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Technical Application of Current Sensors in New Energy Vehicle Battery Management System

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Update time : 2022-05-08 11:12:22

As the core of new energy electric vehicles, on-board battery is directly related to vehicle life, mileage, vehicle economy, and safety, all of which depend on the performance of the battery management system. The monitoring accuracy and execution reliability of the battery management system depend on various sensors, so the research and analysis of the current sensor technology is particularly necessary.

1. New energy electric vehicle battery management system

The battery management system is a system that monitors the voltage, current, load, temperature and other states of the vehicle battery, and can provide safety, communication, cell balance and management control for it, and provide a communication interface with the application equipment. The BMS can monitor the total voltage and current data of the battery system, obtain the voltage of single cells, cell groups, and battery modules, and grasp the temperature and shape of the battery pack.

2. Application of current sensor in battery management system

2.1 Hall current sensor

The changing magnetic field of the Hall-effect sensor is converted into a changing voltage, which is an indirect measurement. It can be divided into two types: open-loop and closed-loop, the latter has higher precision. The Hall current sensor simplifies the circuit. It only needs to connect the positive and negative poles of the DC power supply, and pass the measured current bus through the sensor to complete the isolation detection of the main circuit and the control circuit. The output signal of the sensor is the secondary side current, which is proportional to the primary side current (input signal), the value is small, and A/D conversion is required. Hall current sensor integrates the advantages of transformer and shunt, and has a simpler structure, but it is susceptible to interference and is no longer suitable for the increasingly sophisticated and complex new energy electric vehicle power supply environment.

2.2 Fluxgate current sensor

The principle of the fluxgate is that under the influence of the excitation current, the size of the excitation current changes the inductance strength, which in turn changes the magnitude of the magnetic flux, and the magnetic flux opens or closes like a door.

The accuracy of ordinary Hall current sensor is between 0.5% and 2%, while the fluxgate current sensor is made by the principle of fluxgate, and the accuracy can reach 0.1% or even higher, so it is also called high precision current sensor. In terms of structure, there are also two types of open type and non-open type, that is, there are two types of open-loop and closed-loop. This paper focuses on the closed-loop fluxgate current sensor, that is, amplifying the second harmonic signal of the fluxgate excitation current, driving the compensation coil, so that the magnetic flux of the magnetic core and the magnetic flux of the primary current are canceled, and the “zero magnetic flux” is maintained. The magnetic flux is not zero for the HPIT series, and it is a symmetrical shape without second harmonics.

Fluxgate current sensors are divided into 4 categories in terms of structure, which are single magnetic ring, double magnetic ring, double magnetic ring (shielded), and multi-magnetic ring (nested). Due to the high sensitivity of the fluxgate principle, the strict correspondence between the closed-loop magnetic balance and the turns ratio output, the overall magnetic core closure, and the probe compensation to eliminate oscillation harmonics affecting the output cleanliness, etc., closed-loop fluxgate current sensors are widely used. It is used in various new energy electric vehicle products, such as Tesla Model 3, BYD, Ideal ONE, Xiaopeng P7 and other best-selling models.

2.3 Tunneling magnetoresistance effect current sensor

The tunneling magnetoresistance (TMR) current sensor is a new generation of magneto-sensitive elements. Compared with Hall devices, anisotropic magnetoresistance (AMR), and giant magnetoresistance (GMR), it has the advantages of low energy consumption, low temperature drift, The advantages of high sensitivity can significantly improve the sensitivity and temperature characteristics of current detection, so it is used to completely replace the Hall sensor in the battery management system of a new generation of new energy electric vehicles. The TMR current sensor no longer needs to perform temperature compensation when detecting current, and reduces the total temperature drift in the environment of -40°C to 85°C from 1% to 2% to 0.1% to 0.2%.

For example, in the current detection and control of on-board chargers, it can accurately detect the current of copper bars or wires and use a smaller chip, and the accuracy, linearity, response speed and temperature drift characteristics are more optimized. The car brings excellent safety and economy.

3. Development trend of battery management system sensor technology

3.1 The trend of functional integration

New energy electric vehicles have been developing in the direction of light weight, and at the same time, the requirements for the integration of components are more stringent. BMS is a management system with complex structure and integrated functions, and its size is small, so the sensors are required to have multi-functional integration, so that the battery system can be fully monitored with a minimum number of sensors. When an abnormality occurs, the fault point can be found faster and more accurately.

3.2 The trend of precision monitoring

In the future, the monitoring data accuracy of sensor technology will become more and more precise, and more accurate data will be required for the collection of data such as current, voltage, temperature and humidity, so as to improve the user's accurate grasp of the working conditions of the battery system. The next step is to start from theoretical simulation and experimental research at the same time, and research and explore a new generation of high-efficiency and high-precision BMS sensors for monitoring.

3.3 Product security trends

Functional safety is a basic requirement for new energy electric vehicles and an inevitable trend in the development of sensor technology. On the one hand, it is necessary to ensure the safety of the sensor product itself, and on the other hand, it is the safety of the entire BMS supported by the sensor, which will directly or indirectly affect the driving safety, and affect the user's driving experience and personal safety.

4. Summary

With the continuous upgrading of the new energy electric vehicle industry at home and abroad, more and more sensor technologies will be applied to new energy electric vehicles and BMS. BMS will also be upgraded from the current “hardware + algorithm” system to “data + active management” system. HANGZHI current sensors work closely with new energy electric vehicle industry to ensure the system updating.

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