Total sales of electric cars in 2021 doubled the previous year’s sales. Electric cars are environmentally friendly and can save you a lot of money. But how do electric cars work? Is it similar to internal combustion engines? What is the physics behind electric cars? We are ready to bring answers to your questions.
Physics behind the battery of electric cars
Battery for EVs is like gasoline in internal combustion engines. We can not transfer electricity to a driving car. So, we need to store energy in some form. Gasoline is storing energy to carry in cars. But electric cars use batteries to do that.
Most electric cars use lithium-Ion batteries. The same type of batteries in your mobile phones, laptops, and many other consumer electronics. The key to electricity generation of Lithium-Ion batteries is electrochemical potential. A general list of the electrochemical potential of elements is below.
How it works
Using two metals with different electrochemical potentials we can create an external flow of electricity. But lithium-ion batteries have a different mechanism. A cell of a lithium battery has metal oxide, electrolyte, and graphite.
Lithium has only one electron in its outer shell and wants to lose it to get into a stable stage. So, the element is highly reactive.
When an external power source and the lithium-ion cell are in contact, the positive terminal will attract electrons in the metal oxide. Then they will eventually make their way into graphite layers through the external circuit. Electrolyte prevents electrons from getting back into lithium oxide. Anything cannot go through it except lithium ions. Lithium ions will flow into graphite layers crossing the electrolyte because the negative terminal attracts them. When all lithium is trapped in graphite, the battery is fully charged.
Using the stored energy of a lithium-ion battery to work is very simple. Lithium in the graphite of a charged battery is unstable. When an external circuit such as a motor is connected, lithium will come to bake to metal oxide crossing electrolyte. The electron will come back through the external circuit where the load is in. Graphite in a lithium-ion battery is not doing any chemical reactions.
The practical battery
A practical lithium battery has a few more parts for increasing efficiency and avoiding danger. The electrolyte is made of an organic salt of lithium. If the temperature of the battery increases too much this salt will dry up creating a short circuit. It can lead to a fire. Therefore, an additional layer called a separator is coated on electrolyte to prevent such a scenario. The separator is lithium permeable because of microporosity. Graphite and metal oxide are coated on copper and aluminum foils. They act as current collectors here and positive and negative sources can be connected to them. Then all three sheets are wound around a steel axis. Finally, we get the famous compact cylindrical cell.
An EV battery cell has a 3.6V or close voltage. Cells are connected in a combination of parallel method as well as serial method. Also, Lithium-ion battery packs are described by that pattern. Tesla Model S battery is a 22S72P. So, it is made by connecting 22 parallel cells connected in series. The total cell count is 1584 (22 X72). All cells together have a total capacity of 99 kWh.
The battery management system of an electric car serves the below purposes.
- Managing the temperature
- State of charge
- Voltage protection
- Cell health monitoring
It is very important to have a battery management system because factors like heat can decay the performance of cells and put the safety of the car at risk. A glycol-based cooling system is used to control temperature. Using a large number of small cells has an advantage over using a few giant cells.
Physics behind the inverter of electric cars
Electric cars use AC motors. The inverter in an electric vehicle converts DC from the battery to AC. But the motor required a three-phase current. Each phase should be a sin wave because of smooth variation between negative and positive.
To achieve the requirements, the inverter has to switch DC input back and forth very rapidly. It contains transistors to do that. But several types of transistors can be used for this operation. Tesla vehicles have silicon (Si) insulated gate bipolar transistor (IGBT). They are in EV inverters since 1980.
The inverter is not just converting current. It is a variable frequency drive (VFD). By reducing the frequency of the AC we can increase or decrease the rotational torque of the motor. That gives electric cars a huge advantage over gasoline cars because internal combustion engines can not generate useful propulsion torque over a very wide range of rotation speeds.
Modern electric cars can convert their kinetic energy to electricity and store it in the battery. This mechanism takes place, when you take your foot off the accelerator. The motor generates three phases of sinusoidal AC. The inverter converts it to DC and recharges the battery.
Electric cars have only one gear. They don’t have a multi-speed gearbox like gasoline cars because they don’t need to. Electric motors can provide a wide range of speeds with good power and torque. But that doesn’t mean they have transmission.
EV transmission is a very simple single-speed transmission system. It only has one purpose and that is speed reduction. Torque multiplication comes from that. EV transmission systems can do that with a single step. They don’t even need a reverse gear because we can change the direction of the motor by just reversing the power phase.
The second component in an EV gearbox is the differential. There are two types of differentials used in vehicle transmission systems. Open differential system and limited-slip differential system. However, the traction control problem is a major disadvantage of an open differential system. But EVs use an open differential system because it is more rugged and can carry more torque.
EV manufacturers came up with solutions for the traction problem. Selective breaking and cutting down the power supply can effectively overcome the problem. Power cut effective in an EV. Brands like Tesla use algorithms and controllers so they can exclude a complex set of mechanical hardware.
Physics behind the motor of electric cars
An electric motor in an EV converts electric energy to kinetic energy which is next converted into work. So, it is a crucial component of the car. There are two types of electric motors differentiated by the form of current they are required to operate. AC and DC. The reason behind EVs using AC motors is their efficiency.
Different manufacturers use different AC motors for their products. Tesla models s has a three-phase induction motor. But not all EVs are using AC motors either. The Tesla Model 3 has a permanent-magnet DC motor.
Induction and permanent magnet motors
An induction AC motor has two major parts. Stator and a rotor. The stator is the stationary part of the motor. Power input is given to the coil winding located at the stator. So, there will be a coil winding for each phase. Rotor, the rotating part of the motor is usually the shaft and a squirrel cage around it.
The rotation of an electric motor can be explained by Faraday’s laws of electromagnetic induction. When a three-phase current is flowing through windings, a rotating magnetic field is created. According to Faraday’s first law, whenever a conductor is placed in a varying magnetic field, an electromotive force is induced.
In an induction motor, this emf will be induced on the squirrel cage creating a current in the cage bars. SO, now it is a current-carrying loop inside a magnetic field. Following the Lawrence force law, an electromagnetic force should act on the squirrel cage causing it to rotate with the shaft.
The mechanism of the permanent-magnet DC motor in Tesla model 3 is quite different. It brings principles of SynRM and permanent magnet motors together. So, the tesla model 3 motor can provide higher torque at the start and also perform well at high speeds with no back EMF issue.
Additional technologies provided by most EV manufactures
Manufacturers can integrate many technologies into EVs because of their nature. Self-driving is one of the cool features that rarely comes with gasoline cars but is common in EVs. Self-driving is still not at its full potential, but the technology has been going through a lot of upgrades.
Artificial Intelligence made autonomous vehicles possible. It’s a process involved in algorithms, machine learning, and very powerful computational hardware. But for all those work the system needs data about the surrounding. So, EV manufacturers use sensors to collect information about the road, traffic, near objects, and the state of motion of the vehicle.
Early research for self-driving focused on LiDAR technology. But now leaders in the industry like Tesla openly criticize it. They are using cameras for better input.
|Camera||Maximum distance (m)|
|Rearward Looking Side Cameras||100|
|wide forward camera||60|
|main forward camera||150|
|narrow forward camera||250|
|Rear View Camera||50|
|Forward Looking Side Cameras||80|
Physics theories about electricity and electromagnetism are behind the success of electric cars. With the use of renewable energy for electricity generation, electric cars are environmentally friendly. They will be the cars of the future.