Series batteries are widely used in portable tools, notebook computers, communication radio and portable electronic equipment, aerospace satellites, electric bicycles, electric vehicles, energy storage devices. In order to maximize the available capacity of the battery pack and improve the reliability of the battery pack, the performance of the single battery in the battery pack should be consistent, so it is necessary to monitor the single battery, that is, to measure the voltage of the single battery.

There are many methods to measure the voltage of series battery pack. At present, differential detection and current source detection are widely used. In the differential detection type, the resistance values of the two resistance pairs should be strictly matched, otherwise the detection accuracy of the battery pack voltage will be affected. In order to reduce the influence of the leakage current of the detection line on the consistency of the battery pack, the resistance value of the resistance needs to be increased, which will increase the difficulty of mass production and reduce the detection accuracy. In the current detection circuit, only one resistance pair is needed for resistance matching. In order to improve the detection accuracy, it needs small resistance matching, but increases the leakage current of the detection line. In order to reduce the impact of leakage current on the consistency of battery pack and reduce the power consumption of voltage detection circuit, it is necessary to add switch control devices on the voltage detection circuit, often using optocoupler or photoelectric relay. The current mode voltage detection circuit has good performance, but it can not detect when the voltage is lower than 2V. Firstly, the voltage detection circuit is improved to expand the range of voltage detection. Secondly, with the improved voltage detection circuit and the photoelectric relay as the control switch, the factors affecting the voltage detection accuracy are analyzed and tested. Finally, an electronic switch is used to replace the photoelectric relay to improve the voltage detection accuracy.

  Schematic diagram of voltage detection and interface circuit design

Figure 2 Schematic diagram of voltage measurement circuit

The reference voltage of the sampling circuit is 2.5V, so the battery voltage needs to be attenuated twice, so R1 = 2r2 is selected. In the circuit, the capacitor C1 is the decoupling capacitor, the resistor R5 is the current limiting resistor, and the resistor R4 is used to ensure the reliable operation of the circuit. In order to reduce the leakage current of the voltage detection circuit, aqw216 photoelectric relay is added to each single battery voltage detection line as the detection control switch, such as As shown in Figure 2, when the battery voltage needs to be detected, the photoelectric relay is turned on through the control terminal, and the photoelectric relay is turned off after detection, which can effectively reduce the impact of leakage current on the consistency of battery pack during detection.

Ad7674 can provide three different conversion rates to optimize performance for different applications. The three working modes are as follows: warp, which allows sampling rate up to 800 kHz. However, in this mode, only when the time between conversion is less than 1ms, can the conversion accuracy be guaranteed. If the time between two consecutive conversions is more than 1 ms, the result of the first conversion will be ignored. This mode is suitable for applications requiring fast sampling rate. Normal, the sampling rate of this mode is 666 kHz, in this mode, there is no limit on the time between sampling conversion, which can ensure high conversion accuracy and fast sampling rate. Impulse, a low power consumption mode with a sampling rate of 570 kHz.

  Schematic diagram of voltage detection and interface circuit design

Only one C8051F060 chip can be used to complete all kinds of control, multi-channel A / D conversion and D / a conversion, I2C, SPI data bus transmission, RS232, RS485 serial communication and other functions of single-chip microcomputer 8051, thus greatly reducing the types of components, reducing the area of printed circuit board, saving cost and improving system reliability. The configuration of cross switch mode makes the application of I / O port more flexible and convenient. The interface circuit diagram of ad7674 and C8051F060 is the peripheral circuit and interface circuit of ad7674 in high speed acquisition system. The peripheral circuit includes voltage reference input design, analog voltage input design, analog and digital power supply design and interface circuit design. The interface circuit includes ad7674, C8051F060 and CPLD.

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