Regenerative Braking: An Energy Balance

Automotive Engineering · CHAPTER 5 / 5

Level: introductory undergraduate · Study session: 15–20 minutes including practice.

Learning outcome: Estimate recoverable kinetic energy with explicit limits.

Before you begin: Complete earlier chapters in this discipline; revise algebra and units as needed.

1. Understand the idea

Regeneration converts some kinetic energy into stored electrical energy. The theoretical kinetic-energy reduction is an upper bound before conversion losses and operating constraints. Battery state of charge, temperature, motor capability and tyre grip can restrict recovery. Friction brakes remain necessary.

2. Write the model

ΔEk = ½m(v1²−v2²); Erecovered = ηΔEk

Define the symbols from the worked case before substituting values. Keep units consistent, record the assumptions and round only after the calculation.

3. Follow a worked example

Stopping a 1000 kg vehicle from 20 m/s removes 200 kJ of kinetic energy. At an illustrative 60% recovery fraction, stored energy is 120 kJ or 0.0333 kWh.

4. Explore the model

Figure: calculated teaching cases; straight segments connect sampled points, not experimental observations. Read exact values below.

Illustrative scenario sweep — synthetic educational data
Initial speed (m/s)	Kinetic energy (kJ)	Recovered at 60% (kWh)
10	50	0.008333
20	200	0.033333
30	450	0.075

Sweep initial speed and calculate recoverable energy. Compare with battery capacity to put one braking event in context.

5. Reproduce it yourself

Copy the input cases into Excel or a notebook and name each quantity and unit.
Recalculate each result from the stated equation or rule. For qualitative cases, explain the state or partition decision instead.
Change one input, predict the direction of change, then calculate again.
Write one sentence explaining where the model stops being valid.

6. Check your understanding

How much is recovered at 50% of 200 kJ?

Reveal the answer and check your work

100 kJ, or about 0.0278 kWh.

Compare your result, units and reasoning. If they differ, return to the equation and test the worked case first.

7. Common mistake

Recovered energy cannot exceed the available energy removed from vehicle motion.

Further study and academic note

Explore the discipline reference and further learning resources. This is an original IIMEEE introductory teaching note with synthetic examples; it is not a peer-reviewed research result or a professional design specification. Figures were constructed from the listed teaching cases. Resource links support further study and do not imply endorsement.