Qualities of a Regenerative Braking System

Regenerative braking is an essential part of making any vehicle more fuel efficient. The typical (non-regenerative) braking action involves stopping or slowing forward motion through the application of friction pads to the wheels of the vehicle. Unfortunately, this means that the kinetic energy of the moving vehicle is converted into heat and lost. Regenerative braking seeks to convert some or all of that kinetic energy into potential energy, which can then be used to propel the vehicle forward again. The stored energy can either perform this propulsion by itself, or be used to assist the main powerplant (an internal combustion engine, for example) during the take-off process. In either case, the use of regenerative braking leads to increased efficiency.

This is primarily due to the fact that power requirements are greatest during takeoff, as the powerplant has to overcome the static inertia of the vehicle. By diverting some or all of that task to the regenerative braking system, the primary engine is put under less load, resulting in more efficient operation. Additionally, a vehicle’s powerplant is, in general, far larger than is necessary to accomplish its function for the majority of its lifetime – keeping the vehicle moving by counteracting losses due to kinetic friction.

Except in extreme cases (refuse haulers, transit buses) most vehicles spend a vast majority of their time cruising, which generally requires a minimum of power. However, because the engine must be made large enough to get the vehicle moving from a stop, all vehicles are saddled with an engine that is much heavier, larger, and less efficient than it needs to be. Regenerative braking can alleviate this restriction by removing some of the responsibility of initial take-off from the engine and applying power directly to the drivetrain, resulting in a smaller, lighter, and more efficient engine. It also implies that, provided the capacity of the regenerative braking system is large enough, the stored energy can also provide “boost power” to the drivetrain during, for example, passing situations or emergencies.

In order to fulfill its function in the most optimal manner possible, a regenerative braking system (RBS) needs to fulfill certain requirements.

1) It should be lightweight, as any added vehicle weight will detract from any performance or efficiency gains. Additionally, light weight also simplifies retrofitting the RBS onto an existing vehicle platform – while such a retrofit would not see the efficiency gains of a ground-up design which incorporates the RBS along with the resultant smaller engine, it is still possible for an RBS to have a positive effect on vehicle efficiency. The RBS should also have enough energy capacity to capture all of the available kinetic energy. The single parameter which relates to both of those criteria is the energy density – the available energy storage capacity per unit mass or volume. The energy density of the RBS has a profound effect on its usability for different vehicle platforms. As an example, consider an automobile of average mass (1500 kg), moving at 60 kph (36 mph) – this translates into a little over 208 kJ of energy.

Contrast that to a large garbage truck (13000 kg) moving at 30 kph (18 mph), which has around 450 kJ of kinetic energy. As the average speed of a garbage truck during its daily routine is much slower than that of a passenger car, this is not an unfair comparison. What is immediately apparent is that an RBS which has an energy density of only 1 kJ/kg is sufficient to serve the needs of the garbage truck (implying a total RBS weight of ~ 450 kg – less than 0.5% of the truck weight), that RBS would be useless for the passenger car (as it would weigh over 200 kg – almost 14% of car weight). The added mass of the RBS would nullify any benefit gained from regenerative braking. When considering the energy density of the RBS, it is important to include not only the energy density of the storage material, but also any additional weight in ancillary components required to transmit energy to and from the energy storage material. These additional components add mass/weight without providing any direct energy storage capacity, thus de-rating the total energy density of the RBS.

2) The RBS should also be efficient, in both the absorption and release of energy. The benefits of higher RBS efficiency include: more delivered energy during take-off operations (resulting in less engine load), more reserve energy for passing/emergency situations, and a smaller overall capacity/size of the RBS. The last point comes about because inefficiencies must be taken into account when designing the capacity and size of the RBS. Let us consider the refuse hauler case – in order to accelerate the hauler back up to 30 kph, 450 kJ must be delivered after considering all loss effects. Excluding other loss factors (such as tire rolling resistance) an RBS which is 90% efficient would need to have a capacity of 500 kJ – a 40% efficient unit would need over 1100 kJ. Since losses such as friction are intrinsic to the vehicle platform, it is vital that the RBS be as efficient as possible. One means of accomplishing this is to ensure that minimal energy conversion operations (e.g. mechanical kinetic energy to electrical potential energy) are performed.

3) Related to the efficiency of the RBS is its power density in both charging and discharging operations. As its name implies, power density refers to the amount of power that can be delivered per unit mass or volume. The charge and discharge power densities need not be equivalent; indeed, in most cases they are not the same. For instance, a conventional electrochemical battery is typically only capable of being charged at 1/10th its discharge rate. If the power density is not sufficiently high in the deceleration case, some fraction of the input energy is wasted and the total energy absorbed is less than the maximum amount available. Conversely, if the power density is too low during acceleration, additional power is required from the engine, lowering efficiency. It should be obvious that the most power is required at the beginning of each of the acceleration/deceleration actions, as this is when the inertia (resistance to change in velocity) is highest.

4) Another requirement for an effective RBS is its cycle life – that is, the number of charge/discharge cycles it can perform before a noticeable degradation in performance occurs. Higher cycle life implies that the RBS will be able to perform its function for a longer period of time before needing reconditioning or replacement.

5) For the most optimal performance, the RBS should also have high shelf life – that is, once charged, it should be able to maintain its charge for a long period of time without undue loss. While this is not truly necessary during most operations (the recovered braking energy is only held for long enough to assist in the next acceleration event), it is of vital importance in one instance. When the vehicle is stopped for a long period of time (for instance, at the end of the work day), that final braking event provides energy to the RBS.

This stored energy could then be used when the vehicle is started again (for instance, the next day) to provide acceleration, provided it is held with little or no loss. This is a subtle, but important consequence of shelf life – it plays a role in the ultimate size and power of the primary powerplant. Unless the RBS can maintain its stored energy (or have an independent method of being recharged during a long rest), the primary powerplant will still need to be sized to accelerate the vehicle from a dead stop. So while an RBS with low shelf life will improve efficiency once the vehicle is moving (by reducing the load on the primary powerplant), it will not enable a reduction in overall engine size, which is the most effective means of improving overall engine efficiency.

6) The RBS should also possess temperature stability, at least in the range of temperatures likely to be encountered by the vehicle. Fluctuations in the energy or power density of the RBS as a result of changing temperatures will require that the RBS be oversized with respect to both energy and power capacity – this again increases the size and weight of the RBS, reducing its effectiveness.

7) The RBS should also be durable and reliable, because of the intense stresses associated with the deceleration/acceleration events. These are when the forces on the vehicle are the greatest, and comprise the majority of the RBS’s active life cycle. Therefore, there should be intrinsic durability to the design which allows for repeated and reliable actuation under these most stressing conditions.

8) Finally, the RBS should be effective regardless of the specifics of primary propulsion – it should be compatible regardless of whether the vehicle has a gasoline, diesel, electric, or some other powerplant. This either implies that it has a design which makes the mechanical energy of acceleration and deceleration easily converted into the type of the main powerplant, or that its operation is independent of the main powerplant to such a degree that the two can coexist on the same platform. For instance, an RBS that takes the energy from braking and converts it into heat is not useful unless that heat energy can then be used to accelerate the vehicle or otherwise reduce the load of the primary powerplant during acceleration.

Electric Car Components

You have decided to build an electric car. There are a wide variety of electric car components available. Understanding these components and their specific purposes is the first part of building your own. Below is a list of the most common parts that the home electric car builder will need to build a car that meets the needs of the average driver.

Electric Motor

Every electric car needs a motor. Electric motors vary in shape and size, weight and price. They can use AC or DC electricity. A budget builder may choose to use an electric motor from an old forklift or elevator system. There are also lots of electric car-specific motors available for purchase alone or as part of a kit. You will need to choose a motor that will suit your needs for performance and budget.

Motor Controller

The purpose of the motor controller is to adjust the speed at which the motor spins. If 120V were applied directly to an electric motor for example, it would run at full speed. There needs to be a means of adjusting the output of the motor and this is precisely what the motor controller is for. It allows the motor to run at any speed between zero rpm and its max rpm. This part can also be salvaged either from a forklift or golf cart.

Throttle Pot Box

A pot box is a small part that connects to your stock throttle cable. When you push on your throttle, the pot box sends a signal corresponding to the amount of pressure you’re putting on the pedal to the controller which then sends the proper power to the motor.

Adapter Plate

The adapter plate mates the electric motor to a stock transmission. These can be bought for any commonly converted vehicle. Most EV-specific motors have a standard bolt pattern so most adapter plates will work with most motors. If you use a motor from a forklift you will need to have an adapter plate custom built or of course if you’re a decent fabricator you can always do this yourself.

Contactor

This is basically a high-voltage relay. It connects your battery pack to the controller when you turn on the key.

Fuse

A fuse will blow and cut power when too much amperage is drawn.

Manual Switch

There needs to be one (or more) manual disconnects for the main battery pack. This way if all else fails you can manually disconnect the power and safely stop the vehicle.

Batteries

There are many different types of batteries available. The type of batteries that you choose will affect your performance and range.

Charger

There are many different types of chargers available and the charger you need will depend on the batteries you use.

DC/DC Converter

The DC/DC converter takes the voltage of your main (traction) battery pack and reduces it to 12V which keeps your 12V battery charged. An electric vehicle still needs an 12V battery to power all the lights, stereo, horn etc. Keeping this battery charged can be achieved other ways as well. Some EV builders use an alternator that runs off the electric motor and others use a separate 12V charger to charge this battery.

Gauges

You will need to know what’s going on under the hood and this is where your gauges come in. Most basic EV builds use a high-voltage ammeter and voltage gauge (for traction pack voltage) and a low voltage gauge (12V system).

Heater

Although a heater is not necessary to drive the car, it is a creature comfort that we have all become accustomed to. Being that the stock heater in any gasoline car uses heat created by the gasoline engine to heat the cabin, we need to figure out something else to get heat into the vehicle. There are several ways to do this.

Ceramic Element

– This is the most common means of heating an EV. A ceramic element is placed within the heater core or otherwise inside the heater box and powered with the traction pack voltage. The ceramic element basically becomes the heater core and other than a switch required to turn the element on, the system will function as normal.

Fluid Heater

– A fluid heater uses the stock heater core and circulates fluid through it just like the stock system would. The fluid is heated by an electric element and circulated through the heater core by a small pump.

Space Heater

– It is possible to use a small ceramic space heater in the car with an AC power inverter. AC inverters however are quite expensive and this is not typically a cost effective way of heating an EV.

Heated Seats

– Heated seat covers are widely available nowadays and put out a good amount of heat. The air in the car will stay cold but a heated seat can do wonders to make you feel warm.

Heat The Car Prior To Use

– Simply put a space heater in the car for a while prior to use. It will heat the car and stay warm long enough for a short commute.

These are the primary components required to build an electric car at home. There will be other bits and pieces needed along the way but these are the primary components.