of having a smartphone. The same will happen to cars.
What’s Inside an Electric Car?
The charging port allows the electric car to be connected to an external source of electricity for charging.
An on-board charger converts the incoming AC power supplied via the charging port to DC power for charging the traction battery.
The traction battery bank stores energy for the propulsion of the vehicle. It has a battery management system that monitors and regulates the battery charging characteristics such as voltage, current, temperature, and state of charge.
A power converter regulates the power flowing from the battery to the motor and to the vehicle accessories like light and audio system.
The motor drive controls the speed, torque and rotational direction of the motor. The converter is bidirectional for operating in both driving and regenerative braking mode.
The electric motor is responsible for converting electric energy to mechanical energy for driving the wheels via the transmission. The main benefit of the electric motor over the internal combustion engine is its ability to provide peak torque at nearly all speeds. Learn more by watching this video.
The auxiliary battery provides electricity to start the car before the traction battery is engaged and also powers the vehicle accessories.
The power electronics controller controls the flow of electrical power in the different power electronic converters in the electric car.
Motor power is the power delivered by the motor to the wheels for propulsion.
Nominal battery capacity (in kWh) is the total electric energy that can be stored in the battery.
State of charge (SOC) is the ratio between the amount of energy currently stored in the battery and the total battery capacity.
Range (in km) is the maximum distance that can be driven by an electric car when the battery is full.
Available range is the maximum distance that can be driven by an electric car based on the current state of charge of the battery.
Energy consumption (in kWh/km) is used to measure efficiency.
Fuel economy is the distance traveled per unit volume of fuel consumed by the car. It can be expressed in kilometers per liter or miles per gallon.
Available range = current battery capacity / energy consumption = (SOC / 100) * nominal battery capacity / energy consumption
Efficiency – MPG and MPGe
MPG measures how many miles a gasoline car can travel with one gallon of fuel. The same idea applies to the metric equivalent of km/L. The conversion between the two is:
MPG = 2.350 * km/L
km/L = 0.425 * MPG
For electric cars, MPGe (miles per gallon equivalent) is used to measure the distance in miles traveled per unit of electric energy consumed by the car. The ratings are based on the EPA formula, in which 33.7 kWh of electricity equals 1 gallon of gasoline.
Hence MPG and MPGe gives a fair way to compare the efficiency or fuel economy of any type of car. The higher the MPG or MPGe, the more efficient is the car.
Ideally, an electric car has high range, low energy consumption and a high MPGe.
Types of Vehicles
Vehicles (in this case, cars) can be classified based on drivetrain and fuel type:
Internal combustion engine (ICE) vehicles have a mechanical drivetrain and are fueled by diesel or gasoline.
Alternative fuel ICE vehicles have a mechanical drivetrain but use other fuel types such as autogas, natural gas, biofuel, or hydrogen.
Electric vehicles (EVs) have an electric drivetrain and can be further classified based on their energy sources and propulsion devices.
Hybrid electric vehicle (HEV) sources its energy solely from gasoline or diesel, but uses both an electric motor in combination with a battery and a combustion engine for propulsion.
Plug-in hybrid electric vehicle (PHEV) also uses both an engine and an electric motor for propulsion but can be charged by electricity via a charging port.
Battery electric vehicle (BEV) is purely powered by electricity and the propulsion is solely driven by an electric motor.
Fuel cell electric vehicle (FCEV) sources energy directly from hydrogen using a fuel cell and the propulsion is done using an electric motor.
Solar electric vehicles (coming soon) sources energy directly from solar panels and the propulsion is done using an electric motor.
Electric Vehicle (EV) Charging
To understand how electric cars work, we also need to understand how charging works. Fast and reliable charging infrastructure is crucial for mass adoption of electric cars.
Developing charging infrastructure is a classic chicken and egg problem. Operators don't want to invest in charging stations until there are enough EVs and drivers don't want to switch to an EV until there are enough charging stations.
The same phenomenon was observed in the roll-out of gasoline cars and petrol stations almost a century ago.
Policies and new business models help solve this problem by incentivising operators to install charging stations and driver to go electric.
AC and DC Charging
In the electricity grid, the electric power is alternating current (AC) by nature. However, the electric power of a battery is direct current (DC) by nature. Hence, to charge an electric car from the AC grid, the power must be converted from AC to DC.
Where the power conversion happens is the key differentiator between AC and DC charging.
When AC power from the electricity grid is fed to the car and converted to DC power by the on-board charger, we are talking about AC charging.
This is practical but the size and weight of the onboard charger are dependent on the maximum charging power, typically in the range of 1.9 to 22 kW.
If the charging power has to be further increased, then the charger will become bigger and heavier. However, due to size and weight restrictions inside the vehicle, the on-board charger is limited in size and weight as well.
Therefore, the solution to increase the charging power, is to move the AC/DC converter outside the vehicle, in an off-board charger. This is referred to as DC charging. AC power from the electricity grid is converted to DC power in the charging station (off-board charger), allowing DC power to flow directly from the station and into the battery.
Because the conversion takes place in the more spacious charging station, larger converters can be used allowing for faster conversion. As a result, some DC stations can provide up to 350 kW of power and fully charge an electric car in 15 minutes.
Another key difference between AC and DC charging is the charging curve. With AC charging, the power flowing to the electric car represents a flat line. This is because the relatively small onboard charger can only receive limited power, spread over a longer period.
In contrast, DC charging has a degrading charging curve as the battery initially accepts a quicker flow of power but gradually asks for less as it reaches full capacity.
Think of the EV battery as a glass, the DC charging station as a water bottle, and the water inside that bottle as the power. At first, you can quickly fill the glass with water, but you’ll need to slow down as you get to the top, so the glass doesn’t overflow.
One of the main concerns about EVs is charging time. While fueling a combustion engine car takes around 2 minutes, charging an EV battery takes much longer.
Well, the charging process of an electric vehicle battery has two distinct charging regions: constant current (CC) and constant voltage (CV), as seen below.
When you start charging the battery, it typically charges at a constant current, equal to or less than the nominal current of the battery.
During this period, the voltage of the battery increases as the battery gets charged. This is referred to as the constant current (CC) region. Here, fast charging happens.
When charging a battery, there is a maximum charging current and voltage for safe operation of the battery and to ensure a long lifetime.
When the voltage reaches the maximum set point, usually at a state of charge of roughly 80%, the EV charger changes to the constant voltage (CV) region where the voltage is maintained while the current is gradually reduced to zero.
Charging in this region typically takes a long time as the charging current reduces. Charging is stopped once the current is close to zero.
Power and Energy
When talking about EV charging, it's important not to confuse power and energy.
Energy = Power * Time
The units of energy are joules, watt-hour or kilowatt-hour. While the units of power are watts, horsepower or kilowatts. Simple conversions can be made:
1 kilowatt (kW) = 1000 watts (W)
1 horsepower = 745.7 watts
1 watt-hour (Wh) = 3600 joules (J)
1 kilowatt-hour (kWh) = 1000 watt-hours
To get an overview of the charging capabilities of different electric cars, check out this tool made by EVBox.
Understanding Charging Time
High currents lead to faster charging, but can also cause the battery and the charger to overheat or to degrade faster, causing a shorter life-span.
The maximum charging power is dependent on the maximum voltage and current that the battery can handle and the charger can provide.
Charging power (W) = battery voltage (V) * charging current (amp)
The energy delivered to the battery is the charging power integrated over time, which, assuming zero losses, is defined as:
Energy delivered (kWh) = charging power (kW) * charging time (h)
The C-rate is the ratio of the charging power to the nominal energy capacity of the battery.
C-rate (%) = charging power (kW) / nominal battery capacity (kWh)
The higher the charging current, the higher the C-rate. As the C-rate increases, the battery losses and temperature increase which eventually reduces the battery lifetime. Hence, to ensure the longevity of the EV battery pack, lower C-rates are preferred.
The main disadvantages of conventional (uncontrolled) charging are:
Peak loading on the electricity grid
No correlation to renewable energy generation
No correlation to energy prices
Smart charging is the key to overcoming these issues. Firstly, if many cars are connected at the same time, smart charging can plan and spread the charging over the day.
Secondly, smart charging allows for EV charging from renewable energy. Wind and solar power generation have their maximum at the start and middle of the day, respectively. Meanwhile, energy prices and load on the grid are reaching its peak at the end of the day.
With smart charging, EVs can be controlled to charge from the wind generation in the morning. Then the charging can be increased to charge from both the solar and wind generation in the afternoon. Finally, the charging power can be reduced in the evening when the loading on the grid and the energy prices are high. This is shown in the graph below.
Thus, by making sure electric cars are charged when energy prices and the load on the grid are minimal, smart charging reduces the cost of charging.
If you have read until this point, I hope that you now have a better understanding of how electric cars work. If you want to learn, check out the two other articles I wrote about this course: business and policy.
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