What is the BMS and How does it work?

bms

BMS (Battery Management System), commonly known as battery nanny or battery housekeeper, is mainly to intelligently manage and maintain each battery unit, prevent overcharging and over-discharging of the battery, prolong the service life of the battery, and monitor the status of the battery.

BMS

The BMS battery management system unit includes a BMS battery management system, a control module, a display module, a wireless communication module, electrical equipment, a battery pack for powering electrical equipment, and a collection module for collecting battery information of the battery pack. The BMS battery management system is respectively connected with the wireless communication module and the display module through the communication interface, the output end of the acquisition module is connected with the input end of the BMS battery management system, and the output end of the BMS battery management system is connected with the control module is connected to the battery pack and the electrical equipment respectively, and the BMS battery management system is connected to the Server through the wireless communication module.

What are the architectural components of BMS?

The battery management system is closely integrated with the power battery of the electric vehicle. The sensor detects the voltage, current, and temperature of the battery in real-time, and also performs leakage detection, thermal management, battery balance management, alarm reminder, and calculates the remaining capacity (SOC). , discharge power, report the state of battery deterioration (SOH) and the remaining capacity (SOC), and also control the maximum output power according to the voltage, current and temperature of the battery to obtain the maximum mileage, and use the algorithm to control the charger for optimal current. Charge, and communicate in real-time with the onboard master controller, motor controller, energy control system, onboard display system, etc. through the CAN bus interface.

BMS is mainly composed of a BMU master controller, CSC slave controller, CSU balance module, HVU high voltage controller, BTU battery status indication unit and GPS communication module. , IoT smart home, light hybrid vehicles to master-slave separate electric vehicles (pure electric, plug-in hybrid), electric ships, etc., and then to the three-layer energy storage system (EMS).

Application of BMS in Electric Vehicles

The use of battery management systems (BMS) in electric vehicles can be traced back to the management of NiMH batteries in Toyota HEV models. Different from the management of lithium batteries, because the NiMH battery has the characteristics of high consistency, good safety, and low cell voltage (1.0~1.7V), the BMS of the NiMH battery usually does not need the equalization function, and does not need to control the contactor. , and do not need to collect voltage for each battery (6 batteries can be connected in series as a whole for voltage monitoring). Although the BMS hardware function of NiMH battery is relatively simple, the difficulty lies in how to estimate the SOC and how to control and adjust the charging and discharging interval to avoid the rapid deterioration of the battery due to the complicated correspondence between the memory effect and the external characteristics of the NiMH battery and the SOC. With the application of lithium battery technology, the power battery system has higher energy density, larger capacity and longer running time, which also puts forward new requirements for the function of BMS. From the perspective of topology architecture, BMS is divided into two types: Centralized and Distributed according to different project requirements.

Centralized BMS Architecture

Centralized BMS has the advantages of low cost, compact structure, and high reliability. It is generally used in scenarios with low capacity, low total pressure, and small battery system volume, such as power tools, robots (handling robots, power-assisting robots), IOT smart homes ( Sweeping robots, electric vacuum cleaners), electric forklifts, electric low-speed vehicles (electric bicycles, electric motorcycles, electric sightseeing cars, electric patrol cars, electric golf carts, etc.), light hybrid vehicles.

The BMS hardware of the centralized architecture can be divided into high-voltage area and low-voltage area. The high-voltage area is responsible for the collection of single battery voltage, the collection of total system voltage, and the monitoring of insulation resistance. The low-voltage area includes power supply circuit, CPU circuit, CAN communication circuit, control circuit, etc. With the continuous development of passenger car power battery systems to high capacity, high total pressure and large volume, distributed architecture BMS is mainly used in plug-in hybrid and pure electric vehicles.

Distributed BMS Architecture

The distributed BMS architecture can better realize the hierarchical management of module level (Module) and system level (Pack). The slave control unit CSC is responsible for voltage detection, temperature detection, balance management (some have independent CSU module units) and corresponding diagnostic work for the monomers in the Module; the high voltage management unit (HVU) is responsible for the Pack’s battery. The total voltage, total bus voltage, insulation resistance and other states are monitored (the bus current can be collected by Hall sensors or shunts); and the CSC and HVU send the analyzed data to the main control unit BMU (Battery Manangement Unit), and the BMU Carry out battery system BSE (Battery State Estimate) evaluation, electrical system state detection, contactor management, thermal management, operation management, charging management, diagnosis management, and management of internal and external communication networks.

At present, mainstream mass-produced electric vehicles generally use distributed BMS architecture, such as BMW i3/i8/X1, Tesla Model S/X, GM Volt/Bolt, BYD Qin/Tang, Roewe e550/e950/eRX5 and so on. The advantage of the distributed BMS architecture is that it can be efficiently configured according to the series-parallel design of different battery systems. The wiring harness distance between the BMS and the battery is shorter, more uniform, and more reliable, and it can also support larger batteries. System design (eg MW-scale energy storage system).

Another reason why distributed BMS has become a mainstream application solution is that it better meets the trend of power battery system module design. With the wide application of power battery systems in the automotive field and the increase in production scale, a unified standard battery module is gradually put on the agenda in the industry. If there is no standard Module as the support for the promotion of industrialization, the old electric models will encounter the embarrassing situation that no battery spare parts can be replaced after several years of use, and the power batteries retired from the automotive field will face the situation that they cannot be effectively used in cascade . The standardized Module needs to highly integrate some functions of the battery management system (single state acquisition and management) with the battery, so as to achieve the requirements of high space utilization, high reliability and strong versatility. Therefore, the slave control unit CSC has gradually become one of the indispensable key components in the standard Module.

Core Functions of BMS

Cell monitoring technology

  • Single-cell voltage acquisition
  • Single-cell temperature collection
  • Battery pack current detection

The accurate measurement of temperature is also very important for the working state of the battery pack, including the temperature measurement of a single cell and the temperature monitoring of the battery pack heat dissipation liquid. This requires a reasonable setting of the location and number of temperature sensors to form a good fit with the BMS control module. The monitoring of the temperature of the cooling liquid of the battery pack focuses on the temperature of the fluid at the inlet and outlet, and the selection of the monitoring accuracy is similar to that of the single battery.

SOC (State of Charge) technology: Simply put, how much power is left in the battery

SOC is the most important parameter in BMS, because everything else is based on SOC, so its accuracy and robustness (also called error correction capability) are extremely important. If there is no precise SOC, no amount of protection functions can make the BMS work normally, because the battery will often be in a protected state, and it will not prolong the life of the battery.

The higher the accuracy of the SOC estimation accuracy, the higher the cruising range of the electric vehicle can be for the battery of the same capacity. High-precision SOC estimation can maximize the performance of the battery pack.

Balance technology

Passive balancing generally uses resistance heat release (capacitive carrier) to release the “excess power” of high-capacity batteries, so as to achieve the purpose of balancing. The circuit is simple and reliable, and the cost is low, but the battery efficiency is also low.

Transferring excess power to high-capacity cells during active equalization charging and transferring excess power to low-capacity cells during discharge can improve the use efficiency, but the cost is higher, the circuit is complex, and the reliability is low. In the future, as the consistency of cells improves, the need for passive equalization may decrease.

BMS software architecture

High and low voltage management

Generally, when powered on normally, the VCU will wake up the BMS through the hard wire or 12V/24V of the CAN signal. After the BMS completes the self-check and enters the standby state, the VCU sends the high voltage command, and the BMS controls the closed relay to complete the high voltage. When power off, the VCU sends a high voltage command and then disconnects to wake up 12V. It can be woken up by the CP or A+ signal when the gun is plugged in and charged in the power-off state.

Charge management

  • Rushing
    Slow charging is to convert AC into DC to charge the battery by the AC charging pile (or 220V power supply) through the on-board charger. The specifications of the charging pile are generally 16A, 32A and 64A, and it can also be charged by household power. The BMS can be woken up by the CC or CP signal, but it should be ensured that it can sleep normally after charging. The AC charging process is relatively simple, and can be developed in accordance with the detailed regulations of the national standard.
  • Fast charge
    Fast charging is to charge the battery with DC output from the DC charging pile, which can achieve 1C or even higher rate charging. Generally, 80% of the power can be charged in 45 minutes. Wake-up by the auxiliary power A+ signal of the charging pile, the fast charging process in the national standard is more complicated, and there are two versions in 2011 and 2015, and the charging pile manufacturers have different understandings of the technical details that are not clear in the national standard process. It is a great challenge, so fast charging adaptability is a key indicator to measure the performance of BMS products.
  • Estimation function
    SOP (State Of Power) mainly obtains the available charge and discharge power of the current battery through the temperature and SOC look-up table, and the VCU determines how to use the current vehicle according to the transmitted power value. It is necessary to consider both the release of battery capacity and the protection of battery performance, such as partial power limitation before reaching the cut-off voltage. Of course, this will have a certain impact on the driving experience of the whole vehicle.

    SOH (state of health) mainly represents the current state of health of the battery. It can be represented by the change of battery capacity or internal resistance. When the capacity is used, the actual capacity of the current battery is estimated through the battery operation process data, and the ratio to the rated capacity is SOH. An accurate SOH will improve the estimation accuracy of other modules when the battery decays.
  • SOC (State Of Charge) belongs to the BMS core control algorithm, which represents the current remaining capacity state, mainly through the ampere-hour integration method and the EKF (extended Kalman filter) algorithm, combined with correction strategies (such as open circuit voltage correction, full correction, charging end correction, capacity correction at different temperatures and SOH, etc.). The ampere-hour integration method is more reliable under the condition of ensuring the current acquisition accuracy, but the robustness is not strong. Due to the existence of error accumulation, the correction strategy must be combined, while the EKF has strong robustness, but the algorithm is more complex and difficult to implement. Domestic mainstream manufacturers can generally achieve an accuracy of less than 6% at room temperature. Estimation at high and low temperature and battery attenuation is difficult (inaccurate estimation may cause the estimated mileage to be inconsistent with the actual mileage, which is very dangerous in high-speed driving).
  • SOE (State Of Energy) algorithm is currently not developed by domestic manufacturers, or a relatively simple algorithm is used to look up the table to obtain the ratio of the remaining energy to the maximum available energy in the current state. This function is mainly used for remaining cruising range estimation.

Troubleshooting

According to the different performance conditions of the battery, it is divided into different fault levels, and under different fault levels, the BMS and VCU will take different measures, such as warning, limiting power or directly cutting off the high voltage. Faults include data acquisition and rationality faults, electrical faults (sensors and actuators), communication faults and battery status faults.

Balance control

The equalization function is to eliminate the inconsistency of the battery cells during the use of the battery. According to the short board effect of the wooden barrel, the cell with the worst performance reaches the cut-off condition first during charging and discharging, and the other cells still have some capacity. It is not released, causing battery waste.

Balance includes active balance and passive balance. Active balance is the transfer of energy from more monomers to less monomers, which will not cause energy loss. However, the structure is complex, the cost is high, and the requirements for electrical components are also relatively high, relatively passive. The balance structure is simple and the cost is much lower, but the energy will be dissipated and wasted in the form of heat. Generally, the maximum balance current is about 100mA. Now many domestic manufacturers can achieve better balance by using passive balance.

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