How EV Batteries Store Energy

An electric vehicle doesn’t burn fuel. Instead, it stores energy in a large battery and uses that energy to turn an electric motor. Most of today’s electric cars use lithium-ion batteries, the same family of technology found in mobile phones and laptops.

Cells, modules and packs

The battery in an electric car is not a single block. It is made up of many small units called cells, which are joined together into groups called modules, and then into one large battery pack. Joining cells together adds up the energy they can store and the power they can deliver.

What happens inside a cell

Each cell has two electrodes, kept apart by a thin barrier and surrounded by a liquid or gel called the electrolyte. When the battery is charged, lithium ions are moved from one electrode to the other and stored there, ready to be used. When the car is driven, the ions move back, and this releases energy. Electrons are forced to travel through the outside circuit, which includes the motor, and that flow of electrons is the electric current that turns the wheels.

So the energy is held in a chemical form, not as electricity itself.

Two units worth knowing

Battery capacity is measured in kilowatt-hours (kWh). You can think of it as the size of a fuel tank. Power, measured in kilowatts (kW), tells you how quickly energy can flow, a bit like how fast fuel can be pumped. All else being equal, a battery with a larger capacity can travel further, while higher power allows quicker acceleration or faster charging.

Keeping the pack healthy

A battery management system constantly monitors the voltage and temperature of the cells and keeps them within safe limits. Heat is the enemy of batteries. Very hot or very cold conditions can reduce performance, and over many years of use a battery slowly loses some of its capacity.

Why range changes

The distance an electric car can travel on a full charge is not fixed. It depends on speed, the weather, the load being carried, the road and the style of driving. Many electric cars also recover some energy while slowing down, by using the motor as a generator. This is called regenerative braking.

Key points

• Most electric car batteries are lithium-ion packs made of many small cells.
• Energy is stored chemically, as lithium ions move between two electrodes.
• kWh is the size of the ‘tank’; kW is how quickly energy can flow.
• Range varies with speed, temperature, load and driving style.