How does an Electric Vehicle work?

The EVs could be of several types (BEV, HEV, FCEV, PHEV) and all of them function differently but one common component in them is the battery. So, let us understand how a battery works. A battery does not store electricity but electrical energy in the chemicals inside it. It has 3 main components, 2 terminals or nodes made of different chemicals, typically metals, the anode and the cathode and the 3rd component is the electrolyte. The electrolyte separates both terminals and is present to put the different chemicals of the anode and cathode in contact with one another, in a way that the chemical potential can equilibrate.

During the discharge of electricity, the chemical on the anode releases electrons through the negative terminal in ions in the electrolyte. Meanwhile, at the positive terminal, the cathode accepts the electrons, completing the circuit for the flow of electrons, converting stored chemical energy into useful electrical energy. That’s what generates an electric current. It took a while before humans could improve the battery technology enough, that it could power an entire vehicle to travel practical distances. With time, we got there. From a single charge battery to a rechargeable, lead-acid battery to the lithium-ion battery.

These days the batteries in most HEVs and BEVs are lithium-ion batteries which look like these. 

lithium-ion battery from the BMW i3

The metal case of these batteries holds a long spiral, comprising of 3 thin sheets pressed together. Inside the case, these sheets are submerged in an organic solvent, often ether, that acts as the electrolyte. The outermost sheet is the negative electrode, which is made of carbon (C). The middle sheet is a separator, which is a very thin sheet of micro perforated plastic. And the innermost sheet is the positive electrode, which is made of Lithium Cobalt Oxide (LiCoO2).

When the battery charges, ions of lithium move through the electrolyte, from the positive electrode to the negative electrode and attach to the Carbon. During discharge, the lithium ions move back to the Lithium Cobalt Oxide form the Carbon. 

So basically, it works on the same principle as any other battery but these lithium-ion batteries can store a lot of electric energy as chemical energy and that has helped electric cars take a leap from being a novelty to being a reality. These batteries can be recharged over and over again. 

The lithium-ion batteries used in most of the EVs are quite similar to each other. Each cell contains about 4.2 volts and about 30 amps. The voltage in a battery is like a stored charge it carries and the amps measure capacity – indicates how quickly the energy can flow out of it. The voltage and amperage in a battery can be changed as per need. But if you increase the amperage and don’t increase the voltage adequately, you can run out of juice before it does any good to you. Wiring the cells together can increase the voltage, amperage or both, depending on whether they are wired in series or in parallel. If you wire 2 identical batteries together in a series, the voltage rating doubles while the amperage rating remains the same. Whereas, in a parallel the amperage rating doubles while the voltage rating remains the same. In an EV the batteries are wired in a combination of series and parallels to get the desired output. For instance, the Tesla Model S uses 7104 cells wired in a combination of series and parallels over 16 modules to get the output of 400 volts and 1500 amps.

How does the electric motor work?

Back in the 1800s anyone and everyone was an inventor playing around with electricity and the resulting currents. Soon these people realized that wrapping wires and sending current through them can generate a magnetic field. This tangible physical force generated by the magnets is what electric motors use to actuate motion. Most EVs today, like the Tesla Model S use the induction motor. The motor consists of 2 parts, the rotor and the stator. The rotor is a series of conduction bars, short-circuited by end rings. A 3 phase AC pulse is given to the stator. This alternating current produces a four-pull, rotating magnetic field (RMF). The electricity running through the stator induces current on the rotor’s metal bars. The rotating field of the stator causes movement in the now charged rotor. In an induction motor, the rotor is just behind the RMF. The speed of the rotor is determined by the frequency of the AC current through the stator. When you accelerate, you increase the frequency of the current. An inverter switches the direct current (DC) from the batteries to an alternating current (AC) to drive the motor. It sits right by the motor and can determine the frequency of the current, which determines the speed of the rotor and the amplitude of the current, which affects the power output of the rotor. The only points of contact in this are the bearings that keep the rotor in place. Since there’s no other contact between the rotor and the stator they don’t wear out that easily. Unlike a conventional engine, who’s usable torque dwells within a limited rev range usually up to 8000 RPM, an electric motor, like the one in a Tesla Model S can effectively produce a much higher force to a rev range up to 18000 RPM. So, there’s no need for shifting or torque convertors of any kind. 

Unlike conventional engines that convert up and down or side to side motion of the pistons to rotational movement, the induction motor produces exclusively rotational force, that means almost all of that can be turned into forward motion when the wheels hit the road. Now, the biggest concern after discharging all that energy and spinning the rotors at 18000 RPM, is heat. Thus, so most of the components, including the motor, the frequency drive and the battery are liquid-cooled so that they don’t overheat. Also, in most electric cars, the induction motor, when it’s not producing movement at the wheels can be spun by the wheels which makes it like the alternator in your car, recharging the lithium-ion battery. 

So, that’s the science behind these cars.