The battery stores energy, the motor uses it, and the onboard charger converts wall power into battery power

An electric car runs on a rechargeable lithium-ion battery pack that sits underneath the vehicle, usually between the wheels. When you plug the car in at home or at a public charger, electricity flows into an onboard charger — a box of electronics built into the car — which converts the alternating current (AC) from the wall into direct current (DC) that the battery can store. The battery then powers an electric motor that turns the wheels. There is no engine, no transmission fluid, no spark plugs. The whole system is simpler than a petrol car, which is why electric cars need less maintenance.

The battery is the heart of the system. A typical electric car battery contains thousands of individual cells wired together, similar to how multiple AA batteries might be connected in a flashlight, but far more sophisticated. These cells store chemical energy. When you press the accelerator, a power electronics controller — essentially a smart switch — releases that energy to the motor in the amount you need. When you brake, the motor can reverse and act as a generator, pushing energy back into the battery. This is called regenerative braking, and it is one reason electric cars are more efficient than petrol cars.

Key Takeaways

  • The battery pack stores electrical energy and sits underneath the car; the onboard charger converts wall electricity into the form the battery can store.
  • The electric motor draws power from the battery and turns the wheels; there is no engine or transmission.
  • Regenerative braking captures energy when you slow down and feeds it back into the battery, extending your range.
  • Charging speed depends on the charger type: a home outlet is slowest, a Level 2 home or public charger is moderate, and a DC fast charger is fastest.
  • The battery degrades very slowly over time but is designed to last the life of the car, and most manufacturers warranty it for eight years or more.

The battery pack: what it holds and how much energy it stores

The battery pack is a sealed unit containing hundreds or thousands of cylindrical or pouch-shaped cells. Each cell has a positive terminal (cathode), a negative terminal (anode), and a chemical compound between them. When you draw power, a chemical reaction inside each cell creates electrons that flow out through the negative terminal, through your car's motor, and back to the positive terminal. This flow of electrons is electrical current, and it is what moves your car.

Battery size is measured in kilowatt-hours (kWh). A small electric car might have a 40 kWh battery; a larger one might have 75 kWh or more. The bigger the battery, the more energy it can store and the farther you can drive on one charge. A 60 kWh battery in a typical sedan might give you a range of 200 to 250 miles, depending on the car's weight, aerodynamics, and how efficiently the motor uses energy. Cold weather reduces range because the battery chemistry slows down and the car uses extra energy to heat the cabin.

How the onboard charger converts wall power into battery power

When you plug an electric car into a wall outlet or public charger, you are connecting to an AC power source — the same alternating current that powers your home. The battery, however, needs DC power — direct current that flows in one direction. The onboard charger is the device that does this conversion. It is mounted inside the car, usually near the battery, and it contains transformers and rectifiers that change AC into DC and regulate the voltage and current so the battery charges safely.

The onboard charger also communicates with the charging station to confirm that the connection is safe before power flows. If something is wrong — a loose connection, a ground fault, or a temperature problem — the charger stops the flow. This is why you cannot straightforward wire a car battery to a wall outlet; the onboard charger is what makes charging safe and efficient. Different cars have onboard chargers of different sizes, measured in kilowatts (kW). A 7 kW charger is common in home installations; a 11 kW charger is faster. The bigger the charger, the more power it can accept from the wall, and the faster the battery fills.

The electric motor and power electronics controller

The electric motor is fundamentally different from a petrol engine. It has no cylinders, no fuel injection, no combustion. Instead, it uses the interaction between magnetic fields to create rotational force. When you press the accelerator, the power electronics controller — a computer that manages the flow of electricity — sends current through coils of wire inside the motor. These coils create a magnetic field that interacts with permanent magnets, causing the motor shaft to spin. The harder you press the accelerator, the more current the controller sends, and the faster the motor spins.

Most electric cars have a single motor, though some high-performance models have two — one for each axle — allowing independent control of front and rear wheels. The motor is connected directly to the wheels through a single-speed transmission (no gear shifting needed) or through a differential that distributes power evenly. Because the motor produces maximum torque when ready — unlike a petrol engine that needs to rev up — electric cars feel quick off the line even if they are not the fastest at high speed.

Regenerative braking: capturing energy when you slow down

When you lift off the accelerator or press the brake pedal, the motor reverses its role and becomes a generator. Instead of the battery pushing current through the motor to make it spin, the spinning wheels push current back through the motor to the battery. This is regenerative braking, and it is one of the biggest efficiency gains in electric cars. A petrol car wastes the kinetic energy of braking as heat in the brake pads; an electric car captures some of that energy and stores it back in the battery.

Regenerative braking does not replace friction braking entirely. The car has traditional hydraulic brakes as well, and the power electronics controller decides how much braking comes from regeneration and how much from friction. In city driving with frequent stops, regenerative braking can recover 10 to 20 percent of the energy you would otherwise lose. This is why electric cars are more efficient in stop-and-go traffic than on the motorway, where you coast more and brake less often.

The three types of chargers and how fast they refill the battery

Charging speed depends on the charger type and the onboard charger capacity. A Level 1 charger is a standard 120-volt household outlet (in North America) or 230-volt outlet (in Europe). It is the slowest option, adding roughly 2 to 5 miles of range per hour. A car with a depleted 60 kWh battery might take 24 hours or more to fully charge on Level 1. Most owners use this only as a backup.

A Level 2 charger is a 240-volt installation, either at home or at a public charging station. It delivers 7 to 11 kW of power and adds 25 to 30 miles of range per hour, depending on the car's onboard charger size. A full charge typically takes 8 to 12 hours. This is the most common home charging setup and is sufficient for daily driving because most people drive less than 50 miles per day.

A DC fast charger (also called a Level 3 charger) bypasses the onboard charger and sends DC power directly to the battery. These are found at public stations and deliver 50 to 350 kW of power, adding 200 miles of range in 20 to 30 minutes on newer cars. Fast chargers are useful for road trips but are slower and more expensive than home charging, so most owners rely on them only occasionally.

Battery degradation and how long the battery actually lasts

Lithium-ion batteries degrade over time and with use. Each charge cycle — from full to empty and back — causes tiny chemical changes inside the cells. After several years, the battery holds slightly less charge than it did when new. A car that could drive 250 miles on a full charge might drive 240 miles after five years, and 230 miles after ten years. The degradation is gradual and usually amounts to 2 to 3 percent per year in the first few years, then slows.

Most electric car manufacturers warranty the battery for eight years or 100,000 miles, whichever comes first. Some offer longer warranties. The warranty typically covers degradation beyond a certain threshold — often 70 percent of original capacity — rather than covering any loss at all. In practice, most batteries last well beyond the warranty period. A battery that reaches 70 percent capacity is still usable; it just has less range. Even after the car is retired, the battery can be repurposed for stationary energy storage, so the useful life extends beyond the vehicle itself.

Cold weather accelerates degradation slightly and reduces range temporarily, but does not cause permanent damage. Keeping the battery between 20 and 80 percent charge most of the time, rather than regularly draining it to zero or charging it to 100 percent, can extend battery life. Some cars have settings to limit charging to 80 percent for daily use.

Frequently Asked Questions

Can I charge an electric car at a regular household outlet?

Yes, but it is very slow. A standard 120-volt outlet (North America) or 230-volt outlet (Europe) will add only 2 to 5 miles of range per hour. A full charge can take 24 hours or longer. Most owners install a Level 2 charger at home for practical daily charging.

What happens to the battery if I leave the car parked for months?

The battery will slowly discharge, losing a few percent of charge per month. When you return to the car, you can recharge it normally. Leaving a battery completely empty or completely full for extended periods is harder on it than keeping it at a moderate charge level, so some cars have a storage mode that maintains the battery at around 50 percent.

Does cold weather permanently damage the battery?

No. Cold slows the chemical reactions inside the battery, reducing range and charging speed temporarily. Once the car warms up, the battery recovers. Repeated exposure to extreme cold may accelerate long-term degradation slightly, but normal winter use does not cause permanent harm.

Can the battery catch fire or explode?

Modern electric car batteries have multiple safety systems: fuses, thermal sensors, and a battery management computer that monitors temperature and voltage constantly. A short circuit or physical damage can cause a fire, but this is rare. When it does happen, the fire burns longer than a petrol fire but starts the same way — from a breach in the battery case. Manufacturer testing and real-world data show electric cars are not more prone to fire than petrol cars.

What does it cost to replace a battery if it fails?

Battery replacement costs vary widely depending on the car and the battery size. A replacement battery can cost between £4,000 and £15,000 or more, but most owners never pay this because the battery is warranted for eight years or longer and degradation is gradual, not sudden failure. If the battery fails within the warranty period, the manufacturer covers the replacement.