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I am trying to measure the resistance of a AA alkaline battery. The circuit I am using is shown in a photo below. I have also attached the values I obtained.

Somehow, the values I am getting fluctuate quite a bit and seem to be an order of magnitude larger than that specified in the datasheet (https://data.energizer.com/pdfs/e91.pdf). 1.0-3.3 ohm compared with 150-300 milliohm.

Please could someone show me where I’ve gone wrong with the calculations. Should I be using a different circuit? Is my battery model inadequate? As far as I understand it, battery’s internal resistance should be roughly constant for a certain state of charge and all the values I obtained occurred roughly at the same state of charge (I used a fresh battery and the battery ran for less than a second for my five tests).

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user16378
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  • What's the impedance of your switch? How about the wires? And the contact with the battery? – Hearth Mar 23 '20 at 15:31
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    Maybe the resistance of the battery contacts is distorting your results. I'm quite sure that at Energizer they would use a "four terminal" measurement, I mean they would use **separate** probes connecting to the battery's + and - pole for the load current and measuring the voltage. It is also possible Energizer used a DC current (like 100 mA) with a 10 mA AC signal added to that. Then they would measure the AC voltage directly on the battery terminals. – Bimpelrekkie Mar 23 '20 at 15:33
  • I didn’t actually use a switch. I just plugged and unplugged wires from my breadboard. I did try measure the resistance from resistor to battery holder on each side but I got roughly 3-4 ohms which gave a negative battery resistance – user16378 Mar 23 '20 at 15:52
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    bread boards can add a LOT of contact resistance! – Aaron Mar 23 '20 at 15:54
  • Battery holders are another source of resistance. You should measure voltage directly on the battery terminals. – Bruce Abbott Mar 23 '20 at 16:07
  • Why is the number of significant digits in your voltage measurements changing so much? You have from 2 to 5 significant digits. I am guessing that your experimental setup/technique is introduction significant errors. – Elliot Alderson Mar 23 '20 at 17:53
  • Polarisation in the battery will add a time-varying component to both the voltage and the internal resistance when on load, recovery time off load will as well. I would suggest you turn the load on, then read every 5 seconds for a minute, and see how the readings develop over that time, I expect you will be surprised. And don't even bother trying to measure through a breadboard, use a good battery holder or, preferrably, solder wires to the battery and use a switch. – Neil_UK Mar 23 '20 at 19:00

1 Answers1

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Some comments and suggestions:

  1. Measure the voltage at the battery's terminals, not across the load resistor, and do not use needle probes that are touched to the battery's terminals. Use probes with sufficient surface area that can be tightly bonded to the battery's contacts to minimize contact resistance.

  2. Depending on the probing setup, the Seebeck effect could affect your voltage measurement data.

  3. A resistor's value changes with temperature. When current flows through a resistor it dissipates power, which causes the resistor to heat up, which changes the resistor's resistance. This change in resistance value might need to be taken into consideration in your calculations.

  4. A battery's internal impedance is temperature dependent. As current flows through the battery it heats up, which increases its internal impedance.

  5. When measuring low value resistances (10 ohms or less), a 2‑wire ohms measurement is inadequate and should not be relied upon for accurate resistance measurement. For low value resistance measurement, always use a ohmmeter that performs ohms measurement via a 4‑wire "Kelvin" connection.
    (NB: Simply subtracting out the fixturing resistance in a 2‑wire ohms measurement is usually insufficient. See the section titled "Why not just subtract fixturing resistance?" in the CIRRIS Systems article (1) referenced below.)

(NB: In technical writing, the device that is being measured is commonly referred to as the device under test (DUT). In your case the DUT is the battery.)

REFERENCES

  1. CIRRIS Systems, 4‑Wire Kelvin Testing [Online]. Available: https://www.cirris.com/learning-center/general-testing/special-topics/40-4-wire-kelvin-testing

  2. J. Fischer, "Kelvin Type Connection", Electrical Engineering Stack Exchange, June 15, 2019 [Online]. Available: https://electronics.stackexchange.com/a/443829/79842

  3. J. Lockhart, Real Batteries, Lecture 12, PHYSICS 230.01 [Online]. Available: http://www.physics.sfsu.edu/~lockhart/courses/p230l12.pdf

  4. Electropedia, "Battery Performance Characteristics", Battery and Energy Technologies, 2005 [Online]. Available: https://www.mpoweruk.com/performance.htm

Jim Fischer
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