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?

EDIT: Capacitor used is an 0402 X5R 10uF 6.3V MLCC - Kemet C0402C106M9PACTU (Datasheet) (Spec sheet)

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  • 1
    \$\begingroup\$ What's the leakage current of the capacitor? What type is it? \$\endgroup\$
    – winny
    Commented Feb 2 at 11:06
  • \$\begingroup\$ Did you check the capacitor polarity for these 2 specific boards? \$\endgroup\$ Commented Feb 2 at 11:16
  • \$\begingroup\$ Added capacitor type in edit, ceramic cap so no polarity \$\endgroup\$
    – Mukira
    Commented Feb 2 at 11:17

2 Answers 2


I can think of two possible reasons-

  1. the capacitors could be damaged and leaky.

  2. the ESP32 ADC input (like most unbuffered SAR converters, though this is not a particularly impressive one) draws a certain amount of current every time you make a measurement. There's a small capacitor that needs to be charged through a resistor (14pF and 1kΩ if memory serves). You have a relatively high impedance divider (16kΩ+), so depending on how you are using the ADC (measuring rate etc.) you can have effects like this.


Seems like quite a big capacitor for this use case. Maybe the internal resistance of the capacitor influences the voltage divider. Is the voltage too low or too high?

  • \$\begingroup\$ Changing the capacitor to a 1uF rather than 10uF has resolved the issue. It still doesn't answer the why the 10uF value causes the drop but at least is a solution \$\endgroup\$
    – Mukira
    Commented Feb 2 at 11:38
  • \$\begingroup\$ @Mukira It was already stated by both winny and Spehro: It is leaky. That means it's passing current and loading down your divider. This can be measured. \$\endgroup\$
    – MOSFET
    Commented Feb 2 at 14:35

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