Formula E cars do not simply waste all braking energy as heat. During regenerative braking, part of the car's kinetic energy is converted back into electrical energy and stored in the battery. This matters because every joule recovered can help extend range, improve lap strategy, or allow more aggressive acceleration later.
Regeneration also reduces the load on friction brakes, which changes how engineers design braking systems for racing.
When the driver brakes, the electric motor can operate as a generator instead of as a motor. The spinning wheels drive the motor-generator, creating electrical power that flows through the inverter and back into the battery pack. The braking force comes from electromagnetic resistance, while friction brakes handle extra braking demand or low-speed stops.
In Formula E, energy recovery is a major part of the race energy budget, so teams balance speed, braking points, battery limits, and tire grip.
Key Facts
- Kinetic energy of the car is KE = 1/2 mv^2.
- Regenerative braking converts some kinetic energy into electrical energy stored in the battery.
- Power is the rate of energy transfer: P = E/t.
- Braking force does negative work on the car: W = Fd when force acts along the displacement, and braking work removes kinetic energy.
- Recovered energy is limited by efficiency: E_recovered = η ΔKE, where η is less than 1.
- Total braking torque can come from both regeneration and friction brakes: τ_total = τ_regen + τ_friction.
Vocabulary
- Regenerative braking
- A braking method that converts part of a vehicle's kinetic energy into electrical energy instead of losing it all as heat.
- Motor-generator
- An electric machine that can use electricity to spin the wheels or use wheel rotation to generate electricity.
- Inverter
- An electronic device that controls electrical power flow between the battery and motor by converting between direct current and alternating current.
- Kinetic energy
- The energy an object has because it is moving, calculated with KE = 1/2 mv^2.
- Energy budget
- The planned amount of energy available, used, and recovered during a race or driving event.
Common Mistakes to Avoid
- Assuming regenerative braking recovers all braking energy is wrong because losses occur in the motor, inverter, battery, tires, and wiring.
- Treating regen braking and friction braking as the same thing is wrong because regen produces electrical energy while friction brakes mainly convert kinetic energy into heat.
- Forgetting that kinetic energy depends on speed squared is wrong because doubling speed gives four times the kinetic energy to remove or potentially recover.
- Ignoring battery limits during regen is wrong because the battery can only accept a certain charging power, so extra braking must be handled by friction brakes.
Practice Questions
- 1 A 900 kg Formula E car slows from 50 m/s to 30 m/s. How much kinetic energy is removed from the car?
- 2 If 65 percent of the removed kinetic energy from the first problem is recovered, how much electrical energy is stored in the battery?
- 3 Explain why a Formula E car still needs friction brakes even if it has strong regenerative braking.