Voltage detection and interface circuit design schematic diagram

Series battery packs are widely used in hand-held tools, laptop computers, communication radios, portable electronic equipment, aerospace satellites, electric bicycles, electric vehicles, and 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 cells in the battery pack should be consistent, so the single cells need to be monitored, that is, the voltage of the single cells needs to be measured.

There are many methods for measuring the voltage of series battery packs. Currently, the two most commonly used methods are differential detection type and current source detection type. The differential detection type requires the resistance values ​​of the two resistor pairs to be strictly matched, otherwise it will affect the detection accuracy of the battery pack voltage. In order to reduce the impact of the leakage current of the detection line on the consistency of the battery pack when using this method, the resistance value of the resistor needs to be increased, so that This will increase the difficulty of mass production and reduce detection accuracy. The current detection type detection circuit only requires resistance matching of one resistor pair. It is mentioned that in order to improve the detection accuracy, resistors with small resistance matching are needed, but this increases the leakage current of the detection line. In actual use, in order to reduce the impact of the leakage current of the detection line on the consistency of the battery pack and reduce the power consumption of the voltage detection circuit, it is necessary to add a switching control device to the voltage detection line, often using an optocoupler or a photoelectric relay. The current-type voltage detection circuit has good performance, but cannot detect when the voltage is lower than 2V. First, the voltage detection circuit is improved to expand the voltage detection range. Secondly, an improved voltage detection circuit and a photoelectric relay as a control switch were used to analyze and experiment the factors that affect the voltage detection accuracy. Finally, an electronic switch was used to replace the photoelectric relay, thereby improving the voltage detection accuracy.

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 by 2 times, so R1=2R2 is selected. The capacitor C1 in the circuit is a decoupling capacitor, the resistor R5 is a current limiting resistor, and the resistor R4 is used to ensure reliable operation of the circuit. In order to reduce the leakage current of the voltage detection circuit, an AQW216 ($3.4979) photoelectric relay is added to the voltage detection line of each cell as a detection control switch, as shown in Figure 2. When the battery voltage needs to be detected, the photoelectric relay is turned on through the control terminal. , closing the photoelectric relay after detection can effectively reduce the impact of leakage current during detection on the consistency of the battery pack.

The AD7674 ($39.7200) can provide three different conversion rate operating modes to optimize performance for different specific applications. The 3 working modes are as follows: WARP, allowing sampling rates up to 800 kHz. However, in this mode the conversion accuracy can only be guaranteed if the time between conversions does not exceed 1ms. If the time between two consecutive conversions is greater than 1 ms, the result of the first conversion will be ignored. This mode is suitable for applications requiring fast sampling rates. NORMAL, the sampling rate of this mode is 666 kHz. In this mode, there is no limit on the time between sampling conversions, which ensures both high conversion accuracy and fast sampling rate. IMPULSE, a low-power mode with a sampling rate of 570 kHz.

Only one C8051F060 ($12.4861) chip can complete various controls of the microcontroller 8051, multi-channel A/D conversion and D/A conversion, I2C, SPI data bus transmission, RS232 ($780.5000), RS485 ($49.9800) serial port communication, etc. function, thereby greatly reducing the types of components, reducing the area of ​​the printed board, saving costs, and improving system reliability. And its cross-switch configuration makes the I/O port application more flexible and convenient. The interface circuit diagram between AD7674 and C8051F060 shows the peripheral circuit and interface circuit of AD7674 in a high-speed acquisition system. The peripheral circuit includes the design of the voltage reference input, the design of the analog voltage input part, the design of the analog and digital power supply and the design of the interface circuit. The interface circuit includes the interface between AD7674, C8051F060 and CPLD.