MOSFET is ON if gate-source voltage is > than spec'd Vgs from datasheet charts. (For PMOS > means more negative). this is NOT Vth, the threshold voltage - thats the Vgs where the MOSFET just starts conducting, something like 100 or 250uA, and isn't a useful figure if you're expecting to pass milliamps or amps. As is happens the FDS6575 isn't a bad device, its useful Vgs is -2v though its drain-source resistance only fully meets spec at Vgs = -4.5v.
Your simulations are flawed though because you are assuming your battery is 8.4v which it won't be if it needs charging, it could be as low as 6v. Futher you're assuming your charger is 8.4v, which is won't be iif its a LiPo charger. LiPo is charged at constant current, not constant voltage, so your charger will match your battery volts until it gets to 8.4-ish then switches to constant voltage until charge current is a few milliamps. Depending on the charger it may not even start charging if its not sure if a battery is connected. So in your charging circuit above, with charger disconnected, if Q2 is on then M1 will be conducting. M2 will be off as its source is floating. When you connect the charger the body diode of M2 will conduct and the charger will 'see' the battery volts less 1 diode drop. The charger may start charging with a constant current, its voltage will rise until current starts to flow into the battery. If Q1 is on M2 will start to conduct as its source will rise above its gate voltage and M1 will start to turn off as its drain rises above its source and the body diode starts conducting.
The problem then arises that the charger will think the battery is charged when the battery volts are at 7.7-7.8v as its not aware of the M1 diode in circuit. So the battery is never fully charged. You need to integrate the charger with your battery control circuit. Also, a 2-cell pack needs balancing, so how are you going to accopmplish that with an external charger?
Your simulations are flawed though because you are assuming your battery is 8.4v which it won't be if it needs charging, it could be as low as 6v. Futher you're assuming your charger is 8.4v, which is won't be iif its a LiPo charger. LiPo is charged at constant current, not constant voltage, so your charger will match your battery volts until it gets to 8.4-ish then switches to constant voltage until charge current is a few milliamps. Depending on the charger it may not even start charging if its not sure if a battery is connected. So in your charging circuit above, with charger disconnected, if Q2 is on then M1 will be conducting. M2 will be off as its source is floating. When you connect the charger the body diode of M2 will conduct and the charger will 'see' the battery volts less 1 diode drop. The charger may start charging with a constant current, its voltage will rise until current starts to flow into the battery. If Q1 is on M2 will start to conduct as its source will rise above its gate voltage and M1 will start to turn off as its drain rises above its source and the body diode starts conducting.
The problem then arises that the charger will think the battery is charged when the battery volts are at 7.7-7.8v as its not aware of the M1 diode in circuit. So the battery is never fully charged. You need to integrate the charger with your battery control circuit. Also, a 2-cell pack needs balancing, so how are you going to accopmplish that with an external charger?