Battery management system for electric vehicle – detail each component

It can also provide security, communication, cell balancing and management control, and provide a communication interface with the application device system, as shown in Figure below.

Battery management system for electric vehicle monitors the total voltage and current data of the battery system, obtains the voltage of a single EV battery cell, and battery module, and grasps the internal temperature and configuration data of the battery pack. It is mainly composed of three parts, including hardware architecture, underlying software and application software.

Hardware architecture

Battery management system for electric vehicle hardware includes CPU, power supply and sampling IC, isolation transformer, CAN module, EEPROM and RCTS, the core of which is CPU. EV BMS hardware structure is shown in Figure below. Centralized and distributed are the topologies of BMS hardware Centralized electronic components are summarized in the plate, the sampling chip is communicated by the Daisy link main chip, the link is simple, the cost is low, but the disadvantage is insufficient stability. The distributed system is composed of a motherboard and a slave board. The system has flexible configuration and high channel utilization, which is suitable for all kinds of battery packs. The disadvantage is that the channel is wasted when the number of battery modules is insufficient. The main controller of the battery management system for electric vehicle processes the reported information, comprehensively judges the battery running status, implements control policies, and handles fault information. The high voltage controller collects the reported total voltage and current, provides the load condition (SOC) and health condition (SOH) data for the mainboard, and realizes the detection functions of pre-charging and insulation. The controller has a single battery information collection and reporting, and has a dynamic balance function to maintain the power output consistency of the battery cell. The sampling control harness simultaneously adds redundancy insurance to each voltage sampling line to avoid external battery short-circuit failures.

Underlying software

According to the AUTOmotiveOpenSystemAr－chitecture (AUTOSAR), the architecture is designed to reduce dependence on hardware devices. The battery management system for electric vehicle is divided into many general function blocks. It can be configured for different hardware and has less impact on application layer software. It needs to be connected to the application layer software through RET interface, and should be configured from the application layer due to flexibility requirements such as fault diagnosis event management (DEM), fault diagnosis Communication management (DCM), Functional information management (FIM) and CAN communication reserved interface.

Application layer software

The application layer covers high and low voltage management, charge management, state estimation, balance control and fault management, etc., as shown in figure below.

•  High and low voltage management mainly involves that when power is needed, VCU activates EV BMS by 12V of hard wire (CAN signal), and closes the high voltage on the relay after the latter completes the self-test. When it is necessary to power off, the VCU gives instructions to disconnect the 12V signal, or it is excited by the CP (A+) signal when charging.

•  In charge management, the slow charge process is relatively simple, and fast charge needs to complete 80% of the charge in 45min, which should be stimulated by A+ signal of the charging auxiliary power supply. At present, the national standard has not yet unified fast charge, that is, there are two fast charge versions in 2011 and 2015.

• SOC is the core control algorithm of the state estimation function, which represents the remaining capacity of the battery and is calculated by a specific ampere-hour integration method; SOH is to determine the life state of the battery and the capacity of the battery full state, generally less than 80% of the battery should not continue to use; SOP needs to be derived based on temperature and SOC conversion, which can send a signal in time before the battery criticality to allow the power system to limit some functions; SOE algorithm is used to estimate the remaining range, the current development is relatively simple, so the range of electric vehicles is often inaccurate, commonly known as “air power” phenomenon.

•  The role of balance control is to balance the inconsistent discharge of the single battery cell, because the circuit will be due to the cutoff of the single battery cell with the worst performance, resulting in the waste of the remaining battery storage with complete performance. Balance control is divided into active and passive, in which active control transfers energy between monomers, its structure is complex and the cost is high, while passive control in addition to wasting part of the energy, the advantage is more obvious, currently favored by manufacturers.

•  Fault diagnosis is mainly based on data collection, general fault, electrical equipment fault, communication fault and battery fault, etc., to divide different fault levels and take corresponding measures.

Sensor application in battery management system

The battery management system for electric vehicle mainly uses sensors such as current sensor, temperature and humidity sensor, voltage sensor and position sensor.

Current sensor

• Hall current sensor

The Hall effect (HallEffect) sensor changes the magnetic field to change the voltage, which is an indirect measurement. It can be divided into open-loop type and closed-loop type, the latter of which has higher accuracy. Hall current sensor simplifies the circuit. Only the positive and negative terminals of the DC power supply need to be connected, and the measured current bus will pass through the sensor to complete the isolation detection of the main circuit and the control circuit. The output signal of the sensor is the secondary side current, which is proportional to the primary side current (input signal), and the value is small, requiring A/D conversion. Hall current sensor integrates the advantages of transformer and shunt and has a simple structure, but it is easy to be interfered, and is not suitable for the increasingly sophisticated electric vehicle power supply environment.

• Fluxgate current sensor

The FluxGate principle is that under the influence of the excitation current, the excitation current changes the inductance strength of the easily saturated magnetic core, and then changes the magnetic flux, and the magnetic flux opens or closes like a door. The accuracy of ordinary Hall current sensors is between 0.5% and 2%, while the fluxgate current sensor is made of the fluxgate principle, and the accuracy can reach 0.1% or even higher, so it is also called a high-precision current sensor. In structure, there are also two types of open mouth and non-open mouth, that is, there are two types of open loop and closed loop. This article focuses on the closed-loop fluxgate current sensor, which amplifies the second harmonic signal of fluxgate excitation current and drives the compensation coil, so that the magnetic flux of the magnetic core and the magnetic flux of the primary side current are offset, and the state of “zero flux” is maintained. For HPIT series, the magnetic flux is not zero, and it is a symmetrical shape without second harmonics, as shown in Figure 5.

Fluxgate current sensors are divided into four types from the structure, as shown in Table below, which are single magnetic ring, double magnetic ring, double magnetic ring (shielding), and multi-magnetic ring (nesting). Due to the advantages of high sensitivity of fluxgate principle, strict correspondence between closed-loop magnetic balance and turn ratio output, overall magnetic core closure, probe compensation to eliminate the impact of oscillation harmonic output cleanliness, etc., Therefore, closed-loop fluxgate current sensors are widely used in various types of battery management systems for electric vehicles, such as the BMS of Tesla Model3, BYD Han, Xiaopeng P7 and other best-selling models.

• Tunneling magnetoresistive effect current sensor

Compared with Hall devices, anisotropic magnetoresistance (AMR) and Giant Magnetoresistance (GMR), the tunneling magnetoresistive effect (TMR) current sensor is a new generation of magnetic sensor, it has the advantages of low energy consumption, low temperature drift and high sensitivity, which can significantly improve the sensitivity and temperature characteristics of current detection. It is used to completely replace Hall sensors. The TMR current sensor no longer needs temperature compensation when detecting current, and the total temperature drift under -40℃~85℃ environment is reduced from 1%~2% to 0.1%~0.2%. For example, for the current detection and control of the car charger, it can accurately detect the copper bar or wire current, and the chip size is smaller, the accuracy, linearity, response speed and temperature drift characteristics are more optimized, bringing excellent safety and economy to the electric vehicle.

Temperature and humidity sensor

• NTC temperature sensor

Temperature plays an important role in the performance of battery management system for electric vehicle. In order to further improve the utilization rate of battery, prevent excessive discharge (charge) of battery, control battery working conditions, and increase battery life, built-in NTC temperature sensor is used to monitor the temperature. The NTC temperature sensor is mainly made of oxidized compounds of high-purity metal elements such as Mn through the combination of ceramic technology and semiconductor technology. The working principle is that the number of carriers in these materials is small and the resistance is high. When the temperature rises, the number of carriers correspondingly increases and the resistance correspondingly decreases (Figure 7). It has the advantages of high resistivity, small heat capacity, fast response, good linear relationship between resistance and temperature, being able to bend, low price, long life and so on. There are three commonly used categories: ground ring shell NTC temperature sensor, commonly known as “ground ring type”; Epoxy resin encapsulated NTC temperature sensor, commonly known as “drop head”, “small blackhead”; Thin film NTC temperature sensor.

• HTW humidity sensor

The humidity sensor is a device that converts the ambient humidity into something that can be marked by an electrical signal. The common humidity sensor measures relative humidity. At present, the humidity sensors commonly used in battery management system for electric vehicle include resistive humidity sensor and capacitive humidity sensor. The principle is to coat the substrate with a layer of moisture-sensing material film, and when water vapor is adsorbed on the film in the environment, the component resistivity and resistance value will change, and the humidity can be measured. HTW-211 is a high precision humidity measurement sensor module, which is an accurate and reliable humidity measurement sensor based on HumiChip. The humidity factor is particularly difficult to capture in the battery management system of electric vehicles, but it has a huge impact on the performance and life of the battery. The humidity output of the sensor is temperature-compensated to generate a linear voltage that is fed into the BMS of the electric vehicle with the ADC.

Voltage sensor

The battery pack of the electric vehicle power supply system is connected by hundreds of series cells, so the channel demand for voltage measurement is large. The series battery is the cumulative voltage, but the single battery electromotive force is not the same, can not simply use one-way compensation method to eliminate the error. Battery voltage acquisition requires high accuracy, reaching 1mV, while the current acquisition accuracy is only 5mV. The voltage sensor can convert the measured battery voltage into something that can output signals. The electroluminescent effect voltage sensor used in electric vehicles is to measure the luminous intensity of the luminous material at the measured voltage to obtain the effective value of the measured voltage. Compared with the traditional optical voltage sensor, the voltage sensor based on electroluminescence effect will no longer use the carrier light source, on the one hand to eliminate the instability of the carrier light source measurement, on the other hand, the sensor structure is simplified and the production cost is reduced.

Position sensor

The position sensor is mainly used to detect the position of the coolant surface in the water cooling device in the battery management system for electric vehicle system. The position sensor is installed on the cooling water float, which is used to detect the position of the liquid surface of the expansion kettle by the cooling liquid phase, and obtain the contact between the outlet of the expansion kettle and the liquid. At least three floats are usually required, and position sensors are installed on each float, so that when the vehicle passes through steep slopes and other sections or there are a large number of bubbles in the cooling system, the electric vehicle BMS can adjust and control the main water pump and auxiliary water pump in time to switch operation.

Core chip in the battery management system for electric vehicle

Common core chip in battery management system for electric vehicle mainly involves computing unit (such as MCU), AFE (analog front-end chip), digital isolator, ADC (analog-to-digital converter), CAN bus transceiver, network transformer, current sensor, fuse/cable and other components.