Lithium iron phosphate (lifepo4 battery)-graphite system battery voltage platform 1 is generally 3.15V~3.45V, and research on lithium iron phosphate batteries is generally different from the charge-discharge curve, rate characteristics, temperature characteristics, open circuit voltage characteristics, cycle characteristics, etc. For perspective, the following are the relevant characteristic curves.
3.1 Charge-discharge curve
From the standard charge-discharge curve of the battery, the most basic input/output voltage, power and other characteristics of the battery can be understood, and it can be preliminarily judged whether the battery can meet the needs of the load.
According to the specification of the battery product, the rated capacity of the battery is 60Ah, the nominal voltage is 3.2V, the maximum charging current is 1C, the maximum discharge current is 2C, the charging and discharging voltage range is 2.5V~3.65V, and the operating temperature range is - 20'C~55°C.
The rate characteristic of the battery refers to the voltage and power variation characteristics of the battery under different input/output currents. It is mainly used to judge whether the battery can meet the load power demand, and is the first battery characteristic to be investigated.
The batteries were charged at different rates of 0.15C, 0.3C, 0.4C, 0.5C, 0.8C, and 1.0C, respectively, and the resulting charging curves were shown in Figure 4. In Figure 4, as the charging rate increases, the battery charging voltage platform gradually increases, indicating that the internal resistance of the battery causes the charging voltage to be polarized.
The charging capacity data of batteries with different rates are shown in Table 1. Only when the charging rate reaches 1.0C, the capacity decreases slightly, and the difference in battery charging capacity under other rates is not obvious. The charging capacity is high in the constant current charging stage before charging to 3.65V.
The battery was discharged at different rates of 0.15C, 0.3C, 0.4C, 0.5C, 0.8C, and 1.0C, respectively, and the resulting discharge curves were shown in Figure 5. The discharge plateau of the battery decreases sequentially with the increase of the discharge rate, which is opposite to the trend of the charging curve, which also explains the voltage polarization characteristics caused by the internal resistance of the battery.
The charging capacity data of batteries with different rates are shown in Table 2. The discharge capacity follows the trend of decreasing gradually with the increase of the discharge rate, and the discharge efficiency remains around 100%. Among them, the capacity of 0.15C and 0.3C is low, which may be caused by the measurement system error and the characteristics of lithium iron phosphate battery.
The comprehensive rate charge and discharge data shows that the battery has high input/output energy and stable input/output power at a current lower than 1.0C rate.
3.3 Temperature characteristics
The purpose of examining the temperature characteristics of the battery is to determine whether the input/output of the battery can meet the load energy requirements under different working environments, and it is also an essential part of evaluating the battery characteristics.
According to the common energy storage environment, four temperature points of 15C, 25C, 35°C and 45°C were selected to investigate the temperature characteristics of the battery.
The battery open circuit voltage is strongly related to the battery electromotive force, so it is closely related to the battery state of charge (SOC). To diagnose the battery SOC, it is necessary to study the battery open circuit voltage characteristics.
The battery is charged and discharged in the range of 0 to 100% SOC with a step size of 10% SOC, and the open circuit voltage of the battery under different SOC is obtained at an interval of more than 5 hours. The change curve is shown in Figure 8. In Figure 8, there is an obvious plateau region in the change trend of the open-circuit voltage of the battery with SOC, that is, the open-circuit voltage of the SOC changes smoothly in the range of 10% to 90%, which increases the difficulty for the open-circuit voltage method to estimate the SOC. Under the same SOC, the charging open-circuit voltage is higher than the discharging open-circuit voltage, indicating that there is still a certain error in the open-circuit voltage obtained by the experimental method, and the curve needs to be processed by weighted average when actually estimating the SOC.
Battery cycle characteristics reflect battery life, which is not only a concern of battery users, but also a key research area for battery management systems (BMS). The end of battery life is generally based on the rated capacity of the battery attenuating to 80%. The state of health (SOH) of the battery is used as a measure of the aging degree of the battery, which can be defined as the ratio of the fully discharged capacity of the battery in the current state to the initial capacity of the battery .
The actual battery life test is usually obtained by accelerated battery cycle test. The test condition is a 1C rate charge-discharge test at room temperature, the voltage range is 2.5V~3.65V, and the charging current ends at 0.03C. The test curve is shown in Figure 9. It can be seen from Figure 9 that the initial capacity of the test battery decays from 58.9Ah to 47.1Ah after 2789 cycles, the SOH reaches 79.97%, and the battery life ends. During the battery test, the discharge capacity showed a linear characteristic with the number of cycles.
Lithium iron phosphate battery (lifepo4 battery), as an energy storage battery that should be widely used, has a voltage platform of about 3.2V in the SOC range of 10%~90%. The tested battery has better charge-discharge rate characteristics within IC, temperature The input/output energy is stable in the range of 15C ~ 45°C, and the cycle at room temperature is 1C to the end of 2789 times, and the capacity decay shows a linear trend. It can be seen that the overall performance of the lithium iron phosphate battery (lifepo4 battery) is stable and can be used as a good DC power supply.
