Battery Experts Provide Deeper Explanations for Chevy Bolt Fires
more at the link (a lot more) Autoweek
General Motors has learned some hard lessons in the raft of Bolt fires, even as a measure of disagreement still exists over where the fault lies.
In August Chevrolet announced the third recall of the Bolt EV, covering all 141,000 models, and later confirmed that 16 cars had caught fire.
The Bolt battery pack is not the only one to overheat or catch fire, however, as BMW, Chrysler, Ford, Hyundai, Mitsubishi, and Tesla have all issued fire-related recalls. EV battery experts also underscore that fires in electric vehicles occur at lower levels than gasoline cars.
General Motors is in the process of replacing packs, and Bolt production is expected to resume in early December. LG Energy Solution, which supplies the packs, agreed to pay $1.9 billion of the approximate $2 billion in costs.
General Motors suffered a setback to its big EV ambitions when the company in August announced the third recall of the all-electric Chevrolet Bolt. The company recalled all 141,000 Chevy Bolts, later confirming that 16 cars had caught fire. GM pointed to battery manufacturing flaws as the culprit—specifically a “torn anode tab and a folded separator” in a few individual cells. However, our conversations with several leading battery analysts cast Bolt battery woes in a broader context.
“This isn’t a black-and-white problem like a cell has defects or not,” said Louis Hruska, a technical consultant who worked at Duracell for 12 years, where he held leadership roles in advanced battery manufacturing. He served as Duracell’s director of engineering for rechargeable batteries for four years.
Hruska believes that eradicating tears and folds is a worthwhile goal but that imperfections and variations—sometimes measuring fractions of a millimeter—are relatively common. “The design of the battery system also must be resilient enough to accommodate some imperfections without catastrophic failure,” he said.
The most urgent purpose of an EV’s battery management system is to prevent overheating in one cell from spreading to the rest of the pack. The first safety measure is to produce near-flawless cells, and the second level of protection is to detect problematic cells and modules—and isolate them. “It’s two layers of safety engineering,” said analyst Sam Jaffe, vice president of Battery Storage Solutions at E Source, a research and consulting firm. “You don’t want a defective cell, but sometimes you still get them.”
Jaffe said, “In the Bolt’s case, both layers failed more than a dozen times.”
A General Motors spokesperson this week told Autoweek that “the Bolt EV does not have the design feature to disconnect an individual battery cell.” Instead, GM uses in-vehicle monitors, factory build data, and customer-use data to detect “abnormal behavior.” In that event, the car’s control system could be used to “limit the entire battery pack to the lowest cell capability.”
GM and South Korea’s LG Energy Solution, its battery supplier, are moving quickly to replace the battery packs of all 2017-19 model-year Bolt EVs (and some 2020-22 models). It’s a $2 billion snafu.
EV Cell Manufacturing Is Hard
A battery-cell assembly line can be compared to a newspaper printing press. But instead of using paper, EV battery cells are made with ultra-thin foil rolls run through a metal coating machine. The rolls are typically less than a meter wide but thousands of meters long.
Each metallic layer is “thinner than the aluminum foil used in your kitchen,” said Hruska. “The foil is so thin that it is torn easily if mishandled by the automated process machinery.” The sheets are coated, dried, heated, cut, folded, welded, and stacked like a deck of metallic playing cards.
Each Bolt battery cell, packaged in pouches, has hundreds of layers.