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:
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):
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.