Different readings when measuring the resistance of rechargeable 1.2V battery

Question

I have 2 rechargeable AA batteries. One is fully charged and one is almost used up.

I directly connect the red probe of the multimeter to the positive pole of the battery, and black probe to the negative pole.

Then I got some inconsistent readings with different DC resistance levels.

First I tried with the used-up battery.

For 20 MOhm level, I got: 1.08 MOhm

For 2 MOhm level, I got almost the same: 1.07 MOhm

For 200 KOhm level, I got: 119.5 KOhm

For 20 KOhm level, I got: 11.02 KOhm

For 2 KOhm level, I got: 1.236 KOhm

For 200 Ohm level, I got: 0L, which according to the manual, should be out-of-range.

Then I tried with the fully charged battery.

For every level, I got the same: 0L, which should mean out-of-range.

My questions are:

• Why the readings are so different with different levels for the used-up battery?

• Why the used-up battery can get readings while the fully charged one always get out-of-range 0L reading?

Thanks.

Answer 1

It's really simple: a multimeter cannot be used to measure resistance of anything that is a voltage source.

To measure battery resistance, a battery diagnostic tool must be used.

A multimeter measures resistance by connecting a current source between the input terminals, and the resistance is just a scaled value of the voltage measured on the input. Obviously, if what's measured isn't a resistor but a battery, the battery voltage will appear as additional resistance.

^{simulate this circuit – Schematic created using CircuitLab}

The multimeter scales the voltage reading so that it appears directly in units of resistance, for convenience.

When a battery's "resistance" is measured that way, obviously the battery voltage adds up and confounds the reading:

^{simulate this circuit}

The resistance reads 5.005 kohm, but 5.000kohm of that is caused by battery voltage.

Of course this is just an example - your multimeter's resistance mode test current will depend on the measurement range, and generally is not 1mA.

Now, if you know exactly what current the multimeter applies at various resistance ranges, you could figure out what is the scaling from input voltage to resistance, and subtract the part of displayed resistance that's caused by the battery voltage. What's left would be the battery ESR. Most hand-held multimeters would not have enough resolution to provide usable results that way, though.

Also, the current sources in multimeters have fairly limited compliance voltage ranges. That means that unless the battery has very low voltage - say 1V or less - the current source won’t work, and the “resistance” will be only a scaled battery voltage, independent of any battery resistance.

Answer 2

That's not a real measure. You can measure battery impedance but you can't do it with a multimeter resistance measurement.

An ohmmeter works by placing a small voltage on the probes, and internally it measures the resulting current. By measuring something that's also producing a voltage, you're confusing the current measure and thus getting wrong readings.

NEVER NEVER NEVER attempt resistance measurements on anything "live"... that includes batteries!!!! YOu'll be lucky if you didn't damage it.