Ran into a bit of a strange issue with a voltage divider. I take the voltage of a single cell li-ion and divide it down to a 0 - 3V range so it can be read by an ESP32, I then multiply this voltage by a fixed constant to know what the battery voltage is.

This same circuit has been built on 50+ PCB's but I'm only seeing the issue on 2 specific boards. There is a parallel capacitor for some filtering, but on these 2 specific boards, with this capacitor fitted the voltage reported by the ESP32 is incorrect. Not by a huge amount, in the region of ~200mV.

If I remove the capacitor the voltage is reported correctly.

What I'm struggling to understand is how a parallel capacitor which should in theory be open circuit at DC, is affecting the voltage divider circuit.

Is anyone able to shed any light on why this is happening?