This is what I came with. I did some limited tests with LoPy connected to 5V supply. T2 drain was connected to 3V3 output of LoPy, VBAT to P16 (set as ADC channel) and EN signal to P20 (set as output). I could measure some voltage with the ADC while EN pin was enabled (3V3). When set to 0V, ADC reading was 0V as well and current through T2 drain was around 30nA (measured using uCurrent).
[image: 1504793749529-battery_voltage_measurement_circuit_v1-resized.png]
@daniel @jmarcelino and others please comment if this circuit makes sense in your opinion. The goal is to limit the current through the voltage divider in between measurements - e.g. during deep-sleep of battery powered sensor.