Entry 3

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mp4 animation 1Hz @ 35000

mp4 animation 1Hz @ 4200

Authors

  • Robert Moision
  • Jeanne M. McGraw

Abstract

This plot is an animation showing the response of 24 thermocouples attached to a lithium-ion (Li-ion) battery pack following an intentional overcharging event. The animation captures both the spatial and temporal response of the thermocouples.

Thermal runaway events in Li-ion batteries have been implicated in a number of recent high profile failures in aircraft (Boeing 787 Dreamliner) and automobiles (Tesla Model S). These failures are often quite dramatic, involving a combination of flames and superheated gases shooting from the battery compartment. Of particular interest is how the failure of a single battery cell can increase the temperature of its surroundings to the point that secondary thermal events are observed in neighboring cells. These secondary events can occur many hours after the first event, making it a particularly insidious phenomenon.

The battery being tested consists of three separated sections. The front section contains the battery control electronics and the other two sections hold Li-ion cell packs. These regions are physically separated by walls within the battery housing which run ¾ of the length of the box leaving a channel to allow the wiring harnesses from the cell packs to connect to the battery control electronics. This channel also allows for hot gases, flames, and debris to spread following a thermal runaway event. In this test, a single cell was overcharged to study the thermal propagation to neighboring cells.

In the animation, the initial thermal event occurs around 4300 seconds and involves a very rapid change in temperature to TC #16. This is the result of flames and hot gases escaping from the damaged battery pack near this thermocouple. Within a few seconds, the temperature of the damaged cell, TC# 4, 5, 6, begins to increase as well as the adjoining battery control electronics, TC# 7 and 8. Around 4500 seconds, the temperature peaks in the adjoining battery cell, TC# 1, 2, 3, damaging it in the process. The temperature of the entire pack slowly cools for over 8 hours before the second thermal event is observed, around 35,400 seconds when hot gases escape from the damaged cell to heat TC #15 and the surrounding area.

Initially, we used the combination of a PowerPoint drawing indicating thermocouple position and a series of x-y line plots to show the temperature response of each thermocouple. We found that these static plots made it difficult to convey the rate and location of the temperature spread. We wanted to display the data in a way that could capture the dynamic nature of the test so that extent of temperature change could be easily understood. Ultimately, we used a combination of animations, static plots, and multivariate analysis to present the thermal runaway results. However, the animations had by far the largest impact on the viewing audiences and were responsible for catalyzing a number of fruitful discussions.

The graphics on the plot and thermocouple locations are identical to those on the earlier static PowerPoint schematics. We decided to replicate the format in the animations to maintain continuity with the earlier work. Faithfully recreating the PowerPoint graphics was fairly straightforward. The type, location, size, and fill of each graphical object in the PowerPoint drawing were entered into a text file that was subsequently used to draw and format objects in Matplotlib. Over the course of the testing process, thermocouple locations were sometimes changed, this approach made it trivial to incorporate modifications.

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