How regenerative braking works

Understanding the process of changing one form of energy to another is basic physics. In that conversion, there is always a loss of some energy, mainly as heat. That heat generation is usually unwanted and makes the process inefficient. When you put any vehicle into gear and start to drive it, you are building up kinetic energy. That is the energy stored in a moving object. The heavier the vehicle and the faster it is moving, the more kinetic energy you have. Once you stop accelerating, unless you are going downhill, the vehicle will slow by itself. What is happening? The friction with the tires, outside air, the driveline (engine, transmission, wheel bearings, axles, etc.) is turning the kinetic energy into — guess what? More heat. When you step on the brake pedal (or the vehicle may do that for you if you are not paying attention — No texting while driving!), the brakes are designed to heat up quickly to replace most of the kinetic energy and bring the vehicle to a complete and safe stop. Unless you are pointing downhill, all the kinetic energy is now gone and the cycle repeats itself when you step on the accelerator.

Wheel End

If you remove a wheel to check the brakes on an xEV, it will look no different than a conventional car or truck. The wheels and axels that connect to the drive motor are the same wheels that will slow down the xEV in regenerative braking mode. As a rule, recuperative braking transitions to hydraulic friction at low speeds, typically between 3 to 7 mph as the kinetic energy left in the xEV is very low, so there is little energy to recycle. Plus, the electric motor would stop the vehicle in segments as the electromagnetic fields swapped poles.

Brake Work

A brake job on most xEVs must be done with high quality parts as there is typically no engine and exhaust noise to mask other noises. Pure EVs are always quiet. Everything you have been taught when replacing pads, shoes, drums, and discs applies to xEVs. You will have return visits from your xEV customers if the brakes are not repaired with the utmost care and the best parts. Even a slight squeal will annoy them.

Bleeding the Brakes

Once you start working on these regenerative brakes (many of you have already), the equipment you will need includes a good scan tool to “air bleed” the system, and some OEMs may require pressure introduced at the master cylinder reservoir. Removing trapped air in a brake-by-wire system is much like bleeding a four-channel ABS. If you only work on the wheel end, and you do not let the brake fluid run completely out of the master cylinder, then manual bleeding will work in most cases. If the braking problems are caused by the electronics, get your electrical wizard hat on.

Inverter Functioning as an AC to DC Processor

What is next is a brief overview of the high voltage generator part of regenerative braking. Anytime a permanent magnet (PM) motor/generator has the rotor spinning, it will generate voltage and current if it's connected to a circuit. As you have learned in automotive classes, the definition of watts is volts multiplied by amps. Watts is also a measurement of power. The basic principle of recuperative brakes is to switch the high voltage transistors (located on the inverter circuit board) on and off. These transistors were typically IGBT (Insulated Gate Bipolar Transistor) and lately some OEMs are using SiC MOSFET (Silicon Carbide Metal–Oxide–Semiconductor Field-Effect Transistor).

Using duty cycle, the current produced by the HV generator is controlled by these transistors so that the current heading to the HV battery pack is modulated indirectly with the brake pedal. The HV transistors also keep the voltage high enough to charge the HV battery. In its simplest form, the faster you charge the battery, the faster the xEV will slow down. The slower you put energy back into the battery pack, the slower the xEV will stop. How do we control the deceleration of the car or truck with just a brake pedal, while not using hydraulic pressure? It is the programming of the inverter circuit board that performs that function. With more on-time, more current flows and the xEV stops faster. With less on-time it will take longer to stop. This is assuming that there is no hydraulic function at that time in the braking event. This is pretty straightforward and logical, but there is more to it than that. By controlling the output of the generator, braking functions can be controlled. Your scan tool is also extremely valuable as it will get a lot of data into your head and hands quickly. A technician can never achieve great success without a great scan tool. In ACDC's latest book we go into more detail. We have dedicated an entire chapter to this subject if you want to know more.

How Brake Fluid Is Controlled

Looking at the ACDC HCU (hydraulic control unit) diagram, most OEMs use a two speed 12-volt motor to pressurize the brake fluid into an accumulator, #5 and #6 in the diagram. It will create this pressure before driving or will set a warning lamp if it fails to do so and set codes. The pressure is normally about 2,700 psi, #19. With that much pressure all we need now is to use it, but when?

The HCU has many solenoid valves. Some are on/off valves and others can be duty cycled. The reason for all the valves is to allow the fluid pressure to be precisely metered into each brake line for each wheel brake. Unlike ABS that uses a similar principle to release a locked wheel and allow steering, this system will brake each wheel when needed. It also can be used for traction control (TC), vehicle stability control (VSC), and automatic braking system (ABS). On more modern vehicles, this brake-by-wire system can be used with lane departure or lane assist, adaptive cruise control and automatic braking. To accomplish that, there is a lot going on, so most OEMs use a dedicated CAN bus. When the xEV is in READY, two valves will close, #9 and #10. These on/off valves block the two lines leaving the master cylinder that normally go to calipers or wheel cylinders. Once they are closed the brake fluid in the master cylinder cannot reach anything except a device called a stroke simulator, #4. The stroke simulator is there to fool your foot into thinking it is doing something. It also mimics a vacuum booster. Some “brake feel” is simulated better than others. If valve #8 is opened slowly (duty cycled) while you are stepping on the brake pedal when starting the xEV, it can feel like the booster is being slowly connected to a vacuum source. Some OEMs just open and close it. The new owner of a high voltage vehicle expects it to operate like their old carbon-era vehicle did.

Once the brake master computer has determined it has to use some hydraulic brakes for any reason, a message is sent on the CAN bus to activate one or more linear valves (#11 to #14) that are holding back the brake fluid that is under high pressure. These four linear valves allow the brake fluid to get to the wheels, if it is a four-wheel brake system, or fewer valves for a motorcycle. The exact pressure needed is achieved by “duty cycling” the proper linear valve. There are four more linear valves (#15 to #18) that act as pressure release valves, if the hydraulic pressure exceeds the requested pressure. There are four pressure sensors (#22 to #25), one for each wheel, that are incorporated into the HCU and are used for instant feedback in the event that the pressure to any wheel is not correct. The release valves will release the extra pressure back into the fluid reservoir (#3) when the sensor reads too high a pressure. If the hydraulic line pressure to a certain wheel is too low, the ECU will increase the duty cycle to #11, 12, 13, or 14, depending on the wheel pressure. There is one more pressure sensor that reports the accumulator pressure (#9) and will request a higher speed of the pump motor if the pressure is not reached in the timeframe it is programmed to look for.

So, we have mentioned nine linear valves (#8, #11 to #18) and two on/off valves (#9 and #10), and seven pressure sensors (#19 to #25). The brake pedal position sensor (#1) and the master cylinder (#2) provide three sensor readings that are critical to a brake-by-wire system. The master cylinder will produce hydraulic pressure that is measured by pressure sensors #20 and #21. The brake pedal has a sensor that is really two sensors in a single unit (#1) that check each other so the reading is perfect all the time.

As you can see the BMC has complete control of each wheel. It is just a matter of the Master Brake Computer knowing what the xEV is doing every microsecond and then making extremely fast decisions so the HV battery stays charged up and the wheels do the right thing.

If you work in a part of America where you see rust, service the calipers frequently. Our four year old Tesla had caliper issues already.

Thanks for studying more about your future.