Data acquisition methods
Single cell voltage detection method
The battery cell voltage acquisition module is a crucial component of the power battery management system. Its performance and accuracy determine the accuracy of the system's judgment of battery status information, and further affect the effective implementation of subsequent control strategies. Commonly used methods for detecting cell voltage include relay array method, constant current source method, isolated operational amplifier acquisition method, voltage/frequency conversion circuit acquisition method, and linear optocoupler amplifier circuit acquisition method.
1. Relay Array Method
Figure 8-6 shows the block diagram of a battery voltage acquisition circuit based on the relay array method. It consists of a terminal voltage sensor, a relay array, an A-D (analog-to-digital) converter chip, an optocoupler, and a multiplexer. To measure the terminal voltage of n batteries connected in series, n+1 wires need to be connected to each node in the battery pack. When measuring the terminal voltage of the m-th battery, the microcontroller sends a corresponding control signal, which selects the appropriate relay through the multiplexer, optocoupler, and relay drive circuit, connecting the m-th and m+1-th wires to the A-D converter chip. Typically, the resistance of the switching devices is relatively small, and the error caused by the resistance of the switching devices is almost negligible after combining with a voltage divider circuit. Furthermore, the entire circuit structure is simple; only the voltage divider resistors, the A-D converter chip, and the voltage reference accuracy affect the accuracy of the final result. The errors of the resistors and the chip can usually be made very small. Therefore, the relay array method is most suitable for applications requiring high individual battery voltage measurements and high accuracy.
2. Constant Current Source Method
The basic principle of parallel battery voltage acquisition using a constant current source circuit is to convert the battery terminal voltage into a linearly changing current signal without using a conversion resistor. This improves the system's anti-interference capability. In a single-stage battery pack, because the battery terminal voltage is relatively low, generally between 2V and 5V, the voltage is relatively stable during discharge, thus improving the system's anti-interference capability. Therefore, a single-channel operational amplifier is often chosen in the design process to achieve this. Due to differences in circuit design and application, constant current source circuits can take many different forms.
The circuit shown in Figure 8-7 is one such example; it is a constant current source circuit composed of a series-select operational amplifier and an insulated-gate field-effect transistor.
As can be seen from the structure of the operational amplifier, this circuit is a multi-stage direct-coupled amplifier circuit with high open-loop gain and deep negative feedback. Its input stage uses a differential amplifier circuit and is integrated on the same silicon chip, resulting in excellent performance matching between the two, and the intermediate stage has high amplification capability. Based on the principle of differential circuits, this circuit has strong common-mode signal rejection capability. Therefore, when using an operational amplifier to measure the voltage of individual cells in a battery pack, the high common-mode rejection and amplification capability will improve the measurement accuracy. An insulated-gate field-effect transistor (IGFET) is a semiconductor device that uses the electric field effect of the input circuit to control the output circuit current. When it operates in the variable resistance region, the output drain current I is linearly related to the input drain-source voltage Us. Furthermore, the gate-source impedance of the transistor is very high, resulting in a very small leakage current, while the drain-source on-resistance is very small, resulting in a very low on-state voltage drop. Figure 8-7 uses a P-channel enhancement-mode field-effect transistor (FET), and a Zener diode is connected to maintain a constant gate-source voltage Ucs. The operational amplifier operates in the linear region. If a low on-resistance FET is selected, the on-state voltage drop is negligible. Therefore,
achievable
In the above equations, the difference between u₁ and u₂ is the battery terminal voltage, and U₁ is the output voltage of the inverting operational amplifier circuit. It is easy to see that the Zener diode connected to the output of the operational amplifier provides feedback, keeping the circuit in a balanced state. V₀ ↑→ |Uz| ↓→ IL ↓→ |VR| ↓→ VI ↑→ |V₀| ↓. Where V₀ is the output voltage of the operational amplifier; VR is the voltage across resistor R₁; and VI is the input differential voltage of the operational amplifier, i.e., VI = U₁ - U₂. When the circuit is in equilibrium, VI = 0. The constant current source circuit has a simple structure, strong common-mode rejection capability, high acquisition accuracy, and good practicality.
3. Isolation Operational Amplifier
An isolation operational amplifier is an electronic component capable of electrically isolating analog signals. It is widely used as isolators in industrial process control and as isolation media in various power supply devices. It generally consists of two parts: an input section and an output section. These are powered separately and coupled by magnetic coupling. The signal is modulated by the input section, passes through the isolation layer, and is then demodulated and restored by the output section. Isolation operational amplifiers are ideal for battery cell voltage acquisition circuits. They isolate the input battery terminal voltage signal from the circuit, thus avoiding external interference and improving system acquisition accuracy and reliability. A typical application example is provided below.
Figure 8.8 shows the application of an isolation operational amplifier in a 600V power battery management system. The battery pack contains 50 horizontal lead-acid batteries with a rated voltage of 12V, and their terminal voltages are acquired one by one by the isolation operational amplifier circuit. The ISO 122 is an isolation amplifier designed with modulation and demodulation technology packaged by Black & Decker (BBB) in the United States, employing precision capacitor coupling technology and a conventional dual-in-line (DIP) pin arrangement. The input and output sections of ISO 122 are located in the sampling circuit, separated by two matched 1pF capacitors forming an isolation layer. The rated isolation voltage is greater than 1500V (AC 60Hz continuous), with high isolation impedance and high gain accuracy and linearity, thus meeting practical application requirements. As shown in Figure 8.8, the input power of ISO 122 is drawn from the automatic battery pack, and the output signal, which has a linear relationship with it, is multiplexed, then automatically divided by two precision resistors controlled by the microcontroller before being sent to the input. The output power is supplied by the power supply module on the circuit board, and the battery terminal voltage is isolated. It should be noted that in the terminal voltage acquisition circuit of the 50th battery, an inverter is added after the isolated operational amplifier circuit to change the output signal from negative to positive. It should also be pointed out that although the isolated operational amplifier acquisition circuit has excellent performance, its high cost has limited its widespread application.
4. Voltage/Frequency Conversion Circuit Acquisition Method
When using a voltage/frequency (V/F) conversion circuit to acquire battery cell voltage, the V/F converter is crucial. It's the component that converts voltage signals into frequency signals, offering excellent accuracy, linearity, and integral input.
Figure 8-9 shows the circuit schematic of the LM331 V/F converter used for high-precision V/F conversion. The LM331 is a high-performance integrated V/F chip manufactured by FS Microcontroller. It employs a new temperature-compensated bandgap reference circuit, providing extremely high accuracy across the entire operating temperature range and at power supply voltages as low as 4.0V.
In this acquisition method, the voltage signal is directly converted into a frequency signal, which can then be processed by the microcontroller's counter port without the need for A-D conversion. Furthermore, to complement the V/F conversion circuit in the battery cell voltage acquisition system, corresponding selection circuits and operational amplifier circuits also need to be designed to achieve multi-channel acquisition functionality. This method involves fewer components, but the voltage-controlled oscillator contains capacitors, and the relative error of capacitors is generally large, with larger capacitors exhibiting even larger relative errors.
5. Linear Optocoupler Amplifier Circuit Acquisition Method
The battery cell voltage acquisition circuit based on a linear optocoupler achieves isolation between the signal acquisition end and the processing end, thereby improving the circuit's stability and anti-interference capability. Figure 8-10 shows the TIL300 linear optocoupler, which consists of an isolated feedback photodiode bifurcated by infrared LED illumination and an output photodiode. Special process technology is used to compensate for the nonlinearity of LED time and temperature characteristics, making the output signal linearly proportional to the servo luminous flux emitted by the LED. The TIL300 has a peak isolation of 3500V, a bandwidth greater than 200kHz, is suitable for isolated amplification of DC and AC signals, and has an output gain stability of ±0.05%/℃. As can be seen from the diagram, the voltage value of a single battery cell (the difference between U1 and U2) is converted into a current signal Ip by operational amplifier A and flows through the linear optocoupler TIL300. After opto-isolation, it outputs a current Ip2 that is linearly related to Ip1. This current is then converted back into a voltage value by operational amplifier A2 for A-D conversion and data acquisition. It is worth noting that the two ends of the linear optocoupler require different independent power supplies, labeled I+12V and ±12V in the diagram. This demonstrates that the linear optocoupler amplifier circuit not only has strong isolation and anti-interference capabilities but also maintains good linearity of the analog signal during transmission. Therefore, it can be used in conjunction with relay arrays or gating circuits in multi-channel acquisition systems. However, its circuitry is relatively complex, and many factors can affect its accuracy.
Temperature Acquisition Methods
Battery operating temperature not only affects battery performance but also directly relates to the safety of electric vehicles. Therefore, accurate temperature parameter acquisition is crucial. Acquiring temperature is not difficult; the key is selecting a suitable temperature sensor. Currently, many temperature sensors are available, such as thermistors, thermocouples, thermistor transistors, and integrated temperature sensors.
1. Thermistor Acquisition Method
The principle of the thermistor acquisition method is based on the characteristic that the resistance of a thermistor changes with temperature. A fixed resistor is connected in series with the thermistor to form a voltage divider, thus converting the temperature level into a voltage signal. This signal is then converted into digital temperature information through analog-to-digital conversion. Thermistors are inexpensive but have poor linearity and generally have relatively large manufacturing errors.
2. Thermocouple Acquisition Method
The working principle of a thermocouple is that a bimetallic body generates different thermoelectric potentials at different temperatures. By acquiring this thermoelectric potential value, the temperature value can be obtained by looking up a table. Since the thermoelectric potential value depends only on the material, the accuracy of thermocouples is very high. However, since thermoelectric potentials are millivolt-level signals, amplification is required, making the external circuitry complex. Generally, metals have high melting points, so thermocouples are typically used for high-temperature measurements.
3. Integrated Temperature Sensor Acquisition Method
As temperature measurement becomes increasingly common in daily life and production, semiconductor manufacturers have introduced many integrated temperature sensors. While many of these sensors are based on thermistors, they are calibrated during manufacturing, resulting in accuracy comparable to thermocouples. Furthermore, they can directly output digital values, making them well-suited for use in digital systems.
Current Acquisition Methods
Common current detection methods include shunts, transformers, Hall effect current sensors, and fiber optic sensors.
The characteristics of each method are shown in Table 8-1.
| Item | Shunt | Transformer | Hall Element Current Sensor | Fiber Optic Sensor |
|---|---|---|---|---|
| Insertion Loss | Yes | No | No | No |
| Arrangement Form | Need to be inserted into the main circuit | Open hole, wire access | Open hole, wire access | - |
| Measurement Object | DC, AC, Pulse | AC | DC, AC, Pulse | DC, AC |
| Electrical Isolation | No Isolation | Isolated | Isolated | Isolated |
| Ease of Use | Small signal amplification, need isolation processing | Relatively simple to use | Simple to use | - |
| Application Scenario | Small current, control measurement | AC measurement, power grid monitoring | Control measurement | Commonly used in high-voltage measurement power systems |
| Price | Relatively Low | Low | Relatively High | High |
| Popularization Level | Popularized | Popularized | Relatively Popularized | Not Popularized |
Among these factors, the high cost of fiber optic sensors limits their application in the control field; shunts are low-cost and have good frequency response, but are cumbersome to use as they must be connected to a current loop; current transformers can only be used for AC measurements; and Hall element current sensors offer good performance and are easy to use. Currently, shunts and Hall element current sensors are most commonly used in the current acquisition and monitoring of electric vehicle power battery management systems.
Smoke Detection Methods
During vehicle operation, due to complex road conditions and inherent battery manufacturing issues, extreme emergencies such as smoke or fire may occur due to overheating, compression, or collisions. If these incidents are not detected and effectively addressed promptly, they will inevitably escalate, threatening surrounding batteries, the vehicle, and personnel in the cargo compartment, severely impacting vehicle operational safety. To prevent such incidents, smoke monitoring has been introduced into battery management systems in recent years and is receiving increasing attention.
Smoke sensors are diverse and can be categorized into three main types based on their detection principles: ① Smoke sensors utilizing physicochemical properties, such as semiconductor smoke sensors and contact combustion smoke sensors; ② Smoke sensors utilizing physical properties, such as thermal conductivity smoke sensors, optical interference smoke sensors, and infrared sensors; ③ Smoke sensors utilizing electrochemical properties, such as current-type smoke sensors and electromotive force-type gas sensors. Because smoke sensors are diverse, semiconductor smoke sensors cannot detect all gases. Therefore, a specific type is chosen to detect one or two specific types of smoke. For example, oxide semiconductor smoke sensors are mainly used to detect hydrocarbon smoke, including O₂, H₂S, CO, H₂, O₃H₂O, Cl₂, OH, CO₂, etc. Due to electrode limitations, these sensors are primarily used to detect inorganic smoke, such as O₂, CO₂, H₂, Cl₂, SO₂, etc.
When smoke sensors are used in power batteries, sensor selection requires understanding the composition of smoke produced by battery combustion. Generally, battery combustion produces large amounts of CO and CO2, therefore sensors sensitive to these two gases should be selected. The sensor's structure needs to be adapted to the vibration conditions of long-term vehicle use to prevent false triggering due to road dust and vibration.
The smoke alarm device in the power battery management system should be installed on the driver's console. Upon receiving an alarm signal, it should quickly issue an audible and visual alarm and fault location, ensuring the driver can promptly detect and receive the alarm signal.
For example, the smoke alarm system used in the Olympic electric bus, primarily developed by Beijing Institute of Technology, uses a battery system powered by a 9V alkaline or carbon-zinc battery, ensuring 24-hour normal operation. The alarm signal is powered by the vehicle's 24V battery power supply, which is supplied separately to ensure the independence of the alarm system. Distributed alarms detect smoke concentration through internal smoke sensors. When the smoke concentration is below the limit, the internal controller of the alarm sets the relay output to open circuit; when the smoke concentration exceeds the limit, the internal controller sets the relay output to short circuit, quickly drawing the +24V power supply to the display panel to form an alarm circuit with the -24V power supply on the display panel, emitting an audible and visual alarm signal. The system structure is shown in Figure 8-11.
