1). The necessity for electric cars to be equipped with battery management systems
(1) Problems with batteries of electric cars
①The large-capacity single battery is prone to overheating. The single battery has a certain temperature tolerance range. In actual applications, if the volume is too large, local overheating will occur, which will affect the safety and performance of the battery. Therefore, the capacity and volume of the single battery are limited. In a harsh environment, the maximum local temperature of a 110mm × 110mm × 25mm 20A·h lithium battery is 135°C; while a 110mm × 220mm × 25mm 50A·h lithium battery has a local temperature of 188°C, which is more likely to cause safety problems. , So it is necessary to monitor and control the temperature.
② The performance of the batteries is not completely consistent. Based on the current plate materials and battery manufacturing level, the performance of the single cells cannot be completely consistent. After the high-power and large-capacity power battery pack is formed in series and parallel, it is harsh The use conditions of the battery are also easy to induce local deviations, which can lead to safety problems. The inconsistencies of battery performance during production and use are shown in Table 1.
|production process||Use process||The difference|
|The production process and material are different||Long time use, material aging is out of sync||Voltage, internal resistance, capacity|
|Different batches produced||Capacity, internal resistance|
|Individual battery internal short circuit||Battery self-discharge||Current, internal resistance|
|Different areas in the battery pack have different temperatures||Voltage, internal resistance, current capacity|
|Series and parallel charging and discharging working current||Uneven voltage distribution|
|Partial system leakage||SOC (state of charge) changes differently|
③The main problems after the battery is assembled are as follows
a. Overcharge/overdischarge. When charging/discharging battery packs connected in series, some batteries may be fully charged or discharged before other batteries. Continued charging/discharging will cause overcharging or overdischarging. The internal side reactions of the battery will cause problems such as a decrease in battery capacity, thermal runaway, or internal short circuit.
b. Excessive current. Parallel connection, aging, low temperature and other conditions will cause the current of some batteries to exceed their capacity and reduce the battery life.
c. The temperature is too high. If the local temperature is too high, the performance of the battery will decrease, which will eventually lead to internal short circuit and thermal runaway, resulting in safety problems.
d. Short circuit or leakage. Vibration, heat, humidity, dust and other factors cause battery short-circuit or leakage, threatening the personal safety of drivers and passengers.
(2) The function of the battery management system
One of the functions of the Battery Management System (BMS) is to avoid the above-mentioned problems in the battery pack. It is necessary to dynamically monitor the working status of the power battery pack, and collect the terminal voltage and temperature, charge/discharge current and battery of each battery in real time. Group total voltage, estimate the SOC, State of Health (SOH) and State of Electroformation (SOE) of each battery. Then by controlling related components, the battery can be prevented from overcharging or over-discharging. At the same time, it can give the battery status in time, find out the box number and box number of the faulty battery, select the problematic battery, and keep the entire battery pack Reliability and efficiency of operation.
In addition, the BMS also needs to set a user-oriented display to convert the estimated remaining power into a mileage that can be driven. At the same time, it also needs to have automatic alarm and fault diagnosis functions to facilitate the driver’s operation and processing. Therefore, the BMS tasks can be summarized as follows: the data acquisition circuit first collects battery status information data, and then the electronic control unit (ECU) performs data processing and analysis, and then sends control instructions to the relevant functional modules in the system according to the analysis results, and sends them to the outside world Send message.
BMS contains multiple processing modules: data acquisition module, SOC estimation module, electrical control module, safety control module, thermal management module, data communication and display module, etc. The main tasks, input signals and actuators of the BMS are shown in Table 2.
|The main tasks of BMS||input signal||operating units|
|Prevent overcharging||Battery voltage, current, temperature||Charger|
|Avoid over discharge||Battery voltage, current, temperature||Motor power converter|
|temperature control||Battery temperature||Hot and cold air conditioning (fans, etc.)|
|Balance of battery component voltage and temperature||Battery voltage and temperature||Balance device|
|Predict battery SOC and remaining driving range||Battery voltage, current, temperature||Display device|
The requirements of charging stations for energy storage devices are large capacity, long life, fast response, and trickle charge. Therefore, the requirements for BMS are different, but the overall function is still similar to that of power battery BMS, which can monitor battery SOC and SOH. Status, dynamic charge/discharge, intelligent management and output control functions.
2). Battery management system structure
The most basic function of the battery management system is to manage the battery pack, and it also includes wire line management, heat (temperature) management, and voltage balance control. Figure 1 shows a block diagram of the BMS structure.
①Battery pack management system: manage the working conditions of the battery, avoid over-discharge, over-charge, over-heating, and promptly alarm for faults, so as to maximize the storage capacity and cycle life of the battery. Its main functions include battery pack voltage test, battery pack current test, battery pack and single-cell battery temperature test, SOC calculation and display, battery pack remaining power display, car online mileage display, automatic diagnosis and alarm, and safety protection, etc. .
②Wire line management system: power battery group and connection, power wiring harness, manual or automatic breaker, sensor type, sensor wiring harness.
③Heat (temperature) management system: battery pack combination mode, battery pack and bracket arrangement, ventilation management system and fan, temperature management ECU and temperature sensor, heat management and application.
④Voltage balance control system: balance the charge of each battery, can extend the battery life, and balance the capacity of the new battery after replacement.
3). The management content of the battery management system
The battery management system mainly performs the following tasks: voltage, current and temperature measurement; calculation of battery SOC; calculation of battery DOD (depth of discharge); calculation of maximum allowable discharge current; calculation of maximum allowable charging current; prediction of battery life index and SOH; fault diagnosis.
(1) Battery measurement and monitoring
(2) Safety management of power battery pack
(3) Thermal management of battery box
(4) Balance management of power battery pack
(5) Battery status fault diagnosis
4). The composition of the battery management system
The basic composition of the battery management system is shown in Figure 2. The basic composition of a battery management system with a temperature measuring device is shown in Figure 3. It uses the principle that the temperature of the damaged battery during the charging process is higher than the normal battery temperature, and uses a temperature sensor to determine and monitor whether the temperature of each battery during the charging process is within the allowable normal range. If it is found that the temperature of a certain battery becomes abnormal, and the remaining power display is abnormal, the corresponding information of the battery will be fed back to the battery management system immediately, and the fault diagnosis system will predict the failure of the power battery pack.