Operating voltage of the battery

Working voltage, also known as terminal voltage, refers to the potential difference between the positive and negative poles of the battery when the battery is working, that is, when there is current flowing in the circuit. In the working state of battery discharge, when current flows inside the battery, the resistance caused by the internal resistance of the battery needs to be overcome, which will cause ohmic voltage drop and electrode polarization. Therefore, the working voltage is always lower than the open circuit voltage, and it is charged with it. On the contrary, the terminal voltage is always higher than the open circuit voltage. That is, as a result of polarization, the terminal voltage of the battery is lower than the electromotive force of the battery when the battery is discharged, and the terminal voltage of the battery is higher than the electromotive force of the battery when the battery is charged. Due to the polarization phenomenon, the instantaneous voltage and the actual voltage of the battery will have a certain deviation during the charging and discharging process. When charging, the instantaneous voltage is slightly higher than the actual voltage, after the end of the charge, the polarization disappears, and the voltage drops; during the discharge, the instantaneous voltage is slightly lower than the actual voltage, after the end of the discharge, the polarization disappears and the voltage rises. Among them, E+, E represent the potentials of the positive and negative electrodes, E+0, E0 represent the balanced electrode potentials of the positive and negative electrodes, VR represents the ohmic polarization voltage, and η+, η represent the overpotentials of the positive and negative electrodes, respectively. [Basic Principles of Discharge Detection] After basically knowing the voltage of the battery, we begin to introduce the discharge curve of the lithium battery. The discharge curve basically reflects the state of the electrode, which is the overlap of the state changes of the positive and negative electrodes. During the entire discharge process, the voltage curve of a lithium battery can be divided into three stages: 1) The battery voltage drops rapidly at the initial stage. The greater the discharge rate, the faster the voltage drops; 2) The battery voltage enters a stage of slow changes. A period of time is called the platform area of u200bu200bthe battery. The smaller the discharge rate, the longer the platform area continues, the higher the platform voltage, the slower the voltage drop. 3) When the battery is nearly discharged, the battery load voltage begins to drop sharply until it reaches the discharge cut-off voltage. When detecting, there are two ways to collect data (1) collect current, voltage and time data according to the set time interval Δt; (2) collect current, voltage and time data according to the set voltage change difference ΔV. The accuracy of charging and discharging equipment includes current accuracy, voltage accuracy, and time accuracy. Table 2 shows the equipment parameters of a certain type of charging and discharging machine. Among them, %FS represents the percentage of the full range, and 0.05%RD means that the measurement error is within 0.05% of the reading. Charging and discharging equipment generally uses numerical control constant current source instead of load resistance as load, so that the output voltage of the battery has nothing to do with the series resistance or parasitic resistance in the loop, but only the voltage E, internal resistance r and loop current of the ideal voltage source equivalent to the battery. I related. If a resistance is used as a load, set the battery equivalent ideal voltage source voltage as E, internal resistance as r, and load resistance as R. Use a voltmeter to measure the voltage across the load resistance, as shown in the upper diagram of Figure 6. However, in actual situations, there are lead resistance and clamp contact resistance in the circuit (unified as parasitic resistance). The equivalent circuit diagram of the upper diagram in Figure 3 is shown in the bottom diagram of Figure 3. In reality, parasitic resistance is introduced unsuspectingly, which makes the total load resistance larger, but the measured voltage is the voltage across the load resistance R, so errors are introduced. When a constant current source with current I1 is used as a load, the schematic diagram and actual equivalent circuit diagram of the constant current source load are shown in Figure 7. E and I1 are constant values, and r does not change within a certain period of time. It can be seen from the above formula that the voltages at points A and B are constant, that is, the output voltage of the battery has nothing to do with the size of the series resistance in the loop, and of course it has nothing to do with the parasitic resistance. In addition, the four-terminal measurement method can achieve a more accurate measurement of the battery output voltage. The constant current source is a power supply device that can supply a constant current to the load, and it can still keep the output current constant when the external grid power supply fluctuates and the impedance characteristics change. Disclaimer: Some pictures and content of articles published on this site are from the Internet. If there is any infringement, please contact to delete. Previous article: How about lithium electric vehicles?