How Do Electric Cars Work? Batteries, Motors and the Missing Gearbox
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Lift the bonnet of an electric car and there is usually a small luggage compartment where the engine would be. The parts that move it are a slab of battery under the floor, a motor the size of a watermelon between the wheels, and a box of electronics that decides how much current to send, and between them they do what an engine, a gearbox, a clutch, an exhaust and a fuel system do in a petrol car, with a fraction of the moving parts. Electric cars are older than petrol ones, outsold them in 1900, and lost for a century to the energy density of oil; the battery is what changed.
The battery
The pack is thousands of lithium-ion cells, the same chemistry as a phone, wired in series and parallel into modules and laid flat under the cabin, where their weight lowers the car's centre of gravity. A typical family car carries 60 to 80 kilowatt-hours, enough to run a house for two or three days, at around 400 volts, and the pack weighs 400 to 500 kilograms, which is the electric car's basic handicap: a kilogram of petrol holds about fifty times the energy of a kilogram of battery, and the motor's efficiency, around 90 percent against an engine's 25 to 30, claws back only part of that. A battery management system watches every cell's voltage and temperature, balances them, and keeps the pack within the band in which it neither overheats nor degrades; most packs are liquid-cooled and are warmed in winter, and manufacturers guarantee them for eight years or 160,000 kilometres to retain at least 70 percent of capacity.
The motor and the missing gearbox
The motor is usually a permanent-magnet synchronous machine or an induction motor, and it turns electricity into torque with a few dozen moving parts against the several hundred in an engine. Two properties of an electric motor remove most of what a petrol car needs. It produces its full torque from a standstill, which is why electric cars leap away from lights and why they need no clutch; and it works efficiently across a range of speeds from zero to about 15,000 revolutions a minute, so a single fixed reduction gear connects it to the wheels and there is no gearbox to shift. The motor also runs backwards as a generator: lift off the accelerator and it slows the car by turning its momentum back into charge, regenerative braking, which recovers a fifth or more of the energy in stop-start driving and lets many electric cars be driven with one pedal.
Between the two
The battery supplies direct current and the motor needs alternating current whose frequency sets its speed, and the inverter between them, a box of silicon carbide switches, chops the DC into a three-phase AC waveform thousands of times a second at exactly the frequency and current the driver's pedal demands. The main parts of the drivetrain:
- •Battery pack: lithium-ion cells, modules, cooling plates and a management system
- •Inverter: converts the pack's DC to variable-frequency AC for the motor and back again when braking
- •Motor: one per driven axle, or one per wheel in some designs, with a fixed reduction gear
- •Onboard charger: converts household AC to DC to fill the pack; DC fast chargers bypass it
- •DC-DC converter: steps the 400 volts down to 12 for the lights, screens and locks
Range and charging
Range is the pack's energy divided by consumption, and consumption depends on speed above all: air resistance rises with the square of speed, so a car that manages 450 kilometres in town may manage 300 on a motorway, and less in winter, when heating the cabin and the battery both draw from the same pack. Charging comes in three speeds. A household socket adds about 10 kilometres of range an hour; a home wallbox at 7 kilowatts fills a pack overnight; and a public DC fast charger at 150 to 350 kilowatts takes a car from 10 to 80 percent in twenty to thirty minutes, slowing above 80 to protect the cells, which is why long trips are planned around that band. Most owners charge at home most of the time and use the fast chargers as petrol stations are used, occasionally.
Costs and the argument
An electric car costs more to buy, chiefly because of the battery, and less to run, because electricity is cheaper than petrol per kilometre and there are no oil changes, exhausts, clutches or timing belts; the crossover in total cost has arrived in most European countries for a car kept five years or more. On emissions, building the battery produces more carbon dioxide than building an engine, and the car pays that back within one to two years of driving on a typical European grid, sooner where the electricity is clean and later where it is coal; over its life an electric car in Europe emits roughly a third to a half of a petrol car's total. The remaining arguments are about the lithium, cobalt and nickel in the packs, the mining that supplies them and the recycling that is only now being built at scale, and about whether the grid can charge a whole country's cars at once, which is a question of when they charge rather than whether.
The takeaway
An electric car stores energy in a lithium-ion battery under the floor, turns it into motion with a motor that delivers full torque from rest and needs no gearbox or clutch, and uses an inverter to convert the battery's direct current into the alternating current that sets the motor's speed, running the motor backwards as a generator to recover energy when braking. Range falls with speed and cold, charging runs from a slow household socket to a half-hour fast charger, and the car costs more to buy and less to run and emit.