Battery management system analysis

　　Against the background that the government is enforcing the parallel management of fuel consumption and new energy vehicle points, a large number of car companies have launched plans to develop and launch new energy vehicles and continue to increase their volumes. In order to meet this series of plans, in the fields of PHEV (including EREV) and EV, automobile companies need to use different combinations of new energy vehicles to comply with policy regulations, adapt to market demand, and cater to consumers. This requires the optimization of vehicle models. The core indicators (cruising range, acceleration to 100 kilometers and charging speed) will be dynamically configured and managed, and can cope with possible changes in battery suppliers in the future. In this process, we will talk in detail about the value of building a battery management system and how to build a battery management system.

　　First part of modular supply

　　To put it simply, with the development of the electric vehicle industry, my country may also follow the German VDA and launch lithium battery standards for automobiles. The standardization of battery cells and modules is imperative. By connecting the battery cells in series, parallel or a mixture of series and parallel, the battery module is ensured to have the same size, and the mechanical properties, thermal properties and safety properties of the battery body are comprehensively considered. While the installation plan remains unchanged, different battery capacities are provided according to different cruising range and power requirements to meet different needs. This kind of modular application can realize large-scale automated production on both the single and module sides, significantly reducing production costs. This makes the supply of the entire battery company use modules as the smallest unit.

　　Low temperature lithium iron phosphate battery 3.2V 20A -20℃ charging, -40℃ 3C discharge capacity ≥70%

　　Modular supply has changed the original construction method of battery companies. Originally, battery monomers were supplied, but car companies needed to build from the monomers. The topology of the entire BMS must be selected based on the battery size. Under the premise of supplying modules, , the basic unit becomes a small module assembly.

　　In this process, the next integrated battery module will contain a larger capacity battery pack than the traditional electric vehicle module. In the past, battery modules generally consisted of 12 battery packs with a capacity of 2-3kWh, but now they are turning to 6-8kWh battery packs that can accommodate 24 cells. This will increase battery capacity within the same battery space and effectively increase the battery life of electric vehicles.

　　The basic situation of soft bags is similar, and it is beginning to develop in this direction.

　　The LECU functions are embedded in the module, and the module temperature collection, unit voltage collection and voltage protection are basically eliminated.

　　·Cell voltage measurement and voltage monitoring: The collection and maintenance of cell voltage, this function is delegated to the bottom layer. Here it is divided into:

　　Collecting cell voltages: accuracy affects comparison of cell differences

　　Judgment of overvoltage and undervoltage: Here is also a logical function that can be implemented below

　　Verification: Complete the diagnosis and processing of the entire function through the judgment of monomer accumulation and module voltage.

　　·Battery temperature: Nowadays, 2-4 temperature points are usually placed in a module to collect the busbar welding temperature and the battery temperature difference within the module.

　　·Communication and signals: transmit temperature and voltage information, as well as basic cell overvoltage and undervoltage

　　·Balanced practical control: primarily includes practical circuits

　　The second part of the battery management function

　　As mentioned before, due to changes in the supply model, battery management functions need to match the entire battery system, and the underlying basic components have become modules. The issues faced by automobile companies here are:

　　·Differences in the needs of the vehicle power system: According to the actual configuration requirements of different vehicles, there are different requirements for the discharge capacity and power characteristics of the battery

　　·Charging characteristics: According to the actual conditions of use, special charging needs can be customized

　　·Regional application characteristics: Depending on the regional environment where the vehicle is used, different thermal management characteristics may even need to be configured.

　　·Differences in modules may require switching of the chemical system of the monomer depending on the needs of the vehicle.

　　In this way, the need for vehicle companies to control BMS is obvious:

　　It is divided into "variable part" and "immutable part". The common parts are:

　　1) Battery parameter detection: including total voltage, total current, insulation detection (monitoring leakage), collision detection, etc.

　　·Total voltage measurement: When calculating the SOC later, the total voltage of the battery pack is often used for calculation. This is one of the important parameters for calculating the battery pack parameters; if the cell voltage is accumulated and measured, the cell voltage sampling of the battery itself has There is a certain time difference and it is impossible to achieve accurate alignment with the data from the battery sensor. Therefore, the battery pack voltage is often collected as the main parameter for calculation. When diagnosing the relay, it is necessary to compare the internal and external voltages of the battery pack at the same time.

　　·Total current measurement: Current measurement methods are mainly divided into two types: smart shunt or Hall current sensor. Since the current values that the battery system needs to handle are often very large instantaneously, such as the discharge current required for vehicle acceleration and the charging current during energy recovery, there is a certain need to evaluate and measure the output current (discharge) and input current (charge) of the battery pack. Range and accuracy.

　　·Insulation resistance detection: It is necessary to conduct insulation detection on the entire battery system and high-voltage system. A relatively simple method is to rely on a bridge to measure the insulation resistance of the positive and negative poles of the bus to the ground wire. Active signal injection can also be used, mainly to detect the insulation resistance of the battery cells to the system.

　　·High-voltage interlock detection (HVIL): used to confirm the integrity of the entire high-voltage system (which can be divided into two parts, the discharge circuit and the charging circuit). When the high-voltage system circuit is disconnected or the integrity is damaged, safety needs to be activated. Measures taken.

　　·SOC and SOH estimation: including state of charge (SOC) or depth of discharge (DOD), state of health (SOH), state of function (SOF), energy state (SOE), fault and safety state (SOS), etc.

　　2) Fault diagnosis and fault-tolerant operation

　　·Fault detection refers to analyzing the collected sensor signals, using a diagnosis algorithm to diagnose the fault type, and providing early warning. Battery failures refer to sensor failures, actuator failures (such as contactors, fans, pumps, heaters, etc.) in various subsystems such as battery packs, high-voltage circuits, and thermal management, as well as network failures and various controller software and hardware failures. wait. The battery pack's own faults include overvoltage (overcharge), undervoltage (overdischarge), overcurrent, ultrahigh temperature, internal short circuit failure, loose joints, electrolyte leakage, insulation degradation, etc.

　　·The failure of the battery management unit also needs to be alarmed with a fault code (DTC). The DTC triggers the indicator light on the instrument panel. In new energy vehicles, there are also corresponding indicator lights to remind the driver of battery failure. Because batteries have certain risks, the connected car system often needs to transmit information directly to deal with sudden accidents. For example, when an accident occurs, when the airbag pops up and the relay is directly cut off by the vehicle controller, the connected car system handles the problem through positioning and early warning, especially battery discharge. Fault diagnosis includes diagnosing faults in battery cell voltage, battery pack voltage, current, and battery pack temperature measurement circuits, determining the fault location and fault level, and making corresponding fault-tolerant controls.

　　·Fail-Safe's fault-tolerant operation mechanism refers to the downgrade operation processing of the vehicle after the vehicle encounters an error during operation. In fact, this function is more like an upgrade and backup for all the above functions. This mechanism includes fault detection, fault type judgment, fault location, fault information output, etc.

　　3)Relay control

　　·There are usually multiple relay systems in the control battery pack to complete the drive supply and status detection of the relay. The relay control is often coordinated with the vehicle controller to confirm the controller, and the collision signal output by the airbag controller is usually coordinated with the relay controller. Disconnect direct hook. The relays in the battery pack generally include main positive, main negative, precharge relay and charging relay. There is also an independent power distribution box outside the battery pack for more detailed maintenance of the entire current distribution. For the relay control of the battery pack, the closing and opening conditions and the sequence of switching are all important.

　　Variable parts:

　　1) Hot processing:

　　·It is necessary to detect the temperature parameters of the battery pack thermal management system (the temperature of the fluid inlet and outlet), and the detection circuit is similar to the single unit detection. According to the temperature distribution information and charging and discharging requirements in the battery pack, the intensity of automatic heating/heat dissipation is determined so that the battery can operate at the most suitable temperature as much as possible and give full play to the battery's performance.

　　·Thermal control: The chemical properties of the battery are greatly affected by the temperature of the environment. In order to ensure the service life of the battery, the battery must operate within a reasonable temperature range, and the vehicle controller can calculate its output based on different temperatures. and maximum input power. CFD simulation analysis is mainly used to control the temperature of the battery system. The core here is to select different internal methods of thermal management, and then ensure that the temperature threshold is available through internal management strategies.

　　2)Charging control

　　An important form of the original battery management system is to monitor the needs of the battery system during the charging process and be responsible for the current input of the entire battery system, including the management and control of regular charging and energy recovery. The variable part now is the planning for fast charging. Due to consumer demand and practical conditions, this area is also in a very high change area.

　　3) Balance management: In actual use of series-connected battery packs, the output capacity of each series connection is different. The battery not only has over-discharge and over-charge constraints, but also has constraints on input and output power at different temperatures and different SOCs. In other words, the constraints of a single battery will affect the entire battery.

　　·Individual differences between individual cells in the battery pack: differences in cell capacity, differences in cell internal resistance, differences in cell self-discharge, differences in operating current and dormant current

　　·As time changes in the battery pack, the cell capacity, cell internal resistance, and cell self-discharge of the battery will all vary.

　　·Customer use: charging time, discharging time

　　·External environment: self-discharge at the same temperature, self-discharge at different SOC

　　·System interaction: the working status of BMS. This factor is related to the working status of BMS.

　　When the actual battery capacity changes significantly and the balancing capability is determined, the upper end of the BMU needs to provide different strategies.

　　Therefore, a possible change in the future is that the battery management system will form a separation between the lower end and the upper end. In order to carry out a large number of projects, save management and change management, the automobile factory needs to form Party B among Party A, the part that specializes in system software, to be responsible The core algorithm and configuration process of the entire battery system management. They are responsible for setting the maintenance and usage thresholds of the battery, and are responsible for the availability and after-sales service of the entire vehicle. The entire BMS management hardware has nothing to do with car companies. It requires very good software and hardware interface files, otherwise it is very easy to make mistakes. The things we can control in the future are also quite limited.

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