Professional Comparison: 12 Lithium Battery Charge and Discharge Curves

By Gerald, Updated on August 15, 2024

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As an energy device that converts “chemical energy-electrical energy”, lithium batteries will inevitably be charged and discharged during use. Reasonable charging and discharging methods can not only reduce the degree of damage to lithium batteries, but also give full play to the performance of lithium batteries, and have important application value.

In China, the prescribed charging and discharging process for lithium battery cycle life test is:

Charging: charging at 1C constant current to the termination voltage, and then charging at constant voltage to 0.05C;
Discharging: discharging at 1C constant current to the termination voltage.

Whether it is a power battery or a consumer battery, the industry and standards recommend constant current and constant voltage charging for lithium batteries, and constant current discharge for discharging.

How to Read Lithium Battery Discharge Curve and Charging Curve?

Part 1. Charge methods

In addition to the CC-CV charging method required by the standard, lithium batteries can also be charged by CC, CV, CP, CP-CV, etc.

1. Constant Current Charge (CC)

Constant Current Charge (CC Charge for short), the current is constant during the entire charging process, and the voltage gradually increases.

Recommendation: Allowed to use .

Figure 1: Constant current charging curve

2. Constant voltage charging (CV)

Constant voltage charging (CV Charge for short), 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 lithium battery is discharged to 3.0V, the instantaneous current of 4.2V constant voltage charging reaches about 17.5C, and then the current gradually decreases, and the charging ends after reaching the set value.

Recommendation: Do not use.

Figure 2: Constant voltage charging curve

3. Constant current and constant voltage charging (CC-CV)

Constant current and constant voltage charging (CC-CV Charge for short), this charging method is 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 again, and finally the current gradually decreases to the set value and the charging ends.

Recommendation: Allowed to use.

Figure 3: Constant current constant voltage charging curve

4. Constant power charging (CP)

Constant power charging (CP Charge for short) keeps the power constant during the charging process. Since the voltage gradually increases during charging, the current will gradually decrease.

Recommendation: Allowed to use.

Figure 4: Constant power charging curve

5. Constant power constant voltage charging (CP-CV)

Constant power and constant voltage charging (CP-CV Charge for short) is a combination of constant power charging and constant voltage charging. First, it is charged to the termination voltage with constant power, and then the voltage is kept constant. When the current decreases to the set value, the charging ends.

Recommendation: Allowed to use.

Figure 5: Constant power constant voltage charging curve

Part 2. Discharge methods

In addition to the CC discharge method required by the standard, lithium batteries can also be discharged by CV, CC-CV, CP, CP-CC-CV, CR, CR-CV, etc.

1. Constant Current Discharge (CC)

Constant Current Discharge (CC Discharge) is the most common discharge method for lithium batteries. The current remains constant during the entire discharge process, and the voltage gradually decreases to the termination voltage, and the discharge ends.

Recommendation: Allowed to use.

Figure 6: Constant Current Discharge Curve

2. Constant Voltage Discharge (CV)

Constant Voltage Discharge (CV Discharge), the voltage reaches the set value at the moment of discharge, and the current is at the peak state. As shown in the figure below, the lithium battery is set to discharge at a constant voltage to 3.0V, and the instantaneous current of discharge reaches 30C-35C, and then the current gradually decreases. After reaching the set value, the discharge ends.

Recommendation: Prohibited.

Figure 7: Constant Voltage Discharge Curve

3. Constant Current and Constant Voltage Discharge (CC-CV)

Constant Current and Constant Voltage Discharge (CC-CV Discharge) is a combination of CC discharge and CV discharge. First, it discharges at a constant current to the termination voltage, keeps the voltage unchanged, and then the current gradually decreases to the set value, and the discharge ends.

Recommendation: Allowed to use .

Figure 8: Constant current and constant voltage discharge curve

4. Constant power discharge (CP)

Constant power discharge (CP Discharge for short) keeps the power constant during the discharge process. Since the voltage gradually decreases during discharge, the current will gradually increase.

Recommendation: Allowed to use .

Figure 9: Constant power discharge curve

5. Constant power constant current and constant voltage discharge (CP-CC-CV)

Constant power and constant current and constant voltage discharge (CP-CC-CV Discharge for short) is a three-stage combined discharge mode of CP, CC and CV, that is, it first discharges at constant power to the termination voltage. When the current at the end of CP is greater than the CC current of the next stage, the CC discharge voltage will first rise and then decrease to the termination voltage, and then the voltage will be constant until the current decreases to the set value, and the discharge ends.

Recommendation: Allowed to use .

Figure 10: Constant power constant current constant voltage discharge curve

6. Constant resistance discharge (CR)

Constant resistance discharge (CR Discharge) is equivalent to an external resistor. According to U=I*(R external + DCR), since the external resistor is large enough, the current value gradually decreases as the voltage decreases during constant resistance discharge.

Recommendation: Allowed to use .

Figure 11: Constant resistance discharge curve

7. Constant resistance constant voltage discharge (CR-CV)

Constant resistance and constant voltage discharge (CR-CV Discharge) is a combination of CR discharge and CV discharge, that is, first discharge with constant resistance to the termination voltage, then keep the voltage constant, and the discharge ends when the current decreases to the set value.

Recommendation: Allowed to use .

Figure 12: Constant resistance and constant voltage discharge curve

In summary, the charging and discharging methods of lithium batteries are diverse, but in the final analysis, they are single-step or combined processes based on CC (constant current), CV (constant voltage), CP (constant power) or CR (constant resistance). The current and voltage changes of these charging and discharging methods are shown in the table below.

Comparison of current and voltage changes in different charging and discharging methods table

At present, the mainstream charging method of lithium batteries is still mainly CC-CV.

This is because lithium batteries have polarization phenomenon (that is, the instant voltage is not a steady-state voltage). In the CC stage, the current is large and the charging speed is fast. After the voltage rises to the upper limit voltage, it maintains a constant potential. The electrons in the external circuit react with Li+ at the negative electrode.

As the negative electrode inserts lithium, the active sites decrease, the number of electrons participating in the reaction decreases, and the current gradually decreases. When the current is reduced to 0 (ideal situation), the polarization is completely eliminated and the lithium battery is fully charged.

However, during the discharge process, lithium batteries usually use CC mode, which may be to make it easier to calculate capacity or estimate SOC. In actual working conditions, constant power discharge is mostly used.

Part 3. Conclusion

The selection of lithium battery charging and discharging methods should take into account the convenience of data processing, charging/discharging efficiency, and the risk of internal damage to lithium batteries. It is necessary to give full play to the performance of lithium batteries and to conduct fast and accurate detection.

At present, lithium batteries are mainly tested in CC-CV charging mode and CC discharge mode, but in actual terminal use conditions, CP mode is the main mode.

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