Regenerative Braking

Before turning to specific vehicle architectures, we pause to consider regenerative braking, which plays an important role in increasing hybrid vehicle efficiency. A key benefit of the vehicle strategies considered in this chapter is that kinetic and potential energy can be recovered during braking. Clearly, energy is required to accelerate a vehicle to a higher speed or to drive up a hill. It makes sense to try to recover and store this energy when stopping the vehicle or when traveling downhill. For highway driving, the braking energy may be a small portion of the total traction energy. However, for urban driving that includes frequent starts and stops, the energy dissipated in braking may be more than 50% of the total traction energy. This is consistent with the data in Figure 12.3 that shows many periods of deceleration. Thus, there is a strong incentive to recover this energy. The kinetic energy of the vehicle is converted to electrical energy with a generator (the same motor that is used to propel the vehicle can be controlled to operate as a generator) and this energy is stored in a battery or capacitor.

ILLUSTRATION 12.1
A 1600 kg passenger hybrid-electric vehicle is moving at 60 km·h−1. The battery has a nominal voltage of 300 V and a capacity of 8 kWh.

To stop the vehicle in 3 seconds, determine the power available in principle from regenerative braking. Assume that the power is constant and neglect rolling resistance and aerodynamic drag during braking.
The kinetic energy is .

This energy must be dissipated or captured in the time that it takes to stop the vehicle (3 seconds). Therefore, the power is

What is the current through the generator? Express the current in terms of both amperes and the battery C-rate (see Chapter 8).
The current is simply the power divided by the battery voltage, .

The capacity in A·h of the battery is Therefore, by definition, the 1C rate is 26.7 A. The C-rate through the generator is .

What is the change in the battery SOC?

Illustration 12.1 provides a simple calculation assuming a constant rate of energy recovery. Alternatively, we could maintain a constant rate of deceleration. Regardless, we can make some general observations. First, note that the power generated can be large during periods of hard braking. Stopping in 3 seconds from 60 km·h−1 is not particularly aggressive; nonetheless, the power that must be absorbed was large compared to the size of the battery—the C-rate is nearly 10. This power must pass through the electric motor/generator. Often, the motor torque from braking is larger than the maximum torque allowed on the motor/generator, and the extra cost, weight, and volume of the larger motor needed to recover all of the energy may not be warranted from a system perspective. Similarly, the current may be larger than that which the battery can accept without exceeding the maximum charge voltage for the cells. This possibility is evident from the relatively high C-rate at which the battery must be charged in order to store the energy from braking. Finally, we can note that in most cases the energy from a single braking event is small compared to the total energy stored in the battery.