LiFePO4 charge-discharge curves analysis refers to the analysis and research of the voltage and current changes of LFP batteries during the charge and discharge process. By analyzing the charge-discharge curve, you can understand the performance and characteristics of the battery and evaluate its capacity, internal resistance, cycle life, and other important parameters.
LFP charge-discharge curves usually take the capacity as the horizontal axis and the voltage or current as the vertical axis. During the charging process, the voltage gradually rises until it reaches the set end-of-charge voltage; during the discharge process, the voltage slowly decreases until it reaches the set end-of-discharge voltage. By observing the shape, slope, and change trend of the charge-discharge curve, the following important analysis points can be drawn:
1. Charging efficiency: Charging efficiency refers to the energy loss of the battery during the charging process. The charging efficiency can be evaluated by observing the slope and changing trend of the charging curve. The larger the slope, the higher the charging efficiency.
2. Discharge characteristics: Discharge characteristics refer to the voltage change of the battery during the discharge process. By observing the slope and changing trend of the discharge curve, the discharge characteristics of the battery can be evaluated. The larger the slope, the better the discharge characteristics.
3. Capacity evaluation: The battery’s capacity can be evaluated by observing the charge-discharge curve’s area. The larger the area, the greater the capacity.
4. Internal resistance evaluation: The battery’s internal resistance can be evaluated by observing the voltage and current changes of the charge-discharge curve. The faster the voltage drops, the greater the current change, indicating the greater the internal resistance.
5. Cycle life evaluation: The battery’s cycle life can be evaluated by observing the charge-discharge curve’s shape and change trend. If the change of charge-discharge curve is small and the shape remains stable, the cycle life is longer.
.The following are 12 kinds of common LiFePO4 charge-discharge curves.
LiFePO4 Charge Curves
1. CC Charge Curve
Constant Current Charge (CC Charge: the current is constant during the charging process, and the voltage gradually increases.
Suggestion: Allow
2. CV Charge Curve
Constant Voltage Charge (CV Charge): the voltage reaches the set value at the moment of charging, and the current is at the peak state. As shown in the figure below, after the LFP battery is discharged to 3.0V, the instantaneous current of 4.2V constant voltage charging reaches about 17.5C. Then the current gradually decreases, and the charging ends after getting the set value.
Suggestion: Do not use
3. CC-CV Charge Curve
Constant Current and Voltage Charge (CC-CV Charge): a combination of CC charging and CV charging. During the charging process, the current is kept constant first, and when the voltage is charged to the upper limit voltage, the voltage is kept constant. Finally, the charging ends after the current gradually decreases to the set value.
Suggestion: Allow
4. CP Charge Curve
Constant Power Charge (CP Charge): keep the power constant during the charging process because the voltage gradually increases during charging, so the current will gradually decrease.
Suggestion: Allow
5. CP-CV Charge Curve
Constant Power and Constant Voltage Charge (CP-CV Charge): It is a combination of constant power charging and constant voltage charging. First, charge at constant power to the end voltage, then keep the voltage constant. When the current decreases to the set value, the charging ends.
Suggestion: Allow
LiFePO4 Discharge Curves
1. CC Discharge Curve
Constant Current Discharge (CC Discharge): LFP batteries’ most common discharge method. During the whole discharge process, the current remains constant, the voltage gradually decreases to the termination voltage, and the discharge ends.
Suggestion: Allow
2. CV Discharge Curve
Constant Voltage Discharge (CV Discharge), the discharge instantaneous voltage reaches the set value, and the current is at the peak state. As shown in the figure below, set the constant voltage discharge of the LFP battery to 3.0V, and the instantaneous current of the discharge reaches 30C-35C. The current gradually decreases, and the discharge ends after getting the set value.
Suggestion: Do not use
3. CC-CV Discharge Curve
Constant Current and Voltage Discharge( CC-CV Discharge): a combination of CC and CV discharge. First, discharge with a constant current to the end voltage, keep the voltage constant, gradually reducing the current to the set value, and the discharge ends.
Suggestion: Allow
4. CP Discharge Curve
Constant Power Discharge (CP Discharge): keep the power constant during the discharge process because the voltage gradually decreases during discharge, so the current will gradually increase.
Suggestion: Allow
5. CP-CC-CV Discharge Curve
Constant Power and Current and Voltage Discharge (CP-CC-CV Discharge): It is a three-stage combination discharge mode of CP, CC and CV, that is, discharge with constant power to the end voltage first, when the current at the end of CP is greater than the CC current in the next stage, At this time, the CC discharge voltage will first rise and then decrease to the end voltage, and then the voltage will be constant until the current decreases to the set value, and the discharge ends.
Suggestion: Allow
6. CR Discharge Curve
Constant Resistance Discharge (CR Discharge): It is equivalent to an external resistor. According to U=I*(R+DCR), since the external resistor is large enough, the current value will gradually decrease as the voltage decreases during the constant resistance discharge process.
Suggestion: Allow
7. CR-CV Discharge Curve
Constant Resistance and Voltage Discharge (CR-CV Discharge): It is a combination of CR discharge and CV discharge, that is, first discharge with constant resistance to the end voltage, then keep the voltage constant, and the discharge ends when the current decreases to the set value.
Suggestion: Allow
Conclusion
LiFePo4 charge and discharge curves analysis is an important method to evaluate the performance and characteristics of batteries or capacitors. It can help us choose a suitable LFP battery and understand its usage and life.