The Vbat, at least what I see in the sources for H4, is being sampled such the Vbat voltage divided by 2 (internal on-chip divider, most probably disconnected when not in use) is converted via the ADC channel 17. There are no external dividers needed.
The rather unusually high noise in the vbat data readings I presented in my other post could be caused by something missing in the H4 code, ie. a small delay should be inserted after activating the divider - before reading the vbat, or something like that..
