BMS battery management system high-precision current detection

qwqwqw2088

BMS battery management system high-precision current detection [Copy link]

1. Development trend of automobile electrification

在全球环境保护和可持续发展议题日益受到重视的背景下，新能源汽车已逐渐成为未来交通领域的关键发展方向。特别是电动汽车领域的先锋人物埃隆·马斯克，他通过公开其首款电动汽车Roadster的全部设计文件，显著促进了这一趋势的发展。Roadster项目在2008年推出，该车型采用锂离子电池技术，单次充电的续航能力超过200英里。

Tesla's decision to open source the design and engineering data of the initial Roadster project reflects its commitment to knowledge sharing and industry innovation. Musk once publicly stated: "I don't care about patents. In my opinion, patents are just a protective umbrella for the less competitive. They can't really promote technological progress, but may hinder the pace of innovation of other participants." This open attitude not only reflects the contribution to technological progress, but also injects new impetus into the development of the entire electric vehicle industry.

Figure 1. Tesla’s open-source Roadster data

2. Analysis of key characteristics of BMS battery management system

The working principle of power batteries is based on internal chemical reactions. When discharging, the anode undergoes an oxidation reaction to release electrons, while the cathode undergoes a reduction reaction to receive electrons. The electrons flow in the external circuit to form a current to supply the external load. When charging, this process is reversed. The key parameters of power batteries include open circuit voltage (the voltage of the battery when it is not connected to an external load or charger), nominal voltage (the stable voltage of the battery when it is working normally), battery capacity (the total amount of electricity that the battery can provide during a complete discharge process, in "A·h"), internal resistance (the internal resistance of the battery to the flow of current) and cycle life (the number of charge and discharge cycles that the battery can withstand before its performance drops to a specified percentage). These parameters determine the performance, efficiency and service life of the battery, and are an important basis for monitoring and management of the power battery management system.

Figure 2. Conventional electric vehicle battery management system hardware components

The power battery management system BMS is designed to achieve comprehensive management and monitoring of battery packs. It has very rich functions and can ensure instant protection of batteries in cases of overcharging, over-discharging, over-current and abnormal temperature to prevent them from being damaged.

Elon Musk's public documents show Tesla's in-depth research on electric vehicle power systems, especially its technical insights into key electrification components such as batteries, motors, and electronic control units. The following are several core functions of Tesla's Battery Management System (BMS):

1. Voltage and current monitoring: Tesla's BMS system continuously monitors the voltage and current of the battery pack to track the battery's state of charge (SOC) and state of health (SOH) in real time. This monitoring is critical to evaluating the energy flow of the battery pack during charging and discharging.

2. Temperature management: The strategic placement of temperature sensors within the battery pack allows for accurate monitoring of the temperature of individual battery cells and the entire battery pack. Effective temperature monitoring is critical to protecting the health and safety of the battery, as extreme temperature changes can adversely affect battery performance and life.

3. Battery balancing: Tesla BMS includes a battery balancing function to ensure that each battery cell in the battery pack is charged and discharged evenly, preventing overcharging or undercharging, thereby optimizing the performance and life of the battery.

4. Fault detection and diagnosis: BMS is designed to identify and diagnose any abnormal or fault conditions within the battery pack, such as cell failure, voltage imbalance, temperature abnormality, etc., which may affect the performance and safety of the battery.

5. Central Control Unit: As the core of the BMS, the central control unit is responsible for collecting data from various sensors and modules, processing information and making decisions based on complex algorithms to optimize battery charging and discharging, thermal management and overall performance.

6. Data Logging and Reporting: Tesla BMS also has data logging capabilities that can collect and store key data about battery performance over time, which is of great value for fault diagnosis, performance evaluation, and future technology improvements.

Figure 3. Battery Monitor Board – processor electrical schematic

Figure 4. Battery Monitor Board – CAN Interface Electrical Schematic

The electrification trend is the core of the development of new energy vehicles. Elon Musk further promoted the development of the electrification trend by publicly releasing the documents of the first-generation Roadster, demonstrating the entire process of electric vehicles from design to manufacturing.

Figure 5. Energy storage system high voltage box (PDU)

The BMS system is also an important component of the energy storage system. It can manage the battery system safely, reliably and efficiently. The early BMS system only had the function of detecting battery voltage and temperature, mainly monitoring the battery. With the development of technology and the need for battery safety protection, the BMS system has more functions. It can not only monitor the voltage and temperature of the battery, but also control and manage the battery according to the battery status, and can be compatible with the energy storage converter (PCS) to achieve the charge and discharge management of the battery cluster. The master control module and the slave control module are interconnected and communicated through the CAN bus. The BMS system collects the voltage and temperature of the battery module single cell (supporting lithium iron phosphate and ternary lithium) to calculate the SOC, SOH, the highest single cell voltage/temperature, the lowest single cell voltage/temperature, insulation resistance and other data. Not only can the passive balancing of the battery cell be achieved, but also the protection of the battery cell against over-voltage, under-voltage, over-temperature/under-temperature, over-current and over-discharge through the three-level fault protection and the control of the main circuit relay can be achieved.

Figure 6. Battery parameter error and sampling cycle requirements collected by energy storage battery management system - "GB/T 34131-2023 Battery management system for power energy storage"

3. Current detection technology

BMS is the bridge between the battery, the core component of new energy vehicles, and the whole vehicle. Benefiting from the development of new energy vehicles, BMS, as a core component, has also developed rapidly. BMS is divided into master-slave BMS and all-in-one BMS according to the different control structures. Regardless of the control structure, total current detection is essential.

The current sensor is generally located in the main positive or main auxiliary circuit of the power battery system high-voltage box assembly (BDU/PDU) to measure the current of the entire battery pack. The current signal will be sent to the BMS for charge and discharge control, battery SOC estimation, and overcurrent and overcharging protection.

As a key parameter in the power battery management process, current detection and current acquisition scheme affect the system cost and acquisition accuracy. The acquisition accuracy directly affects the accuracy of SOC and the protection of the battery system, and indirectly affects the vehicle's cruising range and user experience:

1. Ensure safety;

2. Record abuse information;

3. Used for battery pack SOC (state-of-charge) and SOH (state-of-health) estimation.

4. Current range, the peak current of the battery pack can reach 1200-1500A;

5. In order to improve the accuracy of SOC estimation and the utilization rate of the battery pack, the sensor accuracy requirement reaches 0.5%;

6. The customer also made some high demands on BDU active cooling and thermal control;

7. The electric control voltage is gradually increased from 400V to 800V to improve the working efficiency of the motor and reduce copper loss and cost.

Commonly used current detection technologies include resistance shunt sampling and fluxgate sensor sampling, and the corresponding detection methods are contact measurement (shunt) and non-contact measurement (fluxgate).

A shunt is made based on the principle that voltage is generated across a resistor when a DC current passes through it. A shunt is actually a resistor with a very small resistance. Its measurement is simple and the DC measurement accuracy can reach a relatively high level. Of course, there is also a shunt measurement method with a chip - IVT, which is an advanced form of shunt.

Figure 6. Shunt

The principle of measuring current with a shunt is relatively simple, and the theoretical basis is Ohm's law I=U/R.

When the shunt is connected in series in the circuit under test, the current flowing through the shunt is equivalent to the circuit current. The shunt resistance is known (to reduce power consumption, it is generally a micro-ohm resistance). By detecting the voltage across the shunt, the measured current is calculated according to the formula R=U/I.

Therefore, the key point affecting the accuracy of shunt testing lies in the stability of resistance. When a large current passes through the shunt, heat will be generated, causing the temperature of the shunt to rise. To ensure the detection accuracy of the shunt, the material used to produce the shunt must have a small temperature drift, and the resistance value must be less affected by temperature. Since manganese copper has the advantages of good temperature performance and small temperature drift, it is often used as a material for producing shunts. Traditional shunt production uses a brazing production process.

The main features of the shunt method are:

1. Contact measurement,

2. The shunt has no offset at zero current and is basically unaffected by temperature, which can avoid the drift caused by the coulomb meter (but the offset may be introduced by the measurement circuit);

3. The resistance of the shunt changes with temperature, so the temperature must be measured and the resistance calibrated;

4. There is energy loss in the shunt itself, which is dissipated in the form of heat;

5. The sensor is a small signal and needs to be amplified before collection, and the circuit needs EMI protection;

qwqwqw2088