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I'm working on a three cell LIPO battery cell voltage monitor using three differential op-amps.

I had the circuit I designed below printed on a PCB and have assembled the board, but I am getting strange results on the output of op-amp 1 (Op1_output.) Op2_Output and Op3_Output work as expected.

When connected to the 3 cell LIPO (cell voltage for each cell = 3.8V,) Op1_output is reading 4.6V as opposed to the expected 3.8V. I have tested all the connections and also soldered 3 separate boards in case there was an issue with my connections, but the results are always the same.

I know that I can simply read the 3.8V directly because that cell is the first in the series, but I'm curious as to what my issue is, as creating the circuit in a simulator is yielding the expected results.

Any insight is greatly appreciated.

Op-amp datasheet

Circuit_Diagram

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  • \$\begingroup\$ examine R10 in the schematic \$\endgroup\$
    – jsotola
    Jan 27, 2022 at 2:44
  • \$\begingroup\$ What's the minimum potential difference between the opamp rails? \$\endgroup\$
    – jonk
    Jan 27, 2022 at 3:15
  • \$\begingroup\$ @jsotola - R10 is connected to B3+, which the same as Vcc. Should be fine. It’s op1 that’s malfunctioning. \$\endgroup\$ Jan 27, 2022 at 4:42
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    \$\begingroup\$ Very odd opamp choice. 5532s are more at home in mixing desks and hi-fi amplifiers than battery monitoring circuits! Look at "Input common-mode voltage range" in the datasheet and note that you are operating out of spec. You may be looking for rail to rail opamps instead. \$\endgroup\$
    – user16324
    Jan 27, 2022 at 13:58
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    \$\begingroup\$ That’s a big topic. Try searching ‘selecting an op amp’ and see what comes up. In this particular case, you’d select one for single-ended and lower voltage with inputs that go to ground. So, an LM324 or a newer cousin. \$\endgroup\$ Jan 28, 2022 at 3:28

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According to the 5532 datasheet you linked, the input range needs to be Vss+2V to Vdd-2V. Since you’re using ground as your negative supply, this limits the input range to no less than 2V.

With the R3/R4 voltage divider between OP1 and GND, this value isn’t being met on the (+) input of OP1: it’s only 1.9V above GND if BT1 is 3.8V, and as low as 1.5V at BT1 end of discharge (3.0V).

So OP1 is failing. As to exactly why, that is a result of this particular op-amp’s input stage design.

Try an op-amp that includes GND in its input range. The LM324 can do this. It’s popular and cheap.

So why doesn’t the sim fail? The Falstad simulator uses an ‘ideal’ op-amp which has no such input range limitations. However, if you try the Falstad 741 model it will fail; it’s similar to the 5532. There’s also an LM324 model you can try.

Or, you can get rid of the resistors R1, R2 and R4 and just wire the op-amp as a follower. Then the (+) input will be the same as the BT1 voltage, 3.0 to 3.8V or so, and (-) will follow it.

Still another option is to just measure BT1 directly. Since it’s referenced to GND there isn’t a need to use a differential amp. Save the one part, solve the problem. Win-win.

Not sure what your intention is with the 7809 driving an LED, but the overhead voltage for that regulator is 2-3V. That means it’ll be at only be outputting ~7V at the end-of-discharge voltage of 9V (3.0V per cell).

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  • \$\begingroup\$ Ah of course, I figured it was something obvious that I was missing. That fixed it! Thank you. This is the first proper circuit I designed and I think I got overwhelmed by all the new processes and data. Thanks again! \$\endgroup\$ Jan 28, 2022 at 2:36

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