How to Measure Internal Resistance of a Battery

[neilparker62]

Table of Contents
1IntroductionExperimental BriefRevised Experimental BriefTable of Measurements and Calculated ValuesRegression Analysis: Measured vs Ohm’s Law calculated...

Continue reading...

 

[nsaspook]

https://data.energizer.com/pdfs/batteryir.pdf

 

[neilparker62]

https://data.energizer.com/pdfs/batteryir.pdf

I would say the "alternate method" I suggest is similar to the dual pulse method although I am not really using a stabilization current. Since the measurement is "touch/release" with the multimeter leads, I am effectively applying a 2-3 second 'pulse' with an $\approx 50mA$ load. I have adopted the practice of measuring the battery voltage under load (first) and open circuit voltage thereafter. So as to remove "surface charge" effects. Although I'm not sure how applicable 'surface charge' is with AA batteries - seems more applicable to heavy duty lead acid batteries.

Generally I expect my "pre-pulse" open circuit voltage to correspond to my "post-pulse" open circuit voltage. 50 milli-amps is chosen to try and ensure that we specifically don't have any appreciable change in battery emf over the period of measurement. The method doesn't work for AAA batteries because they drain too quickly at 50mA.

 

[Nik_2213]

Slightly tangential, I remember measuring the internal resistance of Uni lab's wall-power via current & voltage of an incremental dummy load, a bank of 60 watt incandescent lamps.

And, yes, the Lab Tech did warn us NOT to 'just' use the resistance scale of the big Avo analogue multi-meters supplied: He'd expended half a CO2 extinguisher on that hapless perp. Even an Avo's fusing had its limits...

FWIW, when it came to wiring up two long-legged germanium transistors etc as a free-running ~1 kHz multi-vibrator, the lecturer was appalled to find I'd briskly plugged his components into my S-Dec, a palm-sized solderless breadboard. I was done before most of my fellow students had laid out their nail-bed bread-boards, never mind tinned a chunky soldering iron. I had to do mine over the traditional way. No sweat, I'd brought along several of my favourite hair-clip heat-sinks, was soon done...

He was left speechless when I mentioned that three stages made a fun 'Flip-Flap-Flop', four ran as 'two-pair', five did 'two pair plus a runt pulse' and six did 'two prials'...

 

[neilparker62]

Some recent measurements using low resistance measurement techniques described in this article.

Internal resistance: Varta AA rechargeable batteries (2100 mAhr): 47 milli ohms

Pair of battery jumper leads: 47 +- 12 milli ohms
Standard meter leads (pair) : 116 +- 12 milli ohms
Gold plated meter leads (pair): 70 +- 12 milli ohms. These are about twice as long as the other lead pairs mentioned here.
Custom designed meter leads (pair): 47 +- 12 milli ohms
Custom designed pair with 3 wires in parallel (pair): 24 +- 12 milli ohms

28 meters 10 mm^2 roofing cable for a solar power installation: 70 +- 12 milli ohms
2 meters galvanized steel wire: 426 +- 12 milli ohms.

I have requested a modification on the Major-Tech MT870 meter which should reduce the attainable resolution (currently about 24 milli ohms) by a factor of 3.