Standardizing the measurement of internal resistance

[ZapPat]

I have never trusted the CBA to make measurements of cell impedance, or above 20A, for many of the reasons cited, but also because the standard method for measurement of cell impedance is to do it at 1 kHz and thus avoid many of the inherent errors that are not just specific to the CBA.
i have found that if i measure the cell voltage with the CBA2 at 10A and then again at 20A and subtract the difference that the measurements are fairly consistent and in line with the advertised specs and the measurements made at 1 kHz with my other test setups.

Most good cells have both AC impedance and DC resistance specs in their datasheets. The numbers on these specs are often very different, and the AC impedance numbers are always quite a bit lower than the DC resistance. I've seen the DC resistance being anywhere between 1.2 times and even up to almost 1.8 times the AC impedance spec, depending on cells. Here are a couple examples of this from a few datasheets I have:
A123 2.3Ah cell = 8mohms AC / 10mohms DC
Valence 2.4Ah cell = 8mohms AC / 12mohms DC
Valence 1.1Ah cell = 15mohms AC / 25mohms DC

What this means is that the cells can supply current more efficiently during very short periods (like under 1ms for the standard 1kHz test) than for constant loads (DC). However, the DC specs are really what we want to know for EV use, since this DC resistance is the source for our much dreaded voltage sag under load. The controller's capacitors filter out most of the higher frequency current pulses, making their AC impedance not very important for us.

Many companies mix up their cell's DC and AC resistance/impedance specs, and only give the AC impedance specs, but often calling it "resistance" anyways. This gives them better numbers for marketing of course, so buyers beware.

Anyone wanting to read more about internal resistance testing could read this sandia labs report; there's some good info inside about their test procedures using both AC and DC currents.
View attachment Sandia labs Lifebatt test report.pdf

Pat

 

[bearing]

...
Most good cells have both AC impedance and DC resistance specs in their datasheets. The numbers on these specs are often very different, and the AC impedance numbers are always quite a bit lower than the DC resistance. I've seen the DC resistance being anywhere between 1.2 times and even up to almost 1.8 times the AC impedance spec, depending on cells.
...
Pat

Maybe the AC impedance is lower only because cells acts like a capacitor. Cylindric cells are made up of rolls of sheets. The area of between the anode and cathode are by this design large, and the distance between them small - that should make a good capacitor.

 

[bobmcree]

...
Most good cells have both AC impedance and DC resistance specs in their datasheets. The numbers on these specs are often very different, and the AC impedance numbers are always quite a bit lower than the DC resistance. I've seen the DC resistance being anywhere between 1.2 times and even up to almost 1.8 times the AC impedance spec, depending on cells.
...
Pat

it has been my belief for a long time that the reason the ac spec is quoted is that the number is lower. the test is also much easier to reproduce, which makes comparing measurements more valid. when we make measurements of very low values of resistance or voltage, errors are introduced in the dc measurement from dissimilar metals in contact at varying temperatures and lots of other sources of error are also at play, including the difference between the cell voltage when looking at the surface charge on the electrodes and the often significantly lower voltage a second or two later. ac measurements eliminate some of these errors and produce measurements that are much easier to compare.

i have an old analog hp 4328a milliohmeter that uses the ac 4 terminal technique and it measures very low resistance values very accurately. this meter has been the standard in the battery business for over 20 years. i have found the measurements produced by this meter to be very much in line with the performance of battery systems under real world conditions. the new digital units are about $5000 but the old analog ones are around for a few hundred once in awhile.

DC measurements are certainly valuable, and i agree both numbers are useful. dc measurements however are an attempt to pin down the performance of a dynamic system under static conditions, and will always have uncertainties.