Testing Lithium-based Batteries
With the large number of lithium-ion batteries in use and the population growing rapidly, developing an effective testing method has become an urgent task. QuickSort™ (Cadex) is a further development of QuickTest™ using a generic matrix. The simplification was made possible by limiting the battery population to single-cell Li-ion from 500 to 1,500mAh. (Larger cells and higher voltages will need a different generic matrix.) Rather than capacity readout in percentage, QuickSort™ classifies the battery health as Good, Low or Poor.
Electrochemical dynamic response,the method used for QuickSort™, measures the mobility of ion flow between the electrodes on a digital load. The response can be compared with a mechanical arm under load. A strong arm resembling a good battery remains firm, and a weak arm synonymous to a faded battery bends and becomes sluggish under load. Figure 1 illustrates the concept of the technology.
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Figure 1: The electrochemical dynamic response measures the ion flow between the positive and negative plates. This process can be compared to a mechanical arm under load. Courtesy of Cadex |
The test takes 30 seconds, is 90 percent accurate regardless of battery cathode material and can be performed with a state-of-charge range of between 40 and 100 percent. QuickSort™ requires the correct mAh, and setting a wrong value does not shift the reading on a linear scale from good to poor, as one would expect, but makes the sorting less accurate. The system does not rely on internal resistance per se. This would produce unreliable readings because modern lithium-ion maintains a low resistance with use and time. Read more about How to Measure Internal Resistance. At the conclusion of the test, however, an overall resistance check is performed.
Lithium-ion batteries have different diffusion rates, and in terms of electrochemical dynamic response, Li-ion polymer with gelled electrolyte appears to be faster than Li-ion containing liquefied electrolyte. Li-polymer may need a different matrix to produce accurate readings.
Scientists explore new ways to evaluate the health of a battery with scanning frequencies ranging from several kilohertz to milihertz. High frequencies reveal the resistive qualities of a battery, which presents a bird-eye’s view in landscape form. By lowering the frequency, diffusion begins to provide insight into unique battery characteristics that allow capacity estimation, sulfation detection and revealing of dry-out condition.
Evaluating batteries at sub one-hertz frequency needs long test times. At one milihertz, for example, a cycle takes 1,000 seconds and several data points are required to assess a battery with certainty. Low-frequency tests can take several minutes for one measurement, however, with clever software simulation, the duration can be shortened to just a few seconds.
Research engineers at Cadex are working on a technique called Low Frequency Pulse Train (LFPT), also known as diffusion technology. Diffusion works with most chemistries and the information retrieved provides vital information relating to battery capacity and underlying deficiencies. This knowledge enables the all-important state-of-life estimation, the ultimate goal for advanced battery management systems (BMS).
There is a critical need for practical battery testers that can examine the state-of-health of batteries in medical equipment, military instruments, computing devices, power tools and UPS systems. There are currently no instruments that can reliably predict battery state-of-life on the fly, although many device manufacturers may claim their instruments will do so.
