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From Brake pads to Batteries : Regenerative Braking

As the world moves towards a greener future, the need for electric vehicles will only keep on growing. As we move away from the sweet sounds of an IC engine and move on to the silent electric motors, the entire powertrain of the vehicle changes. The goal should be to optimize the efficiency and performance of any powertrain. A good powertrain in the case of a combustion car is usually the one which optimizes the mileage and peak power output, but when it comes to EVs, it narrows down to two things, maximizing the performance and range. In high-performance EVs, such as the RFR23, you would want to enhance the performance. This could be done by getting better components, but the easiest way is to cut down on weight. Most of the weight of an EV comes from its battery pack. To cut down on weight we can’t just reduce the battery capacity and decrease the range. After designing a battery pack, we need to be able to get the most energy out of it before it discharges. Now, how do we do that? What if I told you there’s a way to charge the battery while the car is moving and use this energy later on? This is the essence of regenerative braking aka regen.

So how do we go about this? Let’s say the car is accelerating, and the motor is producing some torque which gets transmitted to the wheels. Now let’s say the driver wants to brake. He takes his foot off the accelerator pedal and hits the brakes. Now the motor is no longer doing any work, but since the vehicle is moving, the rolling of the wheels would keep the rotor of the motor rotating. Now this is where the cool stuff comes in. We know that a potential difference is induced between the ends of a conductor when it's subjected to varying magnetic fields. This is what happens when the rotor rotates, the magnets’ orientation with respect to the coils on the stator changes, hence, changing the flux and inducing a back EMF. Long story short, the motor behaves as a generator and this current can be used to charge the accumulator. Now, the battery getting charged isn’t the only thing occurring. According to Lenz’s law, the induced current will oppose the change in flux which induced it. So as the rotor rotates, the induced current in the stator coils will try to slow the rotor down, effectively providing a braking torque countering the rotation of the wheel and slowing down the car. So not only can we charge the battery but also get additional braking force from regenerative braking.

Now coming to the interesting part? How do we implement this in our car? let’s go over the entire control flow for the regenerative braking process. For this, we need an overview of the braking system of our car. The car has 4 hydraulically actuated brakes. Each brake calliper is connected to the brake pedal via a circuit of brake fluid. Between the front circuit and the rear circuits, there exists a valve, called the proportioning valve which decreases the fluid pressure by a factor of K, called the brake bias. So let’s say you actuate the brakes, if the mechanical braking force applied by the caliper at the front two tires is F, the rear will be F/K. The normal forces will be different during different deceleration rates and will be different at rear and front tires owing to load transfer but let’s not get into more vehicle dynamics, that is for some other day. Now the braking torque due to regen would be the difference in the maximum braking torque and max available braking torque . In any motor, the torque is proportional to the current with the proportionality constant called the torque constant. So when the rotor rotates, it produces a sinusoidal current waveform along the phases owing to the distributed windings in our PMSM motor. The inverter has special algorithms to implement regen braking and invert this 3-phase current to DC . Now, this DC voltage may be lower than the accumulator voltage, but for charging the voltage should be almost equal to the accumulator’s. For this, you have a DC-DC step-up converter to increase the voltage, and this in turn will decrease the current. Your motor controller should be able to handle this current, and the cells in your battery pack should be rated for this charging current. In our case, our motor controller has a continuous current rating of 200Arms, which corresponds to a peak torque of about 150 Nm, and the cells are rated for 15A of current for charging.

Max rear braking force = μ NrearMax

front braking force =μ NfrontAvailable

rear braking force = K μ NfrontRegen torque = max rear braking torqueavailable rear braking torque Regen torque=μ ∗(Nrear ​−K Nfront​)∗ Rtyre

So, when and where can we implement regen? Every corner or every time where we brake or lift the foot off the pedal at high speeds? The answer is no. For this, we need to take a small dive into cell chemistry. When we use high currents to charge lithium-ion cells, it accelerates aging and decreases cell capacity. This happens due to lithium plating on the graphite layers. If you use it at low speeds and between small braking and accelerating cycles, it will lead to something called Micro cycling which is basically charging and discharging in small and quick cycles. So, it’s necessary to specify the operating range of speeds and accelerations for regenerative braking. For example, let’s say you apply regen at very high speeds, you won’t actually be achieving enough braking torque, owing to lesser torque.

Mercedes AMG Petronas F1 Team’s MGU-k unit

Electric cars are not the only ones utilizing this technology, even combustion cars utilize this technology. They do this with the use of a KERS or kinetic energy recovery system, where a generator is coupled to the crankshaft using a flywheel. In formula one – this is called the MGU-k, called the motor generator unit - kinetic, where the stored energy can be used to produce short bursts of extra power necessary for overtakes. Regenerative braking is a giant leap in the world of green mobility and has already spread its presence to different sectors of automotion, and as groups like us push the boundaries in research and testing, it’s only going to get better.

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