@bobcroft Yes, in the meanwhile I figured out, that it is a NTC 100K (also known as 104AT).
I am just struggling with the I2C-ADC. The considered part (ADS1115) can measure 2-5V (Why starting at 2V??). Even more than the ADC of the ESP. But if I would find one with 0-5V it would be a better resolution. Even if I supply the whole stuff only with 3,3V.
Resistance divider is what i thought of. Charge/discharge unit never used before. Will have a look on this too.