Oristo, according to STM32F072 datasheet, the safe voltage for VBAT is 3.6 V and it should not exceed 4.0V otherwise it leads to permanent damage.
Li-Ion battery has max voltage 4.2 V. Today I tested it for a new battery percent calculation function for battery indicator and found that max voltage is about 4.172 V, so it very close to 4.2 V specification for Li-Ion.
4.2 - 3.6 = 0.6 V
So, the diode D2 should have at least 0.6 V voltage drop for safe STM32F072 operating conditions.
May be your diode has smaller voltage drop and it leads to "dead" effect, what do you think?
