What to do with regenerative energy at 100% battery state of charge
What is regenerative energy?
Regenerative energy is generated through the electrical braking mechanism of a vehicle’s drive motor, which also functions as a generator to decelerate the vehicle. Heavy commercial vehicles operating on long slopes must adhere to legal requirements for endurance braking devices. The service brakes, which are the primary braking system, are limited to short periods of use. On steep descents, they can overheat and lose their friction power.
Battery and fuel cell electric vehicles can use electric braking if the battery absorbs the generated energy. However, electric braking becomes ineffective once the battery reaches 100% state of charge (SOC). Various solutions have been proposed to address this issue.
Potential solutions
Use of an additional retarder
One option is to add a hydraulic retarder, commonly used in conventional trucks. This device is installed on the drive shaft between the transmission and the road wheels. It contains vanes attached to the drive shaft, enclosed in a static chamber. When retardation is needed, a liquid (usually water or oil) is pumped into the chamber, creating viscous drag that slows the vanes and, consequently, the vehicle. However, this system adds extra weight and complexity, particularly in electric vehicles where a drive shaft may not be present.
Increasing battery capacity
Another solution is to increase the battery size to prolong the time regenerated energy can be absorbed. However, this approach adds weight, increasing the braking force required to maintain speed downhill. Additionally, it incurs extra costs with uncertain returns on investment, as this battery capacity would primarily be used for braking.
Smart telematics instead of engineering solutions
Smart telematics has also been proposed to extend the use of electric braking before the battery reaches full capacity. Modern telematics and route planning can inform drivers to minimize regenerative energy use and rely on mechanical brakes, allowing for better energy management. However, this approach diverges from the goal of preserving energy in all its forms for reuse.
A comprehensive solution
The initial concept behind the Integrated Brake Chopper and Resistor (iBCR) was created by Accelera™ by Cummins to address this common customer concern: what to do with regenerative energy when the battery reaches 100% state of charge (SOC)?
The iBCR is designed to enhance the quantity of recuperative braking in commercial vehicles, providing greater independence from the vehicle’s batteries. By incorporating the iBCR into battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), or fuel cell electric vehicles (FCEVs), manufacturers can achieve new levels of design flexibility and energy efficiency.
The iBCR significantly boosts overall system efficiency, offering substantial cost savings for commercial vehicles. It also helps original equipment manufacturers (OEMs) comply with regulations for heavy-duty vehicles by enabling the electric drive motor to function as an endurance braking device during energy recovery.
How does the iBCR work?
The brake chopper limits the DC bus voltage by diverting excess braking energy from the circuit. It acts as an electrical switch, activated when the DC bus voltage exceeds a specified threshold. The DC bus is the main circuit connecting the traction battery, traction inverter, and auxiliary inverters.
When activated, the brake chopper redirects excess energy to a liquid-cooled resistor, dissipating the energy as heat.
Additional benefits of the iBCR
- Support for vehicle heating: The heat generated by the resistor can be utilized to support the vehicle’s heating system, allowing OEMs to eliminate traditional heaters and free up valuable space in the vehicle’s architecture.
- Relief for mechanical brakes: The load on conventional brakes can be reduced, and the retarder function can be simulated through a smart combination of electric motor controls and the iBCR.
Why trust this new technology?
The development of the iBCR builds on over 25 years of experience in designing traction systems for hybrid vehicles. Accelera’s long-term discussions with customers and expertise in brake chopper design have culminated in creating the first integrated brake chopper and resistor. The iBCR has already been tested by select customers and is commercially available. Visit the iBCR product page or contact Accelera for more information.
