The voltage measurement of the battery on the BAT_ADC pin 34 come from the signal 'BAT' (see schematics), which itself come from the '+5V' signal. When supply by the battery, '+5V' is equal to the battery voltage ('VBAT' througt Q4) so you measure the battery voltage.
But when supply with USB, in order to charge the battery, then you measure the USB voltage (5V) and not the battery voltage, so you don't know if the battery is charged or not.
Why not measuring directly the battery voltage 'VBAT', so that you can follow the battery charging?
The voltage measurement of the battery on the BAT_ADC pin 34 come from the signal 'BAT' (see schematics), which itself come from the '+5V' signal. When supply by the battery, '+5V' is equal to the battery voltage ('VBAT' througt Q4) so you measure the battery voltage. But when supply with USB, in order to charge the battery, then you measure the USB voltage (5V) and not the battery voltage, so you don't know if the battery is charged or not. Why not measuring directly the battery voltage 'VBAT', so that you can follow the battery charging?