Basics Of Regenerative Braking

Regenerative braking is nothing new, it’s been around 
just about as long as there has been electric trolley 
cars and electric trains. Although, the car industry 
has only recently developed a regenerative braking 
system. Not that it hasn’t been around for a very 
long time in the automotive industry, it’s just only 
become a large part of the industry with the advent 
of the modern hybrid and electric car. 

Regenerative breaking takes the forward motion of the vehicle and turns it into electrical energy. This is called the “kinetic” energy. The law of energy states: “Energy cannot be created only transfered.” Thus, in standard friction brake systems that most cars and trucks use today the energy from applying the brakes is wasted into thin air as heat. Where in the regenerative brake systems that same energy is sent to the batteries and stored for later use. This is accomplished by the mechanical connection between the tires and the battery packs by way of the traction motors. You can visualize the regenerative braking as if it was an alternator on a typical mechanical engine. An accessory belt from the engine crankshaft turns the alternator and as it turns it produces the required voltage for the electrical systems. In the hybrid systems its “alternator”(electric motor) is bolted between the engine and transmission (this is the most common setup), or it can be mounted inside the transmission. 

With the electronic throttle closed (foot off the gas pedal and/or the brake pedal applied) and the vehicle still moving, the kinetic energy can be captured not only as stored energy for the battery, but to aide the friction braking system. This is really noticeable when on a long down hill coast in a vehicle with regenerative braking. You can actually feel the car slowing down under it’s own electrical power being produced by the forward movement of the car. Just like the traction motor of a trolley or train. 

Regenerative braking systems inherently work better at certain speeds than at others. In fact, they're most effective in stop-and-go driving situations. Slower speeds and battery usage go hand in hand. The Toyota Prius for example, has unique requirements for their brake system. On 2001-04 models, the hydraulic brakes are not used until the vehicle is below 7 mph unless the vehicle has to make a hard stop. In fact, on many light-braking events, the rear friction brakes will be lightly applied while the front may not be applied at all until the vehicle is at a full stop or nearly stopped. For this reason, it is normal for a hybrid to wear the rear brake pads two and three times faster than the front pads.

It’s common to see a hybrid with 70,000-100,000 miles on the odometer that still has the original set of front brake pads. This comes back to the earlier point about regenerative braking handling the majority of braking force. But this does not mean that other components, like hardware, rubber seals/boots, or the that brake pads can’t fail for other reasons other than worn beyond usable condition. Weather and time still play a major part to the overall condition of the components. Which means that it’s still a good practice to check the brake components during a routine inspection of the vehicle. Under average driving conditions, the pads may not reach operating temperatures, and corrosion can build up between the backing plate and friction material due to water intrusion and the various road salts and ice removal chemicals used across the country. This corrosion between the friction material and backing plate can cause the friction material to separate. 

Depending on the request for braking, the system electronically calculates and decides how to apply friction and regenerative braking to meet the demand. Light braking to a panic stop all will have different electronic calculations that various sensors as well as the ABS controller will watch for and direct the appropriate action to be taken. Not only are we seeing a more controlled braking system but a lot less wear on the friction brake components and how they interact with the driver.  

Which leads to a lot of new drivers that aren’t used to regenerative braking thinking there is something wrong. The technician may get a complaint that the brake pedal isn’t always in the same place or that they feel a vibration during certain braking conditions. This is basically normal operation on most of the regenerative braking systems. The vibration is usually the hydraulic motor for the ABS and the vibration can be felt through the brake pedal. The pedal grab point is still the same but the pedal travel can change somewhat depending on the distance and speed traveled before the brake pedal was applied.  

Although brake pad wear and brake pad replacement maybe few and far between when it comes to the regenerative braking system vehicles, it’s still a good idea to perform periodic maintenance checks on the entire braking systems. The older the vehicle is and the environment they are in can be just as detrimental to the brake system. A few basic checks will keep your customer on the road and more at ease to know their brake system is good to go...well...I mean good to stop. 

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