1、 Working principle
Normally, in new energy vehicles with distributed electronic and electrical architecture, the control strategy for energy recovery is implemented by VCU, which is responsible for integrating and processing information from various sensors of the vehicle, and comprehensively controlling and managing the vehicle's operating status. Energy recovery function is one of its components.
Specifically, VCU will match an appropriate energy recovery strategy based on the real-time operating conditions of the vehicle, such as vehicle speed, brake pedal position, battery state of charge, battery temperature, etc., and convert the kinetic energy of the vehicle during braking or sliding into electrical energy stored in the battery by adjusting the working state of the drive motor controller (MCU).
During this process, the MCU receives and executes specific instructions issued by the VCU to control the driving motor to operate in recovery mode, thereby achieving the conversion of kinetic energy to electrical energy. In addition, in a good energy recovery strategy, BMS will also exchange information with VCU to ensure the safety of the power battery during the energy recovery process, such as avoiding overcharging and ensuring that the temperature of the power battery is always within an appropriate range. Through the collaborative work of VCU, MCU, BMS and other nodes, the efficient and safe energy recovery effect of new energy vehicles is ensured.
2、 Occurrence process
In the process of achieving energy recovery, there are two main driving environments: one is during braking and deceleration, that is, when the driver steps on the brake pedal, the driving motor will switch from driving mode to generating mode (software implementation process such as changing from positive torque to negative torque), and the energy recovery system will start working, converting the kinetic energy of the vehicle's motion into electrical energy.
The second is that when the accelerator pedal is released and the vehicle is in a coasting state, that is, in a non braking state, when the driver releases the accelerator pedal, the vehicle enters coasting mode, and the driving motor can also enter generator mode to recover the kinetic energy of the vehicle coasting under inertia.

                                                                     Figure 1 Energy Recovery Process

3、 Energy Path
In the process of energy recovery, the source of energy is the kinetic energy of the entire vehicle. This kinetic energy is first converted into electrical energy by the driving motor, and then converted into direct current by the inverter of the MCU for storage in the power battery. During the process, BMS will monitor the status of the power battery and control the charging speed of energy and the balance between each unit of the battery pack, ensuring that the recovered energy is safely and effectively stored in the battery pack. Considering the real-time power consumption of the entire vehicle, in some strategies, in addition to storing energy, it can also be used for auxiliary systems such as air conditioning and heating to improve the energy efficiency and range of the vehicle. The schematic diagram of the architecture and energy recovery path is as follows:

                   Figure 2 Schematic diagram of energy recovery path

4、 Energy recovery strategy
4.1. Involving nodes
As mentioned earlier, in the process of energy recovery, the main nodes involved are VCU, MCU, BMS, and braking system, which provide the following functions:
VCU: As the decision-making center, it collects and processes all necessary parameter information, formulates and executes energy recovery strategies.
MCU: Receive instructions from VCU, control the drive motor to enter power generation mode at the appropriate time, and achieve energy recovery.
BMS: Monitor battery status and report to VCU to ensure that the battery does not overcharge or overheat during energy recovery, while participating in battery balancing control.
Braking system: Cooperate with VCU in joint braking mode to coordinate the ratio of regenerative braking and traditional friction braking, ensuring a balance between braking effect and energy recovery effect.
4.2. System Parameters
In different energy recovery strategies, there may be differences in the judgment conditions and parameters for energy recovery based on the application scenarios and positioning of vehicles. Here, the typical parameters are summarized in the following table:                 

                             Table 1 Energy Recovery Judgment Parameters

4.3. System Interaction Process
The various sensors of the vehicle collect and determine parameters, and transmit the data to the VCU and BMS. Based on the collected data, the VCU calculates the optimal energy recovery strategy and sends control instructions to the MCU, indicating when and to what extent the driving motor should recover energy. After receiving the instructions, the MCU controls the driving motor to switch from driving mode to generating state, thereby converting kinetic energy into electrical energy.
During the process, the BMS monitors the status of the power battery and provides feedback to the VCU on the current SoC, temperature, and other information. It also intervenes when necessary to control the magnitude of the charging current. VCU continuously adjusts the recycling strategy based on the actual recycling effect and changes in battery status to achieve closed-loop control.
4.4. Strategic Key
For the purpose of safe and efficient energy recovery. As the implementation of this function is mainly completed by VCU, it is necessary for VCU to have an intelligent energy recovery adjustment strategy, which requires accurate judgment based on the real-time driving status of the vehicle, dynamically adjusting the strength and strategy of energy recovery, and intelligently switching the recovery level according to the braking intensity of the vehicle and the driver's intention, such as using a low recovery level during light braking and a high recovery level during heavy braking, in order to achieve a balance between comfort and recovery efficiency.
In terms of the grading of recycling efforts, the following example is used to illustrate:
Primary recycling intensity (Level 1):
When the vehicle is driving normally and the speed is low (such as<40km/h), the driver lightly steps on the brake pedal or releases the accelerator pedal to slide. After receiving the signal, the VCU preliminarily judges it as mild energy recovery, that is, sets the recovery force to Level 1. At this point, the system recovers kinetic energy at a lower power to maintain the smoothness of the driving process, while considering the ideal temperature of the power battery and SoC, there is no need to specifically limit the recovery force.
Intermediate recycling intensity (Level 2):
When the vehicle speed increases (such as 40-80 km/h) or the depth of the brake pedal increases, the VCU judges it as moderate energy recovery according to the algorithm. At this time, the recovery force is adjusted to Level 2, and the corresponding recovery power will increase. At the same time, VCU will closely monitor the temperature and SoC of the battery. If it is found that the battery temperature has increased or the SoC is close to the upper limit, the recycling efforts will be appropriately reduced to protect the battery.
Advanced Recycling Strength (Level 3):
When the vehicle is driving at high speeds or decelerating rapidly, the driver may forcefully press the brake pedal. VCU determines this situation as severe energy recovery and sets the recovery force to Level 3. In this state, the system will recover as much kinetic energy as possible, while combining with the vehicle's ABS and ESC systems to ensure driving safety. But if the temperature of the power battery is about to exceed the threshold or the SoC is almost full at this time, the VCU will temporarily reduce or suspend energy recovery, and instead rely on the mechanical braking system. The simple code diagram under this strategy is as follows:

      Figure 3 Implementation Example

Figure 4 Schematic diagram of energy recovery implementation process

In addition to conventional energy recovery strategies, advanced technologies such as predicting driving behavior, such as navigation ADAS、 Path planning and other technologies predict the driver's driving intention and road conditions (such as downhill, traffic lights, congestion, etc.) in advance, intelligently adjust the energy recovery intensity based on this, maximize the recovery of available energy, and achieve efficient energy utilization while achieving safe driving.