Pitfalls of Li-ion battery pack testing

Article By : Bill Schweber

The Samsung Galaxy Note 7 fiasco has given rise to new discussions, ideas and configurations that could help in testing and assessing Li-ion battery packs.

Now that much of the attention, commentary and even jokes by late-night hosts regarding the dangers of the Samsung Galaxy Note 7 smartphone battery pack have faded from the public's consciousness, it may be worth a review to see what is known, not known, or still unclear. There is no question that these battery issues were a major marketing and technical problem, as they led to the phone's recall and complete product withdrawal from the market. It appears that there were two root causes of the battery problems: insufficient clearance around the battery pack to allow for their inevitable thermal expansion; and microscopic burrs that poked through barriers inside the battery.

The issues related to testing of batteries in general and broader concerns about quality-assurance testing of components without destroying them, are complex. Two articles take a look at this from different perspectives. The first, "Why Samsung’s battery fix gets a grade C, for now" (sign-in required) from The Wall Street Journal, is a chronicle and update of what happened and the specifics of Samsung’s test processes. The table provided by Samsung summarises the multi-dimensional testing that needs to be done to establish an acceptable level of confidence. (The associated photos in the article, also provided by Samsung, are even more impressive, as they show racks of thousands of phones and batteries undergoing test. Setting that up must have simultaneously been a test engineer's dream and nightmare.

Figure 1: A summary of the multi-dimensional testing required to establish an acceptable level of confidence for devices. Source: Samsung

The second article, from Medical Design Briefs, isn't specific to Samsung but does look at Li-ion battery packs. "Quality Assurance: Risk Mitigation for Lithium-Ion Battery Packs" looks at some of the test-process flow as well as standards that vendors of these batteries must adhere to in the test process. It also explains how test results are used as part of a quality-assurance feedback loop to understand and improve the product and process.

This is all good and important. But the underlying problem with batteries is that you can only examine them nondestructively as "black boxes" via their terminal voltage and current data, as well as temperature and physical dimensions. If you want to know more, you have to do an autopsy. Also, while aggressive steps such as punctures may enhance assessment of some failure modes, they don't provide insight into the thermal runaway mode that is of most interest (the Samsung batteries had not been physically abused).

But there is hope on the black-box front as well. A team at University College London (UCL) has developed a complex set-up that performed an internal computerised axial tomography (CAT) scan on lithium batteries in real time so they could see what's going on inside as shown in Figure 2. How and what they did and what they found, makes for a fascinating story, as it wasn't just a simple matter of borrowing an available CAT scanner; there's a synchrotron in the picture, as well as an interesting fixture.

Figure 2: (a) Cut-away of battery-containment design attached to the rotation stage for real-time X-ray CAT scan; (b) Arrangement of apparatus thermal runaway experiments; (c) 3D reconstruction with slices in the XY, YZ and XZ planes of a 2.6-Ah battery (Cell 1) with isolated XY slice; (d) 3D reconstruction with slices in the XY, YZ and XZ planes of a 2.2-Ah battery (Cell 2) with isolated XY slice. (Image and caption from University College London and Nature).

You can get details of the story several ways: There's the detailed but hard-to-follow academic paper, with many excellent figures in Nature, "In-operando high-speed tomography of lithium-ion batteries during thermal runaway;" there's the overview by the research team, "Battery Safety;" and there is also the college's well-written press release with an attention-grabbing yet legitimate headline, "Tracking exploding Lithium-ion batteries in real-time". There's also an interview with the lead researchers in NASA Tech Briefs, "3D Imaging Reveals Battery Degradation in Real Time," which explains some of the issues they encountered and overcame with the test set-up—something that's rarely discussed in press releases or formal scientific papers but which is of interest to test engineers.

Getting new insight into subtle test and quality issues is difficult, especially when the "subject" doesn't easily give up its secrets, but with new instrumentation, configurations, ideas and funding (of course), it may be doable. What's on your list of things you'd like to see in a T&M setting, both literally and figuratively?

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