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, power electronics and battery. Potential energy is mass times gravity times height. On a pass descent of the Kesselberg, ADAC measured that cars got back 35 to 50% of the energy they had used on the way up.
Speed holds less than height
Slowing down from speed returns less than most people expect. Motion energy is half the mass times the speed squared: a 2,000 kg car at 50 km/h carries about 54 Wh; at 130 km/h about 0.36 kWh. At this car's own consumption, that carries it about 630 metres at 50 km/h, or about 1.7 km at 130 km/h.
therefore matters most where a car slows down often or descends: in town traffic and in the hills. In ADAC's evaluation of measurements in a WLTP-based cycle, cars recovered 33% of the energy on average in town, 21% out of town and 12% on the motorway.
When the friction brakes take over
When the car slows down, the turning wheels drive the electric motor as a generator, and the energy goes back into the battery. A cold battery reduces regeneration, and a full one makes it much weaker or impossible; then the friction brakes do the work.
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
- LEIFIphysik (FWU Institut für Film und Bild): Grundwissen: Potentielle Energie, E = m · g · h. https://www.leifiphysik.de/mechanik/arbeit-energie-und-leistung/grundwissen/potentielle-energie. Checked 13 Oct 2026.
- ADAC: Rekuperation: So viel Bremsenergie holen Elektroautos wirklich zurück (town 33%, motorway 12%; cold or full battery). https://www.adac.de/rund-ums-fahrzeug/elektromobilitaet/rekuperation-elektroauto/. Checked 13 Oct 2026.
- LEIFIphysik (FWU Institut für Film und Bild): Grundwissen: Kinetische Energie, E = ½ · m · v². https://www.leifiphysik.de/mechanik/arbeit-energie-und-leistung/grundwissen/kinetische-energie. Checked 13 Oct 2026.
- ADAC: Rekuperation: So viel Bremsenergie holen Elektroautos wirklich zurück (town 33%, motorway 12%; cold or full battery). https://www.adac.de/rund-ums-fahrzeug/elektromobilitaet/rekuperation-elektroauto/. Checked 13 Oct 2026.
- ADAC: Rekuperation: So viel Bremsenergie holen Elektroautos wirklich zurück (town 33%, motorway 12%; cold or full battery). https://www.adac.de/rund-ums-fahrzeug/elektromobilitaet/rekuperation-elektroauto/. Checked 13 Oct 2026.
- ADAC: Rekuperation: So viel Bremsenergie holen Elektroautos wirklich zurück (town 33%, motorway 12%; cold or full battery). https://www.adac.de/rund-ums-fahrzeug/elektromobilitaet/rekuperation-elektroauto/. Checked 13 Oct 2026.