I want to build a circuit for a quadcopter where MOSFETs are connected to the primary and secondary battery. When the voltage falls below certain value in the primary battery the voltage monitor triggers the P channel MOSFET and cut the supply from primary and simultaneously the N channel MOSFET connected to the secondary battery turns ON and supplies the power. Batteries are rated 22 V Max current is 250 A.
If you insist on using the batteries separately I would recommend something similar to this, although it can also be done with N-FETs on the low side
The schematic below should work for inverting the output signal for control of the primary pack. The actual values for components should be evaluated based on actual circuit needs(Gate voltages on specific MOSFETs, filtering needs, etc.)