Battery Experts Provide Deeper Explanations for Chevy Bolt Fires

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.

“The design of the battery system must be resilient enough to accommodate imperfections without catastrophic failure.”

Each Bolt battery cell, packaged in pouches, has hundreds of layers.

The safe and effective charging and discharging of batteries require passing a high voltage through these stacked layers of anode and cathode material, with a separator layer to prevent physical contact between the two. “From a manufacturing point of view, you have many thousands of these things to align and connect correctly,” said Hruska.

Sam Abuelsamid, a research analyst for mobility at Guidehouse Insights, said, “These things are hazardous and require very careful manufacturing process control, inspection, testing, and handling during transport.” He said the Chevrolet Bolt’s pack is not the only one to overheat or catch fire. Automakers issuing fire-related recalls also include BMW, Chrysler, Ford, Hyundai, Mitsubishi, and Tesla. “No one is immune,” said Abuelsamid.

EV battery experts also underscore that fires in electric vehicles occur at lower levels than gasoline cars. For example, Abuelsamid said there were 212,000 gas-vehicle fires in 2018, among 290 million vehicles. “That works out to about 10 times the frequency in the Bolt,” he said.

Debating the Bolt’s Pack Design

For most of the past decade, the world’s biggest automakers, including General Motors, ceded the electric car market to Tesla. Elon Musk’s company was the only player focused on producing big-battery, long-range EVs at a large scale. As Tesla sales and market value grew, General Motors was left selling relatively few copies of its 82-mile Chevy Spark EV and the Volt plug-in hybrid. Those models were discontinued in 2017 and 2018, respectively.

GM responded in late 2016 by launching the Bolt, its first long-range, mass-market electric car. Jaffe believes, however, that GM cut corners with the Bolt’s development. “When I saw the Bolt’s pack for the first time, I was taken aback at how close those cells were together,” he said.


Jaffe contrasts the Bolt’s battery spacing, separators, and thermal management system to Tesla’s approach, where heat is more actively pulled away from the cells all the way around. “The Bolt’s pack design is in the Dark Ages compared to what Tesla does,” said Jaffe. He said GM’s new Ultium battery, to be used in future cars, is also superior to what’s used in the Bolt.

Abuelsamid disagrees with Jaffe. “Tesla does have cooling tubes that wind down each row of cells, but everything is stuffed in pretty tight,” he said. “If there is thermal runaway, there’s not much to stop it, which we’ve seen in numerous Tesla fires.”

Trace, Detect, Respond

LG Energy Solution also produces cells for the all-electric Ford Mustang Mach-E. Brett Hinds, Ford’s global chief engineer of energy storage systems, explained how Mach-E cells, coming from LG’s Poland production facility, are evaluated. “Every cell is tested. Every array is tested. Every pack is tested. Every vehicle is tested,” said Hinds.

Despite these measures, Ford of Europe recalled about 20,500 Kuga plug-in hybrid crossovers last year over concerns that battery packs could overheat and cause a vehicle fire. The recall cost about $400 million, according to CNBC. The company responded by changing manufacturing and testing processes at LG and Samsung battery facilities, according to Abuelsamid.

According to a General Motors spokesperson, GM also “tests and tracks every cell, pack, and vehicle, and every cell is digitally imaged before it leaves its manufacturing facility of origin.” Hruska explained that it’s standard practice to test every cell, culling any defective cells at the factory. He warned, however, “It can be difficult to identify cells with latent defects at that stage that show up later as shorted electrodes.”

Hinds said that Ford can’t 100 percent guarantee that a few of the millions of cells going into cars won’t have a defect. “I can’t stand here today and say it’s never going to happen to Ford,” he said. “But we have the systems in place to detect the defect and respond. That’s our sole mission.”

Never Is a Long Time

Warning signs about problems with the Bolt battery started well before the current recall. How do I know? I leased a 2017 Chevrolet Bolt for three years, and during that time, my Bolt received two separate full battery-pack replacements due to cell failure.

In August 2017, I was among about 100 Bolt customers who experienced the car abruptly stopping, leaving me and others stranded on the road. GM replaced that pack in September 2017, announcing that it made “improvements throughout the supply chain” to correct a cell low-voltage problem.

A year later, with the second pack in place, I discovered that my Bolt’s battery was diminished to about half its storage capacity. I informed GM about the reduced range—where remote diagnostics might have caught the problem. In January 2019, my Chevy Bolt got its third battery pack. (I returned the Bolt last year at the end of its three-year lease.)

Tim Grewe, chief engineer of electric propulsion systems at General Motors, told me in 2019 that I was the single “one in a million second-event person.” He explained that GM and LG Energy Solution modified the manufacturing process. “Not only did we fix the issues,” Grewe said, “but we put in extra monitoring to make sure it never happens again.” Nineteen months later, GM acknowledged receiving reports of 12 Bolt fires. The number of confirmed fires has since grown to 16 incidents.

Difficult Lessons

As of this writing, General Motors is in the process of replacing packs, with Bolt production expected to resume in early December. LG Energy Solution agreed to pay $1.9 billion of the approximate $2 billion in costs. LG Energy Solution declined our request for comments about the cause of the Bolt battery issues, the remedy, or its agreement to cover the lion’s share of costs.

Greg Less, technical director at the University of Michigan Battery Lab, said a wholesale recall might ordinarily not have been warranted—but it became a business necessity. “The reason that General Motors recalled every single Chevy Bolt on the road has more to do with the fact that they’ve announced that they’re going all-electric,” he said. “They don’t want to earn the reputation of being the all-electric brand that has their cars burned to the ground. It’s a PR thing.”

Jaffe, the analyst at E Source, is grimmer. “The Bolt will go down in infamy as a failure,” he said. “But from GM’s perspective, that’s okay because they’ve moved on to the Ultium platform.” GM’s all-new, next-generation EV battery platform will be used for the company’s broad portfolio of future electric cars, starting in 2022 with the Cadillac Lyriq and GMC Hummer EV.

A GM spokesperson said that “spacing and cooling elements” used for Ultium-based EVs would “incorporate lessons learned from the total Bolt EV experience.”

Share your thoughts on the Bolt fire situation, the state of EV battery technology, and your inclination to opt for an electric vehicle in the near future.

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