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I want to measure the open circuit voltage of an electrochemical battery using an ADC and an Arduino. I need to use the ADC in differential reading mode (A0 por positive and A1 for negative).

Initially I have used a mounting specified where there is no ground to the battery because I thought that in differential mode it was not needed.

The measurements were not coherent (lots of variations with no sense, huge offset, etc). It looked like this:

enter image description here

After looking deeply into it I had the feeling that there was something wrong with the grounds so I decided to add the same ground (which comes from the building) to the negative of the battery, and suddenly measurements were coherent and made sense for me. This is the way I connected the ground to the battery (red line):

enter image description here

However, this ground connection is generating a current (around 50 mA) into the battery. This current makes the battery to loose capacity, so I cannot use this ground connection.

Any ideas on what to do to have a good measurement? For me it seems that I am doing something really wrong but I don't know where to start looking.

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Whilst the input to the ADC is differential, you still have to comply with the common mode voltage range - that is the actual input voltages have to be within a given range - usually the power rails. Exceed that and what you observed would be the expected result - garbage.

ADCs can have other requirements in the the inputs can’t be too close to the rails - this is also the common mode range. The rails are the electrical limit but for correct operation there may be tighter specs. Read the ADC datasheet.

Why does your multimeter work? It has its own isolated power source (battery) and internally the differential input is tied to the power source. By tying the battery to your ADC gnd, you achieved much the same thing.

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  • \$\begingroup\$ a problem has appeared with this new ground connection, it is introducing a current into the battery cell (around 50 mA) and this affects my battery performance in a not acceptable way. Any idea on how to avoid this current? Or any alternative to the use of this ground connection?. \$\endgroup\$
    – bardulia
    Mar 4, 2022 at 22:41
  • \$\begingroup\$ Where is the current coming from / going to? It’s hard to givre specific guidance as there’s lots of missing information. \$\endgroup\$
    – Kartman
    Mar 5, 2022 at 3:00
  • \$\begingroup\$ What kind of information? Anyways, I have two ideas. One is to add a low pass filter. The other is to reduce sampling data rate to the minimum (8 samples per second instead of current 128), maybe this way I can measure very slow like the multimeter. \$\endgroup\$
    – bardulia
    Mar 6, 2022 at 16:25
  • \$\begingroup\$ Missing information? Battery type, voltage, current and so on. To what accuracy and orecision do you require? I don’t see how a low pass filter would help nor sampling at a low rate - multimeters use an integrating adc unlike the adc you are using. Don’t try to guess a solution - work with facts. \$\endgroup\$
    – Kartman
    Mar 7, 2022 at 1:05

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