3.7 volt 18650 lithium battery

　　Research on 3.7 volt 18650 lithium batterymonitoring device

　　1. Internal resistance measurement technology based on digital filter

　　Online measurement of the internal resistance of each single cell is one of the difficult problems of the detection device, and the measurement accuracy is directly related to the accuracy of the analysis. Online measurement needs to solve the problem of interference between the charger and the electrical load. For large-capacity batteries, the internal resistance of the 3.7 volt 18650 lithium batteryis a micro-ohm level small signal. In this article, digital filtering technology is used to improve the measurement accuracy.

　　The following factors mainly affect online measurement:

　　1) Measure line-coupled high-frequency interference signals;

　　2) 50Hz power frequency interference;

　　3) Low-frequency ripple of charger;

　　4) The voltage of charging or discharging changes slowly;

　　5) Irregular changes in load.

　　For high-frequency interference, on the one hand, it is reduced through hardware low-pass filtering, and on the other hand, smoothing filtering is performed at the effective A/D sampling frequency. The effective signal composition is shown in Figure 1-1.

　　In the research of this article, a special excitation device is designed to feed controlled AC signals to the 3.7 volt 18650 lithium batterypack, and the measurement circuit collects the AC voltage signal of the 3.7 volt 18650 lithium batteryunder test. In order to eliminate the above influencing factors, IIR digital filtering technology is adopted.

　　Differential equation operations can be implemented in a direct manner. Figure 1-2 is a bandpass filter designed using an elliptical filter, M=N=11, with a good falling slope and equal ripples in the passband and stopband.

　　2. Interaction design scheme between monitoring device and charger

　　The interaction scheme between the monitoring device and the charger is a creative working mode to improve the accuracy of deterioration prediction. Its basic structure is shown in Figure 2-1.

　　Monitoring system structure for interactive solutions

　　The measurement theory and method in the float charge state have inherent limitations. Discharge testing can obtain more reliable data. However, current discharge testing either requires manual intervention or is performed passively after an uncertain power outage. The former is difficult to perform regularly. It is carried out sexually, and the risk is relatively high, and the uncertainty of the latter also brings hidden dangers. The interactive solution in this article is a systematic design solution for advanced power supply devices, which can effectively solve the various problems mentioned above.

　　The main principle of the interactive solution is: the 3.7 volt 18650 lithium batterymonitoring unit (BatteryMonitoringUnit--BMU) performs daily inspections, analyzes the collected data and changing trends, and requests the charger (RectifierUnit--RU) to cooperate with the partial discharge test under certain conditions. Since the RU is set to a certain rectified output value lower than the lower limit voltage of 3.7 volt 18650 lithium batterydischarge during partial discharge, the 3.7 volt 18650 lithium batterycan not only provide the load power of the electrical equipment, but also avoid the risk of shutdown due to 3.7 volt 18650 lithium batteryproblems during the discharge process.

　　In the normal float charge state, the BMU continuously detects the voltage and internal resistance of the 3.7 volt 18650 lithium batterypack. If abnormal voltage or internal resistance is found, the partial discharge test process is started to conduct a deeper test. The test process is also set to start at a certain period, such as one month.

　　During the discharge test, the discharge data required by the degradation prediction model are collected, including float voltage, initial drop, normal discharge voltage and other data, and the SOH is accurately known through the calculation of the battery's degradation (SOH) prediction model.

　　In addition, the interactive solution does not exclude passive testing after a power outage. Passive discharge can also trigger predictive calculations. When a discharge occurs, data collection is triggered, and a predictive calculation is started when the discharge depth reaches a certain set value.

　　In this way, based on internal resistance monitoring, the monitoring system achieves long-term continuous and accurate detection of SOH by using three types of discharge tests with different depths:

　　1) A fully discharged 3.7 volt 18650 lithium batteryshould be discharged to 100% depth before being put into operation to confirm that the 3.7 volt 18650 lithium batterypack can meet the design requirements. Otherwise, if there is a quality problem with the product itself, it will affect the accuracy of subsequent monitoring data processing. It should be fully charged before discharging and kept in a floating state for a certain period of time.

　　2) Medium depth of discharge. Medium depth refers to discharge of 30-50% depth. The data processing method of the detection device can calculate the SOH of each 3.7 volt 18650 lithium batteryquite accurately based on the discharge data at this depth. At the same time, it also avoids sudden power outages during deeper discharge processes, exposing the equipment to the risk of power outages. General 3.7 volt 18650 lithium batteryconfigurations often consider the 3.7 volt 18650 lithium batteryCapacity margin, such as doubled. Therefore, medium-depth discharges are safe under normal circumstances, including general power outages.

　　3) For periodic short-term discharge, select a suitable cycle according to the 3.7 volt 18650 lithium batteryapplication, such as 3 months. Generally, the depth of short-term discharge is about 5%, and the detection device starts the FNN operation to predict the SOH of the battery. Because it is a prediction, its reliability is still under study. This also includes whether the input data used in the FNN algorithm is sensitive to all 3.7 volt 18650 lithium batteryfailure conditions. In the FNN operation, there is also the "conservative" side of the algorithm, that is, it is better to underestimate SOH than to give up the risk of overestimating SOH.

　　Therefore, the long-term operation mode of the interactive scheme is shown in Figure 2-2. Generally, a medium-depth discharge is added after multiple short-term discharge tests, or a medium-depth discharge is performed to confirm when the short-term discharge test results indicate that the 3.7 volt 18650 lithium batterymay be seriously degraded. . If the prediction result is confirmed to be correct, the control center will be notified; if the prediction is proved to be wrong, the prediction model will be adaptively adjusted. After the last medium-depth discharge determined that the 3.7 volt 18650 lithium batterywas severely degraded, replacement measures were taken, and a complete discharge was performed before replacement. This set of data is of great significance to the improvement of the SOH model.

　　3. Modular design of monitoring device

　　3.1 Monitoring device design requirements

　　Figure 3-1 Hardware structure of monitoring device

　　3.2 Detection module design

　　The detection module mainly includes 5 parts:

　　1) Measurement circuits for voltage, current, and temperature;

　　2) Channel switching;

　　3) A/D conversion circuit;

　　4) Microprocessor unit;

　　5) Communication interface.

　　The detection module completes data collection and transmits the data to the control module. The high-precision and high-time-efficiency data acquisition module adopts a modular design, taking into account the principles of specialization and generalization, and has flexible configuration. According to the needs of the type and scale of sampling points, each module can be used alone or freely combined to adapt to different needs. monitoring situations.

　　The 3.7 volt 18650 lithium batterypack is composed of multiple single cells connected in series. The general configuration is shown in Table 3-1.

　　The series connection of batteries brings difficulties to the design of the sampling circuit. The current main solutions are as follows:

　　1) Due to the longevity and reliability issues of mechanical contacts, relay switching cannot be used in situations where rapid inspection is required.

　　2) Segmented sampling divides the 3.7 volt 18650 lithium batterypack into segments to reduce the voltage of each segment, using conventional inspection circuits. Since each segment needs to be isolated, the cost increases. Moreover, if a 3.7 volt 18650 lithium batteryopen circuit occurs, the voltage applied to a certain section may still be very high, and the circuit may be damaged if the on-site wiring sequence is incorrect.

　　3) Resistor voltage division: The resistor voltage division method is used in many designs. Since the calibration coefficient can be set for each channel, the common mode error caused by insufficient resistor matching accuracy can be corrected to a certain extent. The long-term stability of this method is limited by the stability of the resistor, and it is difficult to achieve the required accuracy under high common modes. BB Company's INA117 high common mode operational amplifier resistor matching reaches 0.005%, the common mode rejection ratio is 86dB when the temperature coefficient is 1ppm, and the detection error in the 400V common mode range reaches 20mV. For 2V VRLA batteries, the float voltage detection is accurate The voltage should be 10mV or better. Obviously, in reality it is difficult to obtain such high accuracy using the partial pressure method.

　　4) High-voltage resistant electronic switch In this project, high-voltage resistant electronic switch is used to solve the difficulty of inspection. photoMOS is a new type of optically coupled high-voltage electronic switch. It is similar to an ordinary optocoupler, but the output end is a field effect tube, which overcomes the tube voltage drop problem of the transistor and is suitable for the high withstand voltage, high precision, and High speed requirements.

　　The principle of the high common-mode sampling circuit is shown in Figure 3-2, and optical coupling is used for electrical isolation between the A/D and CPU.

　　Figure 3-2 High common mode sampling circuit

　　3.3 Internal resistance module design

　　The internal resistance module is adapted to the distributed structure of the system and accepts the scheduling of the detection module. An excitation signal used to inject the internal resistance measurement into the 3.7 volt 18650 lithium batterypack.

　　The design of the internal resistance module mainly studies and solves the following four aspects:

　　1) The controlled waveform and frequency are controlled by the sampling module CPU and can work at any frequency point and different waveforms within the design range.

　　2) Stability and accuracy must maintain time stability and temperature stability for long-term operation, and the modules can be interchanged.

　　3) The independent excitation signal is not affected by the 3.7 volt 18650 lithium batterycharge and discharge circuit.

　　4) It has a wide working range and can work normally within the lowest discharge limit and the highest charging upper limit of the 3.7 volt 18650 lithium batterypack.

　　The above requirements are mainly reflected in the hardware circuit design.

　　3.4 Control module design

　　The control module is used for data transmission, processing and human-machine interface operation. It has remote (centralized) management RS-485 (RS-232) interface, detection module control port, operation keyboard, display panel, sound and light alarm and alarm output contacts. The control unit displays 3.7 volt 18650 lithium batterydata in real time, intelligently analyzes the data, and provides timely alarms for abnormal 3.7 volt 18650 lithium batteryoperation conditions. Control the work of the detection module through the bus structure and collect the data collected by the detection module. This unit determines and processes the events that occur and issues sound and light alarms, and completes data communication, storage and query functions. These functions are used by operators for on-site event processing.

　　4. Application of monitoring devices

　　During the research process of this article, the monitoring device was actually used in three typical valve-regulated lead-acid 3.7 volt 18650 lithium batteryapplications: telecommunications 48V DC system, power 220V DC system, and petrochemical 400V uninterruptible power supply system, which verified the rationality of the technical solution.

　　Taking the DC system of a telecommunications office station as an example, 3.7 volt 18650 lithium batteryapplications have the following characteristics:

　　1. 48V system, each group consists of 24 2V single batteries connected in series, usually 2 groups of batteries.

　　2. Large-capacity battery, reasonably placed, and good operating environment.

　　3. It is difficult to perform periodic capacity check and discharge.

　　4. Generally, there is a backup oil generator, which will be started after a period of time after a power outage. It is more difficult to detect the problem of 3.7 volt 18650 lithium batterycapacity decline in time.

　　5. 3.7 volt 18650 lithium batterydata can be transmitted to the central control room through the power environment centralized monitoring system.

　　3.7 volt 18650 lithium batterymonitoring uses a control module with 2 sampling modules and 2 internal resistance modules. The system is connected to the power environment centralized monitoring system and networked with the central control room.

　　According to the general usage of valve-regulated lead-acid batteries and the purpose of monitoring and management, the design of the monitoring device mainly considers the following aspects:

　　1) Float charge voltage measurement The operating parameters of the 3.7 volt 18650 lithium batteryare mainly controlled by the charger, especially the float charge voltage of the battery, which directly affects the float charge service life of the battery. The relative difference in float voltage is very small, requiring the measurement circuit to have high accuracy; the high voltage after the 3.7 volt 18650 lithium batterypack is connected in series requires the circuit to have high common-mode resistance.

　　2) Current monitoring detects 3.7 volt 18650 lithium batterycharging, discharging, and current values.

　　3) Monitoring of ambient temperature (or standard 3.7 volt 18650 lithium batterytemperature).

　　4) Internal resistance measurement: measure the internal resistance value of each single cell online.

　　5) The modular structure system must meet most 3.7 volt 18650 lithium batteryapplication scenarios, including applications with different 3.7 volt 18650 lithium batteryconfigurations such as telecommunications, electric power, and UpS. It can be flexibly configured according to different numbers, specifications and placement forms of batteries, which facilitates on-site installation and maintenance.

　　6) Network design. Networking and informatization are the development trends of electronic equipment. System design must have communication interfaces and multiple network solutions. Be suitable for remote management and centralized monitoring.

　　7) Reliability detection devices are used in situations that require high reliability and require long-term stable operation of the device.

　　8) The electromagnetic compatibility detection device should not cause any additional interference to user equipment to ensure long-term stable operation of user equipment and the monitoring system. At the same time, the device is also required to have strong anti-interference ability and remain stable when the high-power power supply device is switched on and off.

　　As shown in Figure 3-1, the device consists of a control module, a detection module, an internal resistance module, related software and auxiliary components. One control module can be connected to multiple detection modules to complete the testing of 3.7 volt 18650 lithium batterypacks with different numbers and voltage specifications. Monitoring and management can manage multiple sets of batteries at the same time.

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