Why does braking charge the battery?
A long descent of about 300 metres. The driver lifts off the accelerator and the car slows down without touching the brake pedal.
The power display swings to the charging side. At the bottom of the hill, the battery shows 1% more than at the top.
The questionWhere does that energy come from, and why only 1%?
Driving: energy flows from the battery to the wheels.
Direct current leaves the battery.
Slowing down: the same chain, run backwards.
The moving car now drives the motor instead of the other way round.
Switch the direction, then select each step.
the example car with two occupants
difference in height
mass times gravity times height
1% of 70 kWh; air, tyres and conversion took the rest
2,000 kg × 9.81 m/s² × 300 m = 5.9 MJ = 1.6 kWh. Part of it was spent against air drag and rolling resistance on the way down, part was lost in motor, inverter and battery.
Speed holds less than height
Slowing down from speed returns less than most people expect. A 2,000 kg car at 50 km/h carries about 54 Wh of motion energy; at 130 km/h about 0.36 kWh. That is enough for a few hundred metres, or a little over two kilometres.
therefore matters most where a car slows down often or descends: in town traffic and in the hills. On a flat motorway at constant speed, there is little to recover.
The brake pedal still matters
- Lifting off lets the motor brake and recover energy. Many cars let the driver choose how strongly, up to driving with one pedal.
- The brake pedal blends both: the motor first, the friction brakes added for strong braking.
- Friction brakes take over completely when the battery cannot accept more, for example when it is full.
Check your understanding
3 questions. Answer all of them to complete this stop. Each answer explains itself.
During regeneration, what does the electric motor do?
Turned by the wheels, the motor generates current. The resistance it creates is what slows the car.
Where does regeneration return the most energy?
Energy comes back only when the car loses height or speed. A full battery cannot take any in.
The car descended 300 m and the battery gained 1%. Why not more?
The climb stored about 1.6 kWh. Driving down at speed costs energy too, and every conversion loses a share.
- Regenerative braking Slowing down with the motor working as a generator, which returns part of the car's energy of motion or height to the battery. Glossary