Hi cedric_calais,
Although not an example of -S3, an example of AD conversion of 1S battery voltage with -C3 can be found in the link below.
The input voltage to a port should be divided so that it is less than 2.2V. The function “alalogReadMilliVolts()” can be used to read the corrected voltage values without having to do anything special.
Note that 2S battery cannot be connected to the battery pad on the back of XIAO.