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I'm trying to understand the effect on a system when a voltage of a system is measured by connecting a voltmeter.

For example let's say we are trying to measure the voltage of a Li-ion battery. The junction is as follows:

Lead(Nickel alloy) | Current collector(Copper) | Anode active material(Lithium-intercalated graphite) | Electrolyte | Cathode active material(Lithium transition metal oxide) | Current collector(Aluminium) | Lead(Nickel alloy)

Lead, current collector and active material would be in thermodynamic equilibrium(Fermi level(a.k.a electrochemical potential) alignment) at each side and the electrolyte works as a border(electrically insulated).

To measure the voltage of the battery, we must connect the voltmeter to both leads of the battery. Each probe will be connected to each side of the battery (anode and cathode lead.)

Assume this voltmeter is a high-end one, there is almost zero current flowing through the voltmeter (infinite input impedance.)

Although it is, I'm concerned that this behavior itself could disturb the voltage of the battery because the Fermi level of each side of the battery will change.

In the situation of anode side when the probe of voltmeter is connected, this makes another metal-metal junction:

Voltmeter | Probe(Stainless steel(?)) | Lead(Nickel alloy) | Current collector(Copper) | Anode active material(Lithium-intercalated graphite) |...

And this would also happen in cathode side when the other probe of the voltmeter is connected to the cathode lead.

So the voltage displayed in the voltmeter would be somewhat distorted because this is the result after some electrons from the voltmeter flow into or out of the lead of the battery since voltmeter measures Fermi level difference between two probes.

For summary, my questions are:

  1. If composition of probes are different from that of a system we want to measure voltage, would the result be somewhat distorted because of Fermi level alignment? If it is, can we calculate how much the Fermi level changes?

  2. Along with question 1, if the two probes of voltmeter are the same material and have dimension, does this effect cancel out so we can measure exact voltage of the system?

  3. Along with question 2, if we have to use additional wire material of which is different from the original one to measure voltage, can we calculate the actual voltage of the system by compensating some constant value?

  4. In the same spirit, does the material of current collector also affect the Fermi level of active material which we actually want to measure?

I've never seen or heard this kind of effect discussed. Is it because the contribution of the effect mentioned above is negligible or am I just wrong?

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although you broke the rules by putting four questions into your post, I think all of them can be answered in one response, as follows:

Any dissimilar metal junction is going to develop a very small voltage (of order ~ millivolts) across it. In a context of measuring voltages of order ~ ten volts across a stack of cells, those millivolt signals will be utterly swamped out.

Furthermore, a sensitive voltmeter is always zeroed and calibrated before use against a precision voltage source, so as to null out contact voltages before taking measurements.

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  • $\begingroup$ I’m very sorry to give four questions at a post. I wouldn’t do that again. But could you give any references for why the voltage difference caused by metal junction would be in order of millivolts ? $\endgroup$ Aug 3 '20 at 8:53
  • $\begingroup$ Look up "thermocouple" on wikipedia. -Niels $\endgroup$ Aug 3 '20 at 15:45

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