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
The measurement and monitoring functions of the battery management system mainly include the following aspects (of which SOC is the most important indicator).
①The technical performance of the battery. Different types, different models and different levels of use of batteries have different performance, including battery capacity, working voltage, termination voltage, quality, external dimensions and battery characteristics (including memory characteristics), etc. Therefore, it is necessary to establish a technical file for the power battery pack. In fact, even batteries of the same model and batch will affect the performance of the entire power battery pack due to manufacturing reasons, differences in electrolyte concentration and usage conditions. Therefore, before installing the battery pack, each battery should be carefully tested, and batteries with little difference in performance should be combined into a power battery pack.
②Management of battery status. The power battery pack of an electric vehicle is composed of multiple single-cell batteries, and its basic state includes the ratio of voltage, current, temperature, SOC, etc. during bidirectional operation of charging and discharging. Under normal circumstances, the voltage, current, temperature, and SOC ratio of the power battery pack should be capable of bidirectional measurement and display.
Due to a variety of reasons, individual batteries in the power battery pack may have performance changes, so that the power battery pack is not sufficiently charged when charging, and the electric energy is quickly discharged when discharging. This requires that the battery management system should be able to automatically detect the status of each single battery in time, and promptly give an alarm when it detects that a battery is damaged, so that the “bad” battery can be removed and replaced.
③Combination management of power battery packs. The power battery pack needs 8~32 12V single cells connected in series (referring to lead-acid batteries) or more single cells (referring to other batteries) in series. In order to be able to be installed in different positions of electric vehicles, usually power batteries The battery pack is divided into multiple small battery packs, and the small battery packs are arranged in a dispersed manner, which facilitates the mechanized installation, disassembly, and maintenance of the battery pack.
If it is found that the temperature of a certain battery is in an abnormal state and the SOC display is abnormal, the online response information of a certain battery is immediately fed back to the battery management system, and the fault diagnosis system predicts the failure of the power battery.
(2) Safety management of power battery pack
The battery pack management system should undertake the overall management of the power battery pack. On the one hand, it ensures the normal operation of the power battery pack, displays the dynamic response of the power battery pack and gives an alarm in time, so that the driver can grasp the situation of the power battery pack at any time; on the other hand, It is necessary to protect people and vehicles to avoid various accidents caused by batteries.
High-voltage cables are required to connect the battery to the battery, and the battery pack to the battery pack. When the total voltage of the power battery pack is high or the high-voltage DC output is adopted, the cross-sectional area of the high-voltage cable is relatively small, which is beneficial to the connection and fixation of the wiring harness, but the high voltage requires more reliable protection.
When the total voltage of the power battery pack is low, the current is relatively large, and the cross-sectional area of the high-voltage cable is relatively large. The high-voltage cable is very hard and cannot be deformed at will, and the installation is relatively inconvenient. A high-voltage cable is also needed to connect the battery boxes in series between the battery boxes. Generally, a manual or automatic breaker is installed in the last output box to cut off the current during installation, disassembly and maintenance. In addition, there are various sensor wiring harnesses in the battery box. Therefore, there are various long-sized wiring harnesses in automobiles, which require reliable insulation between the wires and quick connection.
The total voltage of the power battery pack can reach 90 ~ 400V. High voltage can cause harm to the human body. Effective isolation measures should be taken. Generally, the power battery pack is separated from the vehicle’s seating area, and the power battery pack is placed under the floor or in the car. On both sides of the shelf. Under normal circumstances, when the vehicle is out of use, it usually automatically cuts off the power, and only turns on the power when the car is started. When the car collides or overturns, the battery management system should be able to immediately cut off the power supply to prevent personal accidents and fires caused by high-voltage electricity, and prevent damage caused by electrolyte to ensure personal safety. The airbag can be used to trigger the BMS to control the automatic switch to disconnect.
The safety problem of the battery itself, especially the lithium battery will catch fire or even explode when overcharged. Therefore, the safety problem of the battery use is a problem that major automobile companies and scientific research institutions at home and abroad are currently facing and must solve. It directly affects electric vehicles. Whether the application can be popularized. In terms of safety, BMS mainly focuses on the protection of batteries and the prevention of high voltage and high current leakage. Its necessary functions: over voltage and over current control, over discharge control, prevention of excessive temperature, and collisions Turn off the battery. These functions can be combined with electrical control and thermal management systems to complete. Many systems specifically add battery protection circuits and battery protection chips. For example, the circuit design of the BMS smart battery module also has a single battery disconnect function. The most important thing in the safety management system is to grasp the battery status information in a timely and accurate manner, and send out an alarm signal or disconnect the circuit in time when an abnormal state occurs to prevent accidents.
(3) Thermal management of battery box
The power battery pack used in the car will generate heat when it is working. Different batteries have different heat levels. Some batteries can meet the heat dissipation requirements of the battery pack by using natural ventilation in summer, but some batteries must be compulsory. Ventilation for cooling can ensure the normal operation of the battery pack and prolong the life of the battery.
As for the heat generated by the battery when it is working, it can theoretically be used for heating and windshield defrosting, so that the heat can be managed and applied, but the actual car structure design determines that it is difficult to use this part of the heat or it is not economical in production.
In addition, some storage batteries in the northern winter need to be equipped with an insulated battery box and a constant temperature control system should be designed. The battery is assembled in a system, and the temperature of each battery should be the same or close.
In the temperature management system of the power battery pack, firstly, the support of the power battery pack should be arranged reasonably, and the power battery pack or its grouping should be easy to install, realize mechanized loading and unloading, and facilitate the connection of various wiring harnesses. After the position and shape of the support of the power battery pack are determined, the ventilation ducts, fans, power battery pack ECU and temperature sensor are designed.
Batteries have different working performance at different temperatures. For example, the best working temperature for lead-acid batteries, lithium batteries and nickel-hydrogen batteries is 25-40°C. Changes in temperature will change the battery’s SOC, open circuit voltage, internal resistance and available energy, and even affect the battery’s service life. The temperature difference is also one of the reasons for the battery balancing problem. Ahmad A. Pesaran of the National Renewable Energy Laboratory of the United States pointed out that the main tasks of the thermal management system are: make the battery work in an appropriate temperature range; reduce the temperature difference between each battery module. The use of a car air conditioner can realize the control of the battery temperature, which is also a commonly used temperature control method for electric vehicles, such as using air-conditioning refrigerant to pass into the radiator of the battery.
In electric vehicles, because each battery or each sub-battery group of the power battery pack is arranged in different positions of the vehicle, the heat dissipation environment is different everywhere, so these differences will also affect the charging and discharging performance of the battery and the service life of the battery. In order to ensure that each battery has a good heat dissipation condition and environment, the power battery of an electric vehicle is assembled in a forced cooling system to keep the temperature of each battery consistent or close, and make the surrounding environmental conditions of each battery similar.
The power battery temperature management system has two layouts, horizontal layout and vertical layout, as shown in Figure 1 and Figure 2.
(4) Balance management of power battery pack
Battery packs are different from single cells. Under the current level of battery manufacturing, performance differences between cells will inevitably exist throughout their life cycle. After being combined into multiple battery packs in series, if technical measures are not taken, The inconsistency of the charging/discharging process of the bulk battery will cause the single battery to fail prematurely due to overcharging and overdischarging. In order to avoid premature failure of single cells due to overcharging and overdischarging, and to make the performance indicators of the battery pack reach or approach the level of single cells, it is necessary to balance the single cells in the battery pack. The function of battery pack balancing: Minimize or disappear the deterioration of the charge/discharge performance of each battery cell in the battery pack after multiple cells are connected in series.
It is possible to avoid the performance deterioration of each battery cell inside the battery pack when it is discharged. A simple control circuit can be used. However, it is more difficult to avoid the performance deterioration of each battery cell inside the battery pack during charging. This makes charging equalization become A major problem in battery pack balancing.
There are two types of balance control for multi-cell power battery packs, namely, separate charge balance and charge/discharge combined balance. In a system with a well-designed capacity and discharge power balance, as long as the charge balance control is in place (the charge balance control at this time means: each time the charge balance control can restore the voltage of the worst single cell to full, the worst The performance of the single battery reaches the factory index), in fact, there is no need to discharge equalization. The performance of the battery pack in this balanced mode is determined by the performance of the worst single cell. If the performance of the worst single cell reaches the factory index, the performance of the battery pack can reach the design index. However, if the charge balance control is not in place, the charge/discharge joint balance becomes very important. In this case, the total balance is the sum of the charge/discharge balance, but this method is very unfavorable for the battery because During charging, overcharging may still occur.
The function of discharge equalization makes the discharge energy of the battery pack the average sum of all battery energy. Discharge equalization cannot solve the problem of performance deterioration after single cells are assembled into a battery pack.
For battery pack balancing, there are currently three balancing methods in the industry, namely, single charge balancing, charge balancing plus discharge balancing, and dynamic balancing.
Dynamic balance is the charge/discharge balance during the whole process of using and idle of the battery pack. It can conceal the problem of insufficient charge/discharge balance by extending the equalization time. Under dynamic balance, because the battery is finely balanced at all times, the amount of balance required during charging and discharging is greatly reduced.
In order to overcome the serious impact of battery inconsistency, the requirement of battery balancing is particularly important in battery use. For this reason, in the past ten years, many BMS developers have adopted various methods to balance the battery, which can be summarized as the shunt method (bypass method), cut-off method and parallel method.
① Diversion method. During charging, when the charging voltage of a certain battery exceeds a set value, a part of the current of the battery is shunted by a resistor connected in parallel with the battery, so as to achieve the purpose of reducing the charging voltage of the battery. The structure of this scheme is complex, the volume is large, the heat generation is large when the flow is divided, and the versatility is poor. This shunting method does not necessarily start shunting after the battery is over-voltage. It can start shunting and balancing when the voltage is higher than the average voltage.
②Cut off method. During charging, when the charging voltage of a battery exceeds the set value, the circuit of the battery is cut off by the automatic control switch, and the bypass switch is closed at the same time, the current bypasses this battery and continues to charge the next battery. The number of cut-off switches is twice the number of batteries. The cut-off method requires the cooperation of the charger, which requires the charger to be able to dynamically adapt to charging from one cell to all cells, and to dynamically adjust the charging voltage and charging current after switching the battery to achieve constant current, constant voltage charging and floating charging, etc. The requirements for chargers are relatively high.
③ Parallel method. The parallel method is to connect the batteries in parallel and then in series. This is also the method adopted by some battery manufacturers and battery users who try to use small-capacity batteries to form large-capacity, high-voltage battery packs. After the batteries are connected in parallel, the voltage of each single cell cannot be measured, and therefore, the monitoring of each single cell in the battery pack cannot be implemented. It can be seen that the parallel connection method cannot realize the balanced management of the battery pack.
(5) Battery status fault diagnosis
The fault diagnosis function is an important part of the BMS. The fault diagnosis can grasp the various states of the battery in real time during the working process of the power battery pack, and even locate the battery fault information to the various parts of the power battery system (including Battery module). The fault levels are divided into general faults, warning faults and severe faults. According to the level of the fault, the BMS summarizes the battery status into three types of information: repair as soon as possible, repair immediately, and battery life warning, and transmit it to the dashboard to warn the driver, so as to protect the battery from being overused.
① The important diagnostic contents of BMS are as follows.
a. BMS hardware fault diagnosis during startup. Sensor signal rationality diagnosis, battery pack voltage signal rationality diagnosis, battery module voltage rationality diagnosis, starting process current signal rationality diagnosis, starting process temperature signal rationality diagnosis.
b. BMS diagnosis during driving. Voltage fluctuation diagnosis, no module voltage diagnosis, no battery pack voltage diagnosis, no temperature signal diagnosis, current fault diagnosis, current sensor fault diagnosis, module voltage consistency fault diagnosis, overcurrent fault diagnosis, communication system fault diagnosis, blower fault diagnosis, High voltage control fault diagnosis, module voltage overcharge/overdischarge diagnosis, battery pack voltage overcharge/overdischarge diagnosis, module voltage change rate overcharge/overdischarge diagnosis, battery pack voltage change rate overcharge/over Discharge diagnosis, SOC overcharge/overdischarge diagnosis, sensor temperature overcharge/overdischarge diagnosis, average temperature overcharge/overdischarge diagnosis, sensor temperature change rate overcharge/overdischarge diagnosis, average temperature change rate over Charge/over discharge diagnosis.
②Diagnosis and failure handling strategies are as follows.
a. According to the cause of each fault, the entry and exit conditions of the diagnostic program are set for various fault diagnosis.
b. The time-sharing diagnosis process is adopted to save CPU time and resources.
c. According to the battery charging rate, dynamically adjust the charging diagnosis process parameters.
d. According to the battery discharge rate, dynamically adjust the discharge diagnosis process parameters.
n e. Fault diagnosis is carried out in three different levels (alarm, fault and danger).
f. The fault diagnosis result is sent to the VMS through the CAN bus.
g. The fault diagnosis result participates in the control of the actual working current of the battery.
h. The fault diagnosis result participates in the high-voltage power control.
③The monitoring software has the following functions.
a. Monitor the cell or module voltage of the power battery.
b. Monitor the total voltage of the power battery pack.
c. Monitor current.
d. Battery pack SOC
e. The average operating temperature of the battery pack.
f. Maximum module voltage.
g. Minimal value of module voltage.
i. The minimum value of the temperature sensor.
j. Monitor the maximum allowable charging current and the maximum allowable discharge current.
k. Monitor the fault code status of the battery pack.
l. Display the operating time of the working condition.
m. Store data and use Office software for post-processing analysis.