I'm working on a project where I need to know the charge level of the battery mounted in the ESP32-C3 T-OI Plus (Ver 1.3).
From what I see on the schematic, Vbat is connected to the ADC input through a resistive divider made up of two 100K resistors that cut the voltage in half.

Building on that assumption, I'm using the following Micropython code:

...
#adc
a0 = 2

conversorAD = ADC (Pin(a0))
conversorAD.width (ADC.WIDTH_12BIT)
conversorAD.atten(ADC.ATTN_11DB)
...
def leerBateria ():
#Reads the battery charge percentage
batVolt = 2*(conversorAD.read_uv ()/1000000)
if (batVolt > 4.2):
batVolt=4.2
return (100*(batVolt/4.2))

I think it is correct because it reads values ​​consistent with the battery voltage measured with a multimeter.
What catches my attention is the variability of the values ​​read, for example if I read every 20 seconds I can have (with USB connected)

Bat = 99.57
Bat = 98.67
Bat = 99.57
Bat = 100.00

What could this variation be due to?

Thankfully,
Ernesto.