While sorting batteries, one critical parameter to test for each individual cell is internal resistance. But how do we measure this resistance, and what relevance does it even have?
Why We Care About Internal Resistance
Sometimes found in the battery as self-resistance, internal resistance of a battery stores energy within the battery rather than transferring the energy outward—or vice-versa as the battery discharges. Generally, the lower the resistance the better energy can be both charged into and discharged out of a battery. This quality can be tested while sorting batteries to ensure higher quality, more reliable cells are used for assembly, and generally a more efficient battery pack when complete.
Tools to Test Battery Internal Resistance
There are a few handy devices to test internal resistance when sorting batteries:
Battery Internal Resistance Testers
Usually just plug-and-go style, these specialty instruments are capable of quickly and superficially measuring the battery’s internal resistance upon application of a small AC signal to the battery and measuring across resistors in the circuit. Many come with features that will sort batteries based upon resistive thresholds.Impedance Spectroscopy
In Impedance Spectroscopy, we take it to the next level by measuring the impedance of a battery as a function of frequency, rather than simple resistance. This is necessary for advanced testing methods wherein a deeper understanding of the battery’s internal state is needed i.e. meaning no interruptions of electrolyte, conduction of electrode material, and inside battery faults.
DC Load Test
For a more generalised DC load test – A known current is drawn from the battery and the voltage drop measured. An estimate of internal resistance is intrinsic to Ohm’s law (R = V/I), so no fancy tester is needed for a ballpark number. (As an aside, dedicated battery impedance testers give far more information than this, but for basic sorting of cells, DC load test will suffice, albeit crudely). Now, for the actual testing.
Step-By-Step Guide to Testing Internal Resistance
Prepare the battery cell
To calibrate a test properly, simply charge the cell to an optimal level. This volume is dependent on the work you’re to actually make, however 20~80% state of charge is most common to make predicts for maximum test assurance. The greater the disparity between cell voltage on test, and at completion, the more off you’re going to be!
Connect the battery to the testing device – Many testers, as mentioned, will be polarised. This means connecting the plug end of discharge detectable below to positive cable on cell. For impedance testing, ensure your nature of contact is top notch and setup in general preassembled for best benchmarks to go off.
Put the screen up and run
To say test in this article appears basic, however the testing machines will vary, and you’ll cross-reference after with a preset number determined by schematics for the actual batteries of known good or faulty properties. Most will settle after a few seconds.
Store but don’t forget
Build a denotation for the number. The higher the reading, the more fail likelihood there is as a physical minimum due to build quality. Failure should equal rejection. The more cases of cell of succeeding resistance test further along, the greater the read.
Sort into cells
Forthwith, feeding into charge then passed out again, you’ll arrive at the width of gap measurement to reveal most fruitful and/or back off the Indicative on materials used as cells discharged in Circuit 1. Cells used for lower current spontaneously, less for tiring high C applications. All rounder to move to pack, heavier cells, to recycle depending (features often pooled together for a draw from!). That’s how to think to sort cells man, do extras for yourself, or lend to stockisteral works. Don’t name drop, there’re more about the cells you haven’t even thought of yet!
