There are many ways to charge lithium-ion batteries. The most commonly used methods are constant current charging, constant voltage charging, constant current-constant voltage charging and pulse charging.

(1) Constant current charging

Constant current charging refers to charging the battery with a constant current. When constant current charging is performed with a single current value, if the charging current is too small, the charging time at the beginning of the charging phase will be too long. If the charging current is too large, the charging voltage of the battery will be too large in the later stages of charging, causing a greater impact on the electrodes.

(2) Constant voltage charging

Constant voltage charging refers to charging the battery while keeping the charging voltage constant. Although this method can automatically adjust the charging current according to the change of the battery SOC during the charging process, its disadvantages are also very obvious: the charging speed is slow, and the charging current is too large due to the low battery voltage in the early stage of charging, which will damage the battery, affect the battery life, and even cause the battery to be scrapped.

(3) Constant current-constant voltage charging

Constant current-constant voltage charging is currently the most commonly used charging method for lithium batteries. This method refers to first charging with a constant current until the battery voltage reaches a certain voltage value, and then charging the battery at a constant voltage. During the constant voltage charging stage, the charging current value of the battery will gradually decrease. When the charging current decreases to near zero or 0.02C, the battery is considered fully charged. This method combines the advantages of constant current charging and constant voltage charging: during constant current charging, it ensures that the current in the early stage of charging does not exceed the rated current of the battery; during constant voltage charging, the charging efficiency is improved.

(4) Pulse charging

The pulse charging method refers to charging the battery with a constant pulse current. In the initial stage of charging, the battery is first charged with a small current at a constant current. When the battery voltage is charged to a certain set voltage value, the battery is charged with a pulse current. This method provides sufficient time for depolarization operation, allowing the battery to store more energy.

The figure below illustrates a typical charging process for a potassium-ion battery under constant current/constant voltage charging mode. The charging process is divided into two phases: constant current charging (t0-t1) and constant voltage charging (t1-t2).