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I assembled an 18650 Li-Ion capacity tester with an Arduino Pro Mini. Here is the schematic: enter image description here

When I set the discharging current to 100 mA in the code (which means there has to be 100mV on pin 3 of the opamp and there is), I'm measuring 300mV on pin 2 of the opamp!?

This absolutely does not make sense to me, as the opamp should strive to equalize the inputs.

What am I doing wrong? Has anyone got an idea or experienced the same problem?

But it generally seems like the current value I set differs noticeably from the measured value.

E.g. when I set the discharging current to 500mA, I'm measuring like 600mA...

Anyone got an idea?

Edit: This is what the voltage on pin 1 of the opamp looks like. Referenced to GND. enter image description here

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  • \$\begingroup\$ Broken op-amp maybe? \$\endgroup\$
    – Andy aka
    Commented Jul 29, 2023 at 9:27
  • \$\begingroup\$ It's a new one from STM. I tried using the LMC6482 (it is pin compatible) but it seems like it fully switches the opamp and the current is more than 1 amp ... \$\endgroup\$
    – pomm es
    Commented Jul 29, 2023 at 9:31
  • \$\begingroup\$ What's the output voltage on pin 1 of the op-amp? Is it oscillating? \$\endgroup\$
    – Finbarr
    Commented Jul 29, 2023 at 10:09
  • \$\begingroup\$ Seems like it's oscillating with ca. 30 kHz. I added an image of the voltage on pin 1 of the opamp \$\endgroup\$
    – pomm es
    Commented Jul 29, 2023 at 10:23
  • \$\begingroup\$ Not surprised, with only 1 Ohm to ground the op-amp is driving a high capacitive load. Try adding some resistance between the bottom of C5 and the top of R5. \$\endgroup\$
    – Finbarr
    Commented Jul 29, 2023 at 12:34

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