MOSFET N loss power battery calculation
Q1 is an N-channel enhancement-mode MOSFET, with intrinsic body diode.  If you trace from 3v to the MOSFET, through it's body diode to R3, you'll see that the body diode will always conduct, giving you 3V - body diode drop into R3.
Furthermore, with NEMOS devices, the gate must be more positive than the source to turn it &quot;on&quot;. So unless d5 is significantly more than 3v, it will be &quot;open&quot; or a very high value resistance.
You likely need a PEMOS device.  These are turned &quot;on&quot; from the gate going lower than the source pin.  The PEMOS body diode is also reversed, so will not conduct when the source is tied to 3V.