Charged EVs | Containing single-cell thermal runaway events in EV battery packs

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Below Fireplace: Reevaluating Torch and Grit Testing Parameters

The security of Li-ion batteries depends on containing single-cell thermal runaway (TR) occasions in order that they can’t propagate into adjoining cells, triggering a cascading failure. Whereas the thermal power launched throughout a runaway occasion can take a number of pathways, probably the most important are: 

1. Cell-to-cell conduction
2. Strong-, liquid-, and gas-phase ejecta launched from the misbehaving cell. 

The primary may be managed with high-performance insulation supplies which, when positioned between cells, act as a firewall. Because the world’s highest performing insulation supplies, aerogel-based thermal barriers have successfully solved the problem of cell-to-cell (C2C) conduction. 

The protected dissipation of ejecta is a tougher activity. The high-velocity, high-temperature jets that emerge throughout a cell rupture can rapidly soften and/or erode even stable aluminum. To raised perceive this mixed thermal and abrasive assault, engineers usually flip to the UL 2596 take a look at commonplace for steering. This technique makes use of an array of burning Li-ion cells to evaluate the thermal and mechanical efficiency of battery enclosure supplies. Lately, the usual was revised to incorporate a benchtop experimental technique of simulating publicity to battery-fire ejecta, however with out the batteries. This new torch-and-grit (TaG) technique directs the flame from a propane torch at a goal pattern, whereas periodically injecting high-velocity grit into the stream to duplicate battery ejecta and their results on adjoining parts. 

The UL 2596 commonplace has been very useful within the analysis of pack covers, which usually exist in shut proximity with, and regular to Li-ion cell venting. Nevertheless, outdoors of the instant impingement zone, the UL 2596 take a look at parameters are possible too conservative to simulate publicity all through the remainder of the pack. In different phrases, parts that may survive module- and pack-level thermal propagation testing will usually fail UL 2596, limiting its worth as a screening instrument for non-pack-cover purposes. 

Nevertheless, latest thermal runaway testing means that UL 2596 may be modified to enhance its accuracy for non pack cowl parts. These assessments positioned prismatic cells immediately towards and parallel to a goal pattern such that, when triggered, the ejecta impacted the pattern at an indirect angle. The ensuing “glancing blow” was discovered to be extra typical of what’s skilled by parts farther away from the impingement zone. It additionally means that canting the UL 2596 goal to 15-25° from parallel would possibly present a greater simulation of far-field impingement.

After every take a look at, the ejected stable particles have been collected for evaluation. The staff reviewed the particle measurement, hardness, and chemistry to higher perceive their habits throughout thermal runaway occasions. The evaluation revealed a big proportion of enormous (>700µm), non-spherical remnants of aluminum and copper foils. Whereas their preliminary ejection velocity is excessive, the a number of inelastic impacts they expertise inside the confined area of a battery pack rapidly bleed off momentum. The ensuing crumpled items possess a low ballistic coefficient and can subsequently observe the fluid-dynamic streamlines towards the pack vent – suppose feathers, not cannonballs. 

Smaller particles, possessing a Sauter imply diameter (D₃₂) of 240 microns, have been composed primarily of oxygen, aluminum, nickel, and carbon. Regardless of their metallic composition, these particles have been smooth and simply powdered within the hand. Vickers hardness testing was carried out, displaying a lot of the ejecta being considerably softer than UL 2596’s beneficial grit, aluminum oxide. This implies that additional modifying the take a look at with a softer abrasive would produce outcomes extra acceptable for surfaces outdoors the impingement zone.

Companion with scientists and engineers who deeply perceive the complexities of thermal runaway occasions. Go to our web site to study extra about PyroThin® cell-to-cell barriers and request a meeting with our technical staff to debate your software.