BMS battery management system from shallow to deep all-round analysis

What is BMS?
BMS (BATTERY MANAGEMENT SYSTEM), commonly known as battery nanny or battery housekeeper, is designed to intelligently manage and maintain each battery cell, prevent overcharging and overdischarging, prolong battery life, and monitor battery status.


BMS Battery Management System Module
BMS battery management system unit includes BMS battery management system, control module, display module, wireless communication module, electrical equipment, battery pack for powering electrical equipment and collection module for collecting battery information of battery pack, BMS battery management system is connected to wireless communication module and display module respectively through communication interface, the output end of said collection module is connected to the input end of BMS battery management system The output of said acquisition module is connected to the input of the BMS battery management system, the output of said BMS battery management system is connected to the input of the control module, said control module is connected to the battery pack and the electrical equipment, respectively, and the BMS battery management system is connected to the Server server side via the wireless communication module.

Composition of BMS?
The battery management system is closely integrated with the power battery of the electric vehicle. It detects the voltage, current and temperature of the battery in real time through sensors, and also performs leakage detection, thermal management, battery equalization management, alarm reminding, calculates the remaining capacity (SOC), discharge power, reports the degree of battery deterioration (SOH) and remaining capacity (SOC) status, and also based on the voltage and current and temperature of the battery It also controls the maximum output power with algorithms to obtain the maximum driving range, as well as charging with algorithms to control the charger for the best current, and communicates in real time with the vehicle master controller, motor controller, energy control system, and vehicle display system through the CAN bus interface.

BMS mainly consists of BMU master controller, CSC slave controller, CSU equalization module, HVU high voltage controller, BTU battery status indication unit and GPS communication module, ranging from master-slave integrated architecture for electric tools, electric bicycle, electric forklift, intelligent robot, IOT smart home, light hybrid vehicle to master-slave separated electric vehicle (pure electric, plug-in hybrid), electric ships, etc., to three-tier architecture for energy storage systems.


Battery Management System BMS Electrical Architecture
BMS life cycle form


BMS PCBA circuit board

BMS installed in EV battery pack

BMS installed in the power battery pack
BMS in electric vehicles

The application of battery management system (BMS) in electric vehicles can be traced back to the management of NiMH batteries in Toyota HEV models. Unlike managing lithium batteries due to their high consistency, safety, and low individual voltage (1.0~1.7V), the BMS for NiMH batteries usually does not require equalization functions, control contactors, or voltage acquisition for each cell (6 cells can be connected in series as a whole for voltage monitoring). Although the hardware function of NiMH battery BMS is relatively simple, the difficulty lies in how to estimate the SOC and how to control and adjust the charging and discharging intervals to avoid rapid battery decay due to the memory effect of NiMH batteries and the complex correspondence between extra-voltage characteristics and SOC. With the application of lithium battery technology, the power battery system has higher energy density, larger capacity and longer operation time, which also puts forward new requirements for the function of BMS. From the topological architecture BMS is divided into Centralized and Distributed according to different project requirements.


Master-slave centralized and distributed architecture of BMS
Centralized BMS

Centralized BMS has the advantages of low cost, compact structure and high reliability, and is generally used in scenarios with low capacity, low total voltage and small battery system size, such as power tools, robots (handling robots, power robots), IOT smart home (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.), and light hybrid vehicles.

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

Distributed BMS

The distributed BMS architecture can better realize the hierarchical management at module and system level (Pack). The CSC is responsible for voltage detection, temperature detection, equalization management (some of them are independent of the CSU unit) and corresponding diagnostic work; the HVU is responsible for monitoring the total battery voltage, total bus voltage and insulation resistance of the Pack (bus current can be collected by Hall sensors or shunt); and the CSC and HVU will send the analyzed data to the CSC and HVU. The HVU sends the analyzed data to the Battery Manangement Unit (BMU), which performs battery system BSE (Battery State Estimate) evaluation, electrical system state detection, contactor management, thermal management, operation management, charging management, diagnostic management, and management of internal and external communication networks. The system is designed to be able to manage the internal and external communication networks.

Currently, distributed BMS architecture is commonly adopted by mainstream production EV models, such as BMW i3/i8/X1, Tesla Model S/X, GM Volt/Bolt, BYD Qin/Tang, Rongwei e550/e950/eRX5, etc. The advantage of distributed BMS architecture is that it can be efficiently configured according to different battery system designs in series and parallel, with shorter, more uniform and more reliable harness distance between BMS connections to batteries, and can also support larger battery system designs (e.g. MW-class energy storage systems).

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 widespread application of power battery systems in the automotive field and the climbing scale of production, a unified standard battery Module is gradually on the agenda in the industry. If there is no standard module as the support for industrialization, the old electric vehicle models will encounter the embarrassing situation of no battery spare parts to replace after several years of use, and the retired power batteries from the automotive field will face the situation that they cannot be effectively utilized. The standardized Module needs to integrate some functions of the battery management system (single unit status acquisition and management) with the battery to achieve high space utilization, high reliability and high versatility. Therefore the slave control unit CSC has gradually become one of the indispensable key components in the standard Module.

Core functions of BMS
1) Cell monitoring technology

1.Single cell voltage acquisition.

2.Single cell temperature acquisition.

3) Battery pack current detection.

The accurate measurement of temperature is also quite important for the working status of the battery pack, including the temperature measurement of individual cells and the monitoring of the heat dissipation liquid temperature of the battery pack. This requires a reasonable setting of the location and number of temperature sensors used to form a good match with the BMS control module. The monitoring of the battery pack heat dissipation fluid temperature focuses on the fluid temperature of the inlet and outlet, and the selection of its monitoring accuracy is similar to that of a single battery.

2) SOC (state of charge) technology: simply put, how much electricity is left in the battery

SOC is the most important parameter in the BMS, because everything else is based on SOC, so its accuracy and robustness (also called error correction capability) is extremely important. Without accurate SOC, no amount of protection functions can make the BMS work properly because the battery will be in a constant state of protection, not to mention extending the life of the battery.

The higher the accuracy of SOC estimation precision, the higher the range of EV can be for the same capacity of battery. High precision SOC estimation can make the battery pack work with maximum performance.

3)Equalization technology

Passive equalization generally adopts the resistive heat release method to release the "excess power" of high capacity battery to achieve the purpose of equalization, the circuit is simple and reliable, the cost is low, but the battery efficiency is also low.

Active equalization transfers excess power to high-capacity cells when charging and to low-capacity cells when discharging, which can improve efficiency, but with higher cost, complex circuitry and low reliability. The need for passive equalization may decrease in the future as the consistency of the battery cells improves.


Software core functions of BMS
I) Measurement function.

1) Basic information measurement: battery voltage, current signal monitoring, battery pack temperature detection Battery management system has the most basic function is to measure the voltage, current and temperature of the battery cell, which is the basis of all battery management system top-level calculation and control logic.

2) insulation resistance detection: the battery management system requires insulation testing of the entire battery system and high-voltage system.

3) High voltage interlock detection (HVIL): used to confirm the integrity of the entire high-voltage system and to initiate safety measures when the integrity of the high-voltage system circuit is compromised.

II) Estimation function

1)SOC and SOH estimation: the core and the most difficult part

2) Equalization: When there is an imbalance of SOC×capacity between single cells, it is adjusted by the equalization circuit.

3)Battery power limitation: The battery has a certain limitation on its input and output power at different SOC temperatures.

III) Other functions

1)Relay control: including main +, main -, charging relay +, charging relay -, pre-charge relay

2)Thermal control

3)Communication function.

4)Fault diagnosis and alarm

5)Fault-tolerant operation

BMS software architecture
1.High and low voltage management

Generally, when the power is normally on, the VCU will wake up the BMS through the hard line or CAN signal 12V, after the BMS completes the self-test and enters standby, the VCU will send the high voltage command and the BMS will control the closing relay to complete the high voltage. When the power is down, VCU will send down high voltage command and then disconnect to wake up 12V, when the power is down, the gun can be charged by CP or A+ signal to wake up.

2.Charging management

(1) Slow charging

Slow charging is to charge the battery by AC charging pile (or 220V power supply) by converting AC to DC through vehicle charger, the specifications of charging pile are generally 16A, 32A and 64A, and it can also be charged by home power supply. BMS can be woken up by CC or CP signal, but it should be ensured that it can sleep normally after charging is finished. The AC charging process is relatively simple and can be developed in accordance with the detailed regulations of the national standard.

(2) Fast charging

Fast charging is to charge the battery by DC charging pile output DC, which can achieve 1C or even higher multiplier charging, generally 45min can charge 80% of power. Through the auxiliary power supply of the charging pile A + signal wake up, the national standard fast charging process is more complex, while there are two versions of 2011 and 2015, and charging pile manufacturers for the national standard process is not clear technical details of different understanding also to the vehicle charging adaptability caused great challenges, so fast charging adaptability is a key indicator to measure the performance of BMS products.

3.Estimation function

(1) SOP (StateOfPower) is mainly through the temperature and SOC look-up table to get the current battery charge and discharge power available, VCU according to the power value sent to decide how to use the current vehicle. 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, which of course will have a certain impact on the driving experience of the whole car.

(2) SOH (StateOfHealth) mainly characterizes the current state of health of the battery, which is a value between 0-100%, and it is generally believed that the battery can no longer be used after it falls below 80%. Accurate SOH will improve the accuracy of estimation of other modules when the battery decays.

(3) SOC (StateOfCharge) belongs to the core control algorithm of BMS, which characterizes the current remaining capacity state, mainly through the ATS integration method and EKF (Extended Kalman Filter) algorithm, and combined with correction strategies (such as open circuit voltage correction, full correction, end-of-charge correction, capacity correction under different temperatures and SOH, etc.). The ampere-time integration method is more reliable under the condition of ensuring the accuracy of current collection, but not robust, and must be combined with correction strategies due to the error accumulation, while the EKF is more robust, but the algorithm is more complex and difficult to implement. Domestic mainstream manufacturers can generally achieve accuracy of 6% or less at room temperature, the estimation at high and low temperatures and battery decay is the difficult point.

(4) SOE (StateOfEnergy) algorithm domestic manufacturers are now developing not many, or use a more simple algorithm, look up the table to get the current state of the ratio of the remaining energy to the maximum available energy. This function is mainly used to estimate the remaining range.

4.Fault diagnosis

For the different performance of the battery, distinguished into different fault levels, and in different fault levels BMS and VCU will take different measures to deal with the situation, warning, power limit or directly cut off the high voltage. Faults include data acquisition and reasonability faults, electrical faults (sensors and actuators), communication faults and battery status faults, etc.

5.Equalization control

The equalization function is to eliminate the inconsistency of the battery unit generated in the process of battery use. According to the barrel short board effect, the worst performing unit reaches the cutoff condition first when charging and discharging, and the other units have some capacity not released, resulting in battery waste.

The active equalization is the transfer of energy from more to fewer monomers, which does not cause energy loss, but the structure is complicated, the cost is high, and the requirements for electrical components are also high, relatively speaking, the passive equalization structure is simple and the cost is much lower, but the energy will be wasted in the form of heat, and the maximum equalization current is usually around 100mA. Nowadays, many domestic manufacturers use passive equalization to achieve a good equalization effect.

BMS control method, as the central control idea of power battery, directly affects the service life of the power battery and the safe operation and performance of the electric vehicle. It has a significant impact on the range and determines the future of new energy vehicles. A good battery management system will greatly promote the development of new energy vehicles.
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