Energy takes another route

During propulsion energy goes toward the wheels. What changes when the motor generates during deceleration?

A battery-electric vehicle stores electrical energy in a battery and uses an electric motor for propulsion. A hybrid combines a combustion engine with electric drive. A plug-in hybrid can also recharge its battery from an external supply.

An inverter controls electrical power for an AC traction motor. Under suitable braking conditions, the motor can act as a generator and return some motion energy to the battery. Friction brakes remain part of the vehicle.

Regeneration is limited by battery charge state, temperature, grip and system power limits. A conventional hybrid generally does not plug in; a plug-in hybrid does. The flow shown here is a generic teaching layout, not a BMW high-voltage service diagram.

1. 1
   Battery

   Stored electrical energy is available for propulsion.

2. 2
   Inverter

   Power electronics control energy delivery to the traction motor.

3. 3
   Motor

   The motor converts electrical power into mechanical torque at the wheels.

4. 4
   Regeneration

   During suitable deceleration, energy flows back toward the battery. Not all energy is recovered.

Think it through

Reverse the propulsion arrows for regeneration. Now imagine a nearly full battery: why would the car still need friction brakes?

Show the reasoning

Energy can flow from the wheels through the generating motor toward the battery. A nearly full battery has less ability to accept energy, so regeneration may be limited and friction braking remains necessary.