How can braking return energy to a battery?

Regenerative braking transfers part of a moving vehicle's kinetic energy into electrical storage. The wheels drive an electrical machine in generating operation. Its electromagnetic torque opposes motion, slowing the vehicle while electrical energy flows through power electronics toward the battery.

This does not require the wheels to rotate backward. What changes is the direction of energy transfer: the machine receives mechanical energy rather than supplying driving work. Conventional friction braking instead transfers much of the vehicle's lost kinetic energy into thermal energy.

For a 1,000-kilogram vehicle slowing from 20 to 10 meters per second, the translational kinetic-energy decrease is ½m(vi² − vf²) = 150 kilojoules. The illustration assumes that amount reaches the regeneration chain before its specified conversion losses. Its generator and converter pass on 80 percent, giving 120 kilojoules of electrical energy.

A supplied storage efficiency of 75 percent then stores 90 kilojoules. The combined efficiency is 0.8 × 0.75 = 0.6, not the sum of the percentages. Sixty kilojoules of the selected decrease is not stored, preserving the full 150-kilojoule account.

Actual recovery is constrained by available braking torque, battery acceptance, power limits and other losses. The stated efficiencies are explanatory inputs, not a vehicle specification. The mechanism can recover some previously supplied motion energy; it cannot refill a battery without drawing that energy from the vehicle or another source.