In this circuit, the battery capacity of BAT2 is greater than that of BAT1, VIN_5V is the external power supply, VOUT is the output terminal for powering the system, and the power supply priority is VIN_5V＞BAT2＞BAT1.

 The following three situations explain the working principle

Single power supply case :

When only BAT1 is present, Q2, Q3, and Q5 are turned on, and VOUT draws power from BAT1;

If only BAT2 is present, Q1 is turned on, Q2, Q3 and Q5 are turned off, Q4 is turned on, and VOUT draws power from BAT2;

When only VIN_5V is available, VIN_5V passes through diode D1 to VOUT, and VOUT draws power from VIN_5V.

Dual power supply situation :

When BAT2 and BAT1 exist at the same time, Q1 is turned on, Q2, Q3 and Q5 are turned off, Q4 is turned on, and VOUT draws power from BAT2;

When VIN_5V and BAT2 exist at the same time, Q1 is turned on, Q2, Q3 and Q5 are turned off, VIN_5V passes through D1 to VOUT, BAT2 passes through the body diode of Q4 to VOUT. Since VIN_5V is larger, VOUT draws power from VIN_5V.

When VIN_5V and BAT1 exist at the same time, Q2, Q3 and Q5 are turned on, BAT1 is connected to VOUT, and VIN_5V is connected to VOUT through diode D1. Since VIN_5V is larger, VOUT draws power from VIN_5V.

Three-power supply situation : When BAT1, BAT2 and VIN_5V exist at the same time, refer to the situation where VIN_5V and other power supplies exist at the same time in the dual power supply, and VOUT draws power from VIN_5V.

From this we can know that as long as one of the three power supplies exists, the circuit can work normally; when two power supplies exist, the power supply with high priority will power the system, saving power from the power supply with low priority; when three power supplies exist at the same time, power from two batteries can be saved at the same time; and when two or three power supplies exist at the same time, the power supply with high priority is cut off, and the power supply with low priority can resume power supply in time.