Research and design of portable series 18650 battery 2200mahvoltage detection system

　　Series battery packs are widely used in communications radio stations, portable electronic equipment, aerospace satellites, electric whiteboards, electric vehicles, and UpS. Usually the performance of the single cells in the 18650 battery 2200mahdirectly affects the overall performance of the battery pack. In order to improve the reliability of the battery pack, the performance of the single cells should be consistent. For important equipment, it is necessary to frequently monitor all single cells online, that is, measure the single cell voltage.

　　There are many ways to measure the voltage of a single cell. The simplest one is to use the resistance dividing method: measure the voltage of each series node in turn. Since the measured voltage is the total voltage and the single cell voltage only accounts for a part, the measurement accuracy is as follows. The ratio is reduced; another measurement method is to use an isolated operational amplifier for each single cell. This operational amplifier can convert the battery voltage to a unified reference ground voltage and avoid drift errors and leakage currents caused by resistive voltage division. However, it is large in size and expensive, and is only suitable for those occasions where high measurement accuracy is required and leakage current and cost are not considered. In addition, the third method is to use relay array selection for measurement, which is complicated to control the switches. At present, the most common voltage measurement applications are the operational amplifier differential amplification method and the direct sampling method. However, these methods have the disadvantages of complex circuits and large volumes. They have certain limitations when used in fields that require volume and weight. Therefore, here we propose a measurement method that uses ordinary optocoupler instead of linear optocoupler, and a portable series 18650 battery 2200mahvoltage detection system with C8051F410 embedded microcontroller as the core.

　　2 System working principle

　　The portable series 18650 battery 2200mahvoltage detection system consists of a battery voltage conversion circuit, a C8051F410 microcontroller and an LCD liquid display, as shown in Figure 1.

　　2.1 Battery voltage conversion principle

　　The battery voltage conversion circuit is shown in Figure 2. The conversion circuit is composed of Nantong Optocoupler TLp521-1 and operational amplifier A1. The resistor Rb is the sampling resistor, and the capacitor Cn is used to improve the anti-interference ability.

　　The system uses embedded microcontroller C8051F410. The device uses a high-speed 8051μC core and operates at a speed of up to 50Mi/s, which minimizes power consumption and electromagnetic radiation and improves the system's anti-interference ability. It has a built-in 12-bit A/D converter with zero bit error rate and a sampling rate of up to 200kS/s, a maximum of 24 A/D conversion circuit input ports, simplifying the peripheral A/D conversion channel switching control circuit and achieving high-accuracy analog sampling; in power-saving mode, the typical current is less than 1μA at 32kHz The operating current is only 16μA. Based on the above characteristics, the C8051F410 microcontroller is used as the core of the system to simplify the design of system signal sampling and sampling channel control circuits and reduce the system volume. Use the circuit shown in Figure 2 to convert the 21-cell battery voltage. The system's liquid crystal display circuit uses LCD low-power driver pCF8563 and I2C bus interface, making the connection simple and reliable.

　　3Test data

　　In the voltage measurement experiment, Ra=5kΩ, Rb=5kΩ, the operational amplifier was LM324, and the comparison reference test equipment was Agilent 34401 (DC voltage test accuracy is 0.001V). A 18650 battery 2200mahconsisting of 10 cells was tested. Table 1 shows the system test voltage comparison, and Figure 5 shows the measurement error. It can be concluded from Table 1 and Figure 5 that the measurement error of the 12V 18650 battery 2200mahsystem is less than 0.1%.

　　4 Conclusion

　　After theoretical analysis and experimental verification, this battery monitoring system has advantages: (1) Using low-cost ordinary optocoupler to measure a single cell of the 18650 battery 2200mahcan meet the system accuracy requirements, saving 90% of the device cost, and simplifying the circuit design; (2) ) Due to the high isolation voltage of the optocoupler, the measurement range of the series battery voltage of this system can reach 2000V, which is 3 to 5 times higher than the withstand voltage of the relay; (3) The embedded C8051F410 microcontroller is used, which allows direct input of multiple Up to 24 analog signals, simple measurement circuit and small size.

　　In summary, the system realizes high-precision applications in which linear optocouplers are replaced by ordinary optocouplers, and low-cost circuits are used to solve high-precision and large-range voltage measurement problems. The characteristics of small size, light weight and low cost make the system have good application prospects in realizing portable application of 18650 battery 2200mahtesting equipment.

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