A battery management system (BMS) monitors the voltage, current, load, temperature, and other conditions of a vehicle battery, providing safety, communication, cell balancing, and management control, as well as an interface for communication with application devices. A BMS monitors the total voltage and current of the battery system, acquires the voltage of individual cells, cell packs, and battery modules, and understands the internal temperature and shape of the battery pack. It mainly consists of three parts: hardware architecture, underlying software, and application software.
The main sensors used in BMS include current sensors, temperature and humidity sensors, voltage sensors, position sensors, and gas sensors.
Hall current sensor
Hall effect sensors convert a changing magnetic field into a changing voltage, which is an indirect measurement. They can be divided into open-loop and closed-loop types, with the latter offering higher accuracy. Hall current sensors simplify the circuit; only the positive and negative terminals of a DC power supply need to be connected, and the busbar of the current to be measured passes through the sensor to complete the isolation detection between the main circuit and the control circuit, as shown in Figure 4. The sensor output signal is the secondary current, proportional to the primary current (input signal), and is relatively small, requiring A/D conversion. Hall current sensors combine the advantages of current transformers and shunts with a simpler structure, but are susceptible to interference and are no longer suitable for the increasingly sophisticated and complex power supply environments of new energy electric vehicles.
fluxgate current sensor
The principle of fluxgate magnetism is that when an easily saturated magnetic core is affected by an excitation current, the magnitude of the excitation current changes the inductance, which in turn changes the magnitude of the magnetic flux. The magnetic flux then opens or closes like a door.
Ordinary Hall effect current sensors have an accuracy between 0.5% and 2%, while fluxgate current sensors, made using the fluxgate principle, can achieve an accuracy of 0.1% or even higher, hence they are also called high-precision current sensors. Structurally, they come in two types: open-loop and closed-loop. This section focuses on closed-loop fluxgate current sensors, which amplify the second harmonic signal of the fluxgate excitation current to drive a compensation coil, causing the flux of the magnetic core to cancel out the flux of the primary current, maintaining a "zero flux" state. For the HPIT series, the flux is not zero; it is a symmetrical shape without second harmonics.
Fluxgate current sensors are structurally classified into four types: single magnetic ring, double magnetic ring, double magnetic ring (shielded), and multi-magnetic ring (nested).
Due to its advantages such as high sensitivity based on fluxgate principle, strict correspondence between closed-loop magnetic balance and turns ratio output, overall magnetic core sealing, and clean output due to probe compensation to eliminate the influence of oscillation harmonics, closed-loop fluxgate current sensor is widely used in various new energy electric vehicle products, such as best-selling models like Tesla Model 3, BYD Han, Li Auto ONE, and XPeng P7.
Tunneling magnetoresistive current sensor
Tunneling magnetoresistive (TMR) current sensors are a new generation of magnetic sensing elements. Compared with Hall devices, anisotropic magnetoresistance (AMR), and giant magnetoresistance (GMR), they have advantages such as low power consumption, low temperature drift, and high sensitivity, and can significantly improve the sensitivity and temperature characteristics of current detection.
Temperature sensor
Temperature plays a crucial role in the performance of a BMS. To further improve battery utilization, prevent over-discharge (over-charge), control battery conditions, and increase battery life, an NTC temperature sensor is built in to monitor temperature.
NTC temperature sensors are primarily made from oxide compounds of high-purity metallic elements such as Mn, combined with ceramic and semiconductor technologies. Their working principle is based on the fact that these materials have a low number of charge carriers and high resistance; as the temperature increases, the number of charge carriers increases accordingly, and the resistance decreases (Figure 7). They possess advantages such as high resistivity, low heat capacity, fast response, excellent linear relationship between resistance and temperature, flexibility, low cost, and long lifespan. There are three commonly used types: ground-ring housing NTC temperature sensors, commonly known as "ground-ring type"; epoxy resin encapsulated NTC temperature sensors, commonly known as "water drop head" or "small black head"; and thin-film NTC temperature sensors.
Humidity sensor
A humidity sensor is a device that converts ambient humidity into an electrical signal. Common humidity sensors measure relative humidity. Currently, the humidity sensors commonly used in the BMS (Battery Management System) of new energy electric vehicles include resistive and capacitive humidity sensors. Their principle involves coating a moisture-sensitive film onto a substrate. When water vapor in the environment is adsorbed onto the film, the resistivity and resistance of the element change, thus measuring the humidity.
Humidity is a particularly difficult factor to capture in the battery management system of new energy electric vehicles, but it has a significant impact on battery performance and lifespan. Temperature compensation is applied to the humidity output of the sensor to obtain a linear voltage, which is then input to the BMS of the new energy electric vehicle equipped with an ADC.
Voltage sensor
The battery packs in electric vehicle power supply systems consist of hundreds of cells connected in series, thus requiring a large number of channels for voltage measurement. While the voltage of a series-connected battery pack is cumulative, the electromotive force (EMF) of each individual cell is not identical, making it impossible to simply use unidirectional compensation to eliminate errors. Battery voltage acquisition needs to be highly accurate, achieving 1mV, while current acquisition accuracy is only 5mV.
A voltage sensor converts the voltage of a battery under test into an output signal. Electroluminescent voltage sensors used in new energy electric vehicles measure the intensity of light emitted by the luminescent material at the measured voltage to obtain the effective value of the measured voltage. Compared to traditional optical voltage sensors, electroluminescent voltage sensors no longer use a carrier light source. This eliminates the instability of carrier light source measurements and simplifies the sensor structure, reducing production costs.
Position sensor
The position sensor in the BMS is mentioned in a utility model patent titled "Battery Temperature Control Management System and Electric Vehicle," and it is not yet widely used in new energy electric vehicles.
Position sensors are primarily used to detect the position of the coolant level in the water-cooling unit of the BMS system. The position sensors are mounted on coolant floats to detect the coolant position relative to the expansion tank level, thus determining the contact status between the expansion tank outlet and the coolant. Typically, at least three floats are required, with a position sensor installed on each float. This allows the BMS to promptly adjust and control the switching between the main and auxiliary water pumps when the vehicle traverses steep inclines or when there are numerous air bubbles in the cooling system.
Gas sensor
Thermal runaway in new energy vehicle power batteries typically results in the generation of large amounts of abnormal gases (carbon monoxide, hydrogen, hydrogen fluoride, TVOC, etc.) before the battery catches fire. After detecting the fault using CO and hydrogen sensors, a warning is issued, requiring the vehicle controller to take effective action. The Battery Management System (BMS) comprehensively monitors the battery's health. Different sensors have their own advantages and disadvantages; generally, multiple different sensors are used to detect thermal runaway in the power battery.
Carbon monoxide sensor
To minimize casualties and losses, timely fire detection and early warning are crucial. Thermal runaway in power batteries typically results in the production of large amounts of CO before ignition; therefore, monitoring CO concentration is undoubtedly an effective solution. Once the alarm threshold is exceeded, the alarm is activated, personnel are evacuated, and firefighting efforts begin, thus gaining valuable time.
hydrogen sensor
For new energy vehicles, hydrogen sensors can be used not only to monitor hydrogen leaks in hydrogen storage tanks and fuel cell systems, but also to detect the hydrogen concentration in exhaust gases. New energy vehicles can then use this monitored information to analyze the performance and reaction rate of the fuel cell stack in real time, thereby adjusting relevant input indicators or data configurations to achieve safe and efficient vehicle operation.
Battery Management System Sensor Technology Development Trends
1. Trend of Functional Integration
New energy electric vehicles have been developing towards lightweight design, while simultaneously placing increasingly stringent demands on component integration. A Battery Management System (BMS) is a complex and functionally integrated management system with a small footprint. Therefore, it requires sensors with multi-functional integration to comprehensively monitor the battery system with a minimal number of sensors. This also allows for faster and more accurate location of faults in the event of anomalies.
2. Monitoring the trend towards greater precision
Future products will require increasingly precise monitoring data from sensor technology. More accurate data collection is needed for parameters such as current, voltage, temperature, and humidity to improve users' understanding of battery system operating conditions. The next step requires simultaneous research and development through theoretical simulations and experimental studies to explore a new generation of highly efficient and precise BMS sensors.
3. Trend towards product safety
Functional safety is a fundamental requirement for new energy electric vehicles and an inevitable trend in the development of sensor technology. On the one hand, it's necessary to ensure the safety of the sensor products themselves; on the other hand, it's crucial to ensure the safety of the entire BMS (Battery Management System) supported by the sensors. Both of these factors directly or indirectly affect driving safety, the user's driving experience, and personal safety.
