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Design of a self-discharge detection circuit

by:dcfpower     2021-03-11
u003cpu003e Self-discharge is caused by the internal factors of the battery, and is affected by external factors. Generally, the self-discharge performance of the battery is determined by the performance of the positive and negative electrodes, the polymer separator and the electrolyte that constitute the battery, and is affected by the manufacturing process and production requirements. The self-discharge is not fixed during the life of the battery. It is also related to the aging degree of the battery, SOC and the temperature of the battery environment. Poor charging and discharging systems and working conditions will also affect the self-discharge of the battery. Constitute an influence. u003c/pu003eu003cpu003e Excluding the self-discharge caused by the manufacturing process and incorrect use, the only factors that affect the battery self-discharge are temperature, SOC and battery life. Among them, the influence of temperature on battery self-discharge is obvious. The higher the temperature, the more active the chemical reaction inside the battery, and the chemical reaction that causes battery self-discharge also accelerates as the temperature increases. Therefore, the higher the temperature, the greater the battery's self-discharge rate. During the storage and application of the battery, it is necessary to ensure that the battery is in a lower temperature range to avoid the increase in self-discharge caused by the increase in the outside or the battery's own temperature and the decrease in battery performance. SOC represents the state of charge of the battery. When the battery is in different SOC, its self-discharge is also different. Generally speaking, the greater the SOC of a lithium battery pack, the more obvious its self-discharge. The life span indicates the degree of deterioration of the battery. As the battery continues to age, its internal resistance continues to increase, and the battery's charge retention capability decreases. Our common mobile phone battery standby time is getting shorter and shorter, which is caused by the aging of the battery. In this process, in addition to the serious internal polarization, the self-discharge also increases. u003c/pu003eu003cpu003eu003c/pu003eu003cpu003e The self-discharge detection circuit is the core embodiment of the self-discharge detection method. According to the capacity retention method, it is necessary to monitor the amount of charge required to maintain the battery SOC or voltage. It is necessary to build a charge and discharge control circuit for the battery in the detection circuit to facilitate the detection of the charging current when the voltage is stable. The functional block diagram of the self-discharge detection module is shown in Figure 1 below. u003c/pu003eu003cpu003eu003cpu003eu003c/pu003eu003cpu003eFigure 1 Functional block diagram of self discharge detection moduleu003c/pu003eu003cpu003eFig.1 Self discharge detection moduleu003c/pu003eu003c/pu003eu003cpu003e The reference voltage, as the input of the self-discharge detection circuit, comes from the analog voltage converted by the D/A conversion module according to the digital quantity given by the MSP430F149. The reference voltage value can be set by human keys to measure the self-discharge of the battery under different voltages. First of all, the reference voltage is compared with the real-time voltage of the actual battery through a voltage comparison link to determine the difference between the battery voltage and the reference voltage at this time. If the battery voltage is higher than the reference voltage, the charging circuit stops working, and the battery automatically discharges through the load resistance; if it is lower than the reference voltage, the charging circuit starts to work and charges it until the battery voltage is stable. The charging current at this time is the sum of the charging voltage and the current discharged by the battery through the load resistance and the self-discharge current. The charging current at this time is measured and the discharge current value is calculated. Then the difference between the two is the cell The value of self-discharge current. u003c/pu003eu003cpu003eu003c/pu003eu003cpu003eu003cpu003e In addition, although the voltage sent from the reference voltage module is already DC in principle, it is still unavoidable to be mixed with clutter, which may cause the detection circuit to oscillate , So it is necessary to set a low-pass filter to filter out high-frequency interference. Considering that when fu003e f0, that is, when the signal frequency is greater than the cut-off frequency of the passband, the logarithmic amplitude-frequency characteristic of the first-order low-pass filter only drops at a slow speed of -20dB/decade, and the voltage amplification factor cannot be immediately reduced to Zero, there is a wider transition band, and the filtering performance is poor. Therefore, the second-order low-pass filter circuit is used to ensure the passband gain, while the high-frequency amplitude-frequency characteristics can be quickly attenuated at a speed of -40dB/decade, and the filtering effect is better. u003c/pu003eu003cpu003e At the same time, since the load in this design is an actual battery, and the reference voltage provided by the D/A conversion module cannot be used as a charging source, the self-discharge detection circuit module should also have the function of power amplification, In order to realize the charging action of the battery. The schematic diagram of the self-discharge detection circuit is shown in Figure 2. u003c/pu003eu003cpu003eu003cpu003eu003c/pu003eu003cpu003eFigure 2 Self discharge detection circuitsu003c/pu003eu003cpu003eFig.2 Self discharge detection circuitsu003c/pu003eu003cpu003eu003cpu003e In this figure Among them, the signal DA_Vout is the reference voltage signal generated by the D/A conversion module. The entire circuit uses DA_Vout as the system input, and the battery voltage as the output and feedback, thus forming a feedback system, which finally stabilizes the battery voltage at the reference voltage , The self-discharge current value of the battery can be obtained by measuring the voltage across the sampling resistor Sample Res at this time. In order to prevent the battery from discharging through other circuits such as proportional operational amplifiers, which would cause the self-discharge current value to be inaccurate, a photoelectric switch is introduced, and the pins of the single-chip microcomputer are directly controlled on and off. u003c/pu003eu003cpu003eu003cpu003e
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