I want to build a LiPo protection circuit for my 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. Is this a good approach?

 

One problem with this type of scheme is that the voltage on the primary battery, once the load is removed, will come back up above the threshold voltage. So you can't use a simple voltage comparator. You could introduce hysteresis, so that once the primary is switched off, it won't switch back in unless the voltage recovers beyond the hysteresis amount. Or you could use some sort of latch, so that once the switch is made it's permanent until manually reset.

 

Why both a P-MOSFET and and N-MOSFET?
Post the intended circuit.

 

I have attached the intended idea. The voltage tester output goes high which is because the voltage in primary battery is low(say below 3,7 in any of the cell). The power from the primary should stop but the secondary should kick in immediately to ensure continuous power supply.