You're right that M2's body diode pass throughpasses voltage through, but it gothas a voltage drop of typical -0.7 V and max -1.2 according to the datasheet. Instead of havehaving the diode turn current into heat when charging, the MOSFET is turned on for much lower voltage drop.
In the single cell example a simple Schottky Diode is used to isolate the input, and I'm assuming that is what the MOSFET is doing here as well.
In the single cell example a simple Schottky Diode is used to isolate the input, and I'm assuming that is what the MOSFET is doing here as well.
Correct Correct. That's the main function of the M2 MOSFET, and you. You get better efficiency vsthan with a regular diode.