A battery management system (BMS) is any electronic system that manages a rechargeable battery (cell or battery pack), such as by protecting the battery from operating outside its safe operating area, monitoring its state, calculating secondary data, reporting that data, controlling its environment, authenticating it. The core function of the power battery BMS is to collect data such as voltage, temperature, current, insulation resistance, high-voltage interlocking state, etc. of the system, then analyze the data state and the use environment of the battery and monitor and control the charging and discharge process of the battery system, so as to maximize the use of the dynamics under the premise of ensuring battery safety. The energy is stored by the force battery system. According to the function, BMS can be divided into battery data acquisition, battery status analysis, battery safety protection, battery system energy management control, data communication and storage, fault diagnosis and management, etc.
1:-Battery data capture
Battery data acquisition includes the collection of voltage, temperature, current, insulation resistance, high-voltage interlocking state, and other data, which can provide real-time data of the battery system for BMS and provide a basis for the state analysis, control, and protection of subsequent battery systems.
Voltage collection includes the voltage of each series of cells, the total voltage inside the battery system, and the external total voltage Link of the battery system. The temperature collection includes the temperature of the cell surface and polar ear, the temperature of the liquid-cooled inlet, the temperature of the fast charging pile interface, and the internal temperature of BMS operation. Current acquisition mainly collects the current of the main circuit of the battery system through a shunt or Hall current sensor and estimates the state of the battery system by estimation methods such as time integration. The insulation resistance mainly collects the insulation resistance between the total positive and box of the battery system and the insulation resistance between the totally negative and box of the battery system. High-voltage interlocks include the interlock state of high-voltage interlock and MSD.
2:- Battery status analysis
2.1 Estimation of the charge state (SOC) of the power battery system SOC is defined as the percentage of the remaining charge of the battery and the capacity of the battery. The calculation formula is as follows.
2.2 Estimation of the health status of the battery system (SOH)
SOH is defined as the percentage of the number of charges that have been fully charged and the number of available charges for the battery life cycle. The number of full battery charges is equal to the total capacity of the battery accumulated charging divided by the full battery capacity. The calculation formula is as follows.
SOC and SOH estimation is one of the core technologies of BMS and the main parameter for evaluating the endurance mileage of the whole vehicle. Typical algorithms for SOC estimation include the open-circuit voltage method (OCV method), time integration method, impedance method, extended Kalman filter method, and neural network method. At this stage, most BMS adopts the weighted time integration method, which increases cell parameters such as open circuit voltage, rated capacity at different temperatures, and charging and discharging efficiency at different currents on the basis of the amp-hour integral method, so as to correct SOC. In order to reduce the SOC estimation error, BMS suppliers and scientific research institutions have studied the estimation method of artificial intelligence SOC. Artificial intelligence SOC calculates real-time OCV by collecting real-time data (single voltage, current, temperature, etc.) of the battery system on the basis of the weighted time integration method. Therefore, dynamic OCV correction SOC can be realized.
3:- Battery safety protection
When the battery system has overvoltage, under voltage, ultra-high temperature, ultra-low temperature, overcurrent, low insulation, voltage acquisition line disconnection, temperature acquisition line disconnection, high-voltage interlocking, and other faults, BMS needs to protect the battery system in time, and take protective measures such as power limiting and power immediately according to the severity of the fault. So as to ensure that the battery system makes the best use of its stored energy under the premise of safety.
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In a low-temperature or high-temperature environment, when the battery system needs to be charged and discharged, BMS will first report the temperature of the cell surface and polar ears and the temperature of the liquid-cooled inlet interface to the whole vehicle. By analyzing the temperature data reported by BMS, the whole vehicle will turn on the liquid-cooled system to heat or heat the battery system, so that the battery system is in a suitable ring. Charge and discharge at the temperature. BMS collects the temperature of the fast charging interface to determine whether the fast charging interface is well connected or overcurrent, resulting in overheating of the interface, and to prevent safety accidents.
4:- Battery system energy management
4.1 Charging management
BMS linearly checks the charging power MAP of the battery system according to the current cell temperature of the battery system and SOC to determine the current maximum allowable charging current of the system. When charging, BMS interacts with the information such as the maximum voltage, maximum total voltage, maximum temperature and current, nominal energy, SOC, and current battery voltage currently allowed to be charged by the battery system with the charging equipment (charging pile or car charger), so that the battery system can charge according to the adaptive. Charge the current, charging current, and charging method to ensure that the battery is fully charged. At the same time, the remaining charging time is estimated according to the maximum output capacity of the charger and the charging status of the battery system.
4.2 Discharge management
Discharge management is a linear check of the 10s/30s peak discharge power MAP and continuous discharge power MAP of the power battery system by BMS according to the temperature collected in real-time and estimated SOC, to obtain the peak discharge power value and continuous discharge power value of the current 10s/30s of the battery system, and report it. Give the whole car MCU. MCU compares the motor request power P1 (motor request power is converted from the motor speed of the vehicle) with the peak discharge power Pmax and continuous discharge power Pc reported by BMS. When P1>Pmax, the peak discharge power of the battery system Pmax is discharged and timed. After the timeout, it will be reduced to the continuous discharge power Pc discharge; when P1<Pc, the next peak discharge power Pmax discharge occurs in BMS.
4.3 Balanced management
The main function of balanced management is to reduce the voltage difference between monomer voltages, so as to reduce the consistency of cell discharge and ensure that the battery system makes the maximum use of its stored energy under the premise of safety. Equilibrium is divided into active equilibrium and passive equilibrium. Active equilibrium is to transfer energy from high-voltage monomers to low-voltage monomers when the monomer voltage difference is too large to achieve equilibrium. Passive equilibrium is that when the monomer voltage difference is too large, a resistance is connected in parallel at the strings of monomers of high voltage, thus consuming part of the energy. Finally, achieve the equalization function.
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5:- Data communication and storage
BMS reports the single voltage, temperature, total voltage, current, and SOC, SOH, and fault alarm data of the battery system to the whole vehicle through CAN communication. After the whole vehicle receives the data, the first is to display the real-time total voltage, current, and SOC in the instrument; second, the fault alarm information is analyzed, and then Send commands to BMS through CAN communication, so that BMS can protect and control the battery system. At the same time, some faults light up the fault lights of the dashboard to remind users.
6: -Troubleshooting and management
According to the cell parameters and the functions of the battery system, a corresponding fault threshold table is formulated, including fault name, fault threshold, fault echo, fault detection time, and response time, as well as the protection measures taken by BMS and the protection measures of the whole vehicle. Reference: Dong Yun Peng and others Functional Design of Battery Management System (BMS).
Functional safety is of the highest importance in a BMS. It is critical during charging and discharging operation, to prevent the voltage, current, and temperature of any cell or module under supervisory control from exceeding defined SOA limits. If limits are exceeded for a length of time, not only is a potentially expensive battery pack compromised, but dangerous thermal runaway conditions could ensue. Moreover, lower voltage threshold limits are also rigorously monitored for the protection of the lithium-ion cells and functional safety. If the Li-ion battery stays in this low-voltage state, copper dendrites could eventually grow on the anode, which can result in elevated self-discharge rates and raise possible safety concerns. The high energy density of lithium-ion-powered systems comes at a price that leaves little room for battery management error.
Fig:-Battery Management system tester (BMS tester)
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