1. MPPT current limiting charging. At the beginning of charging, when the system waits for the battery terminal voltage to reach a low level, the MPPT charging method will be used to draw the output power of the photovoltaic cell to the battery terminal. When the sunlight is strong, the output power of the photovoltaic cell rises greatly, and when the charging current can reach the threshold soon, the MPPT charging is stopped and the constant current charging mode is switched to. When the light intensity becomes weak so that constant current is difficult, then switch to MPPT charging mode, and switch freely until the battery terminal voltage rises to the saturation voltage Ur, and the battery enters the constant voltage charging stage. Through the mutual cooperation and automatic switching of the MPPT charging method and the constant current charging method, the solar energy can be fully utilized to quickly charge the battery.

2. Constant voltage and current limiting charging. At this stage, the charging voltage value is constant at Ur, and the charging current will gradually decrease as the electrochemical reaction inside the battery progresses. When the charging current drops to about 0.01C, the constant voltage charging is terminated and the floating charging stage is entered. In the constant voltage stage, current limiting is also done to protect the battery.

3. Float charging. Float the battery with a voltage Uf that is slightly less than the constant voltage charging. This stage is mainly used to supplement the energy consumed by the battery's self-discharge, which marks the end of the charging process.