lithuim ion battery 18650 module based on 48 cells for application research

　　In the management technology of sodium-sulfur batteries, the detection of cell voltage is an indispensable part, which has a very important impact on the safe and stable operation of the entire battery module. According to the detected cell voltage, balance management and alarm analysis are performed. Cell voltage alarms usually adopt a two-level gradient: alarm and lockout (or cut-off), which generally include: cell overvoltage alarm and cell overvoltage lockout. , cell under-voltage alarm, cell under-voltage lockout, cell voltage negative change rate alarm, cell voltage negative change rate lockout, and some will also add cell voltage imbalance alarm and lockout. lithuim ion battery 18650 modules usually contain many single cells. For example, a 5kW battery module contains 48 single cells. Because of the large number of cells, it is of great significance to find a practical detection solution.

　　There are many methods for detecting monomer voltage. Commonly used measurement methods include common mode measurement method and switch switching method. The common mode measurement method uses precision resistors to attenuate the voltage of each measurement point in equal proportions relative to the same reference point, and then subtracts them in sequence to obtain the voltage of each cell. The circuit of this method is relatively simple, but the disadvantage is that there is accumulated error, which reduces the measurement accuracy. . The switch switching method is used in the reference literature, but each unit in this scheme is equipped with two switches, which increases the cost, volume and power consumption of the system. On this basis, this article uses an improved scheme to achieve For cell voltage detection, this solution can effectively reduce the number of switches and the size of the entire detection system.

　　1 Design of single voltage inspection system

　　The research object of this article is a lithuim ion battery 18650 module containing 48 cells. During measurement, the 48 cells are divided into 4 groups: the first group is the cells numbered 01~12, and the second group is the cells numbered 13~24. , the third group is the monomers numbered 25~36, and the fourth group is the monomers numbered 37~48. Conduct parallel measurements on these four groups, that is, the first round measures the cells numbered 01, 13, 25, and 37, the second round measures the cells numbered 02, 14, 26, and 38, and so on, the twelfth round The cells with numbers 12, 24, 36, and 48 are measured in turn. At this point, the voltage detection of all cells in the entire battery module is completed.

　　Taking the first set of measurements as an example, the measurement principle diagram is shown in Figure 1, in which IN+ and IN- are connected to the A/D chip through the signal conditioning circuit. When measuring single cell1 numbered 1, switches S1, S2, O1, and O2 are closed, and the positive terminal of cell1 is connected to IN+ and the negative terminal is connected to IN-. When measuring cell 2 numbered 2, switches S2, S3, E1, and E2 are closed, the positive end of cell 2 is connected to IN+, and the negative end is connected to IN-. The relationship between the measured cell and the switch that needs to be closed is as follows As shown in Table 1, it is not difficult to find that when measuring odd-numbered cells, switches O1 and O2 are closed, and when measuring even-numbered cells, switches E1 and E2 are closed. Therefore, in order to reduce the number of switches O1, O2, E1, and E2 The number of operations and the loss caused by frequent switch operations improve the efficiency of voltage inspection. Measure the odd-numbered monomers and the even-numbered monomers separately, that is, measure the odd-numbered monomers first, and then detect the even-numbered monomers. .

　　In terms of device selection, following the principle of meeting system requirements and having a certain upgrade margin, TMS320F28335 is used as the main controller of the battery module management unit (BMU), and the field programmable gate array (FpGA) Ep2C8Q208C8N is used as the auxiliary control of the BMU. In this way, the ready-made interfaces of TMS320F28335 can be used, such as SpI interface, CAN interface, etc., while avoiding the use of a large number of discrete logic devices, making the circuit small in size and power consumption.

　　The switch in Figure 1 uses Panasonic photoMOS type optocoupler relay AQW214EH. The five GpIO ports of TMS320F28335 are used to control the Ep2C8Q208C8N to output 17 control signals to control the 17 switches in Figure 1 respectively.

　　An AQW214EH can be used as two switches. Figure 2 shows the specific implementation of switches S1 and S2. The implementation principles of the other switches are exactly the same. In Figure 2, cell1+ means connected to the positive electrode of cell1 in Figure 1, and cell2+ means connected to cell2 in Figure 1. The positive pole, S1 and S2 are respectively connected to the corresponding IO ports of the FpGA. When the IO port of the FpGA outputs a low level, the corresponding switch is closed, otherwise, the switch is opened.

　　The above part takes the first group as an example to describe the measurement principle. The implementation principles of the remaining three groups are exactly the same as the first group. These four groups share the 17 control signals output by Ep2C8Q208C8N, so as to ensure that each round of measurement can be detected. The corresponding numbered monomers in these four groups. The four groups of output signals are sent to the A/D chip respectively through the signal conditioning circuit. The A/D chip used in this design is the ADS8325 with 16-bit precision and a maximum sampling rate of 100KSpS. Its serial SpI output is isolated by an optocoupler. Connected to the SpI interface of TMS320F28335, since the SpI clock frequency can reach the MHz level, the time taken to read data from the ADS8325 is basically negligible, and the sampling time of each round will be very short.

　　It is not difficult to find that for a lithuim ion battery 18650 module containing 48 cells, if the solution of assigning 2 switches to each cell is adopted, 96 switches will be needed, that is, 48 pieces of AQW214EH will be needed. The group requires 17 switches, and the four groups have a total of 68 switches, that is, 34 pieces of AQW214EH, which will greatly reduce the size and cost of the circuit.

　　2 Software simulation and testing of single voltage inspection system

　　The auxiliary controller Ep2C8Q208C8N outputs 17 signals to control 17 switches according to the control signal of the main controller. Its input signals are en, oe, a, b, c, which respectively correspond to the 5 control signals of the main controller, where en is Enable signal, high level is active, oe is the odd and even control terminal. When oe is 0, the odd-numbered monomer is measured. When oe is 1, the even-numbered monomer is measured. Regardless of the odd number, Whether the number or even number is detected, there are 6 batteries that need to be detected. For this, 6 states are needed. These 6 states are controlled by signals a, b, c. The output signals are S1, S2,..., S13, O1, O2, E1, E2, these 17 control signals respectively correspond to the 17 switches in Figure 1. When the output is low level, the switch is closed, and when the output is high level, the switch is open.

　　In QuartusⅡ9.1, Verilog HDL language is used to program the auxiliary controller. When designing the program, a reasonable coding method should be selected. Commonly used coding methods are: sequential coding (also called binary coding), Gray code and one-hot code. For small digital system designs it is more efficient to use sequential encoding and Gray code. As far as sequential encoding is concerned, sometimes multiple bits change at the same time. For example, when changing from 011 to 100, every bit of the binary changes. However, it is impossible to ensure complete synchronization of multiple bits in a circuit. Once If they are not synchronized, erroneous logic will be generated, and there is only one bit difference between adjacent Gray codes, which greatly reduces the logic confusion in the circuit when transitioning from one state to the next, and improves the anti-interference ability of the circuit. It also reduces the electrical noise in the circuit, making it more reliable than sequential coding. Therefore, this article uses Gray code for programming.

　　Compile and simulate the written program. The simulation results are shown in Figure 3. Observing the waveforms of a, b, and c, you can find that only one of the three changes changes each time. This is the Gray mentioned above. code, when oe is low level, the switch control signal at both ends of the odd-numbered monomer is low level, thereby realizing the detection of the odd-numbered monomer. When oe is high level, the switch control signal at both ends of the even-numbered monomer is The signal is low level, thus completing the detection of even-numbered monomers. The simulated waveform results are completely consistent with the previous analysis.

　　In order to verify the feasibility of this design, the cells numbered 01~48 were inspected and the test results were uploaded to the monitoring platform. The display results of the monitoring platform are shown in Figure 4. After frequent long-term testing, it was found that the designed The system can accurately detect the voltage values of cells numbered 01~48 without any erroneous logic.

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