A misaligned factory robot may have sparked a Chevy Bolt battery fire

Chevrolet

GM announced last Friday that it is recalling every Chevrolet Bolt it has ever made, including a new electric utility vehicle model that debuted this year. After a series of fires affected Bolt models, the company traced the problem to two concurrent defects in LG Chem car batteries.

The automaker initially discovered the problem with the battery from one of LG’s Korean factories, and recalled the car with the cell last November. But then more Bolts caught fire, and another LG factory became entangled in the investigation, prompting two extended recalls. The problem, GM says, has been traced to the torn anode tab and folded separator.

That’s all GM has said so far. It did not say how widespread the defect was, nor did it say exactly how the fire started. But in the little information that has been released, and in the timing of GM’s recall, there are clues. To elaborate, Ars spoke with Greg Less, technical director of the University of Michigan’s Battery Lab.

“What we saw was a perfect storm,” said Less. The Bolt battery pack consists of pouch-type cells, which are essentially a cathode, anode, and separator layer that is flooded with liquid electrolyte and encased in a flexible polymer pouch. Torn anode tabs, he said, would make projections on what would otherwise be a deflated battery. The projection brings the anode closer to the cathode. “And it’s probably going to be okay if the divider is where it’s supposed to be,” he said.

But in the problematic Bolt battery, the separator is not where it should be. A separator is placed between the anode and cathode to prevent the two electrodes from touching. Torn tabs are not always a problem as the separator will prevent any projections from bridging the anode-cathode gap. However, in a cell with a folded separator, the gap will disappear from at least part of the battery. If the anode bridges the gap, Less says, “You have a short circuit, and it all goes downhill from there.”

“I wouldn’t be surprised if the two defects were caused by the same thing,” he added. “I would imagine that the separator should be folded at the edge near where the anode tab is. What I suspect is that at some point during the handling of the cell, before it is fully packed, some part of the robotic machine catches up. Tabs caught, separators caught—something so rarely caught that it goes unnoticed, and causes this damage.”

Quality control

Infrequent defects may be difficult to identify in the quality control (QC) process. “It can’t happen in every cell, or QC will catch it,” Less said. Quality control programs usually operate by sampling the factory output. “QC is usually quite slow. You can’t see every cell and keep the process cost-effective, I wouldn’t imagine,” he said. One way to check the proper alignment of the battery layers is with an X-ray of the cells. Another way is to tear out the battery manually.

Inside, technicians can look for problems like torn tabs or misaligned seams. In the format Less usually works with, which is at 72×110 mm smaller than the Bolt cell, he says, layers must be carefully aligned. If it is more than 1 mm, the cells must be discarded.

If a manufacturing defect in a Bolt battery is rare, it’s unlikely that GM or LG will be able to find out the true extent of the problem without inspecting each individual package. Remembering every Bolt is probably the only option.

Known issues

The good news, Less says, is that GM and LG were able to dissect some of the batteries and pinpoint the cause of the problem. That’s not always possible.

“This isn’t one of these mysterious cell failures, where you’re like, ‘We didn’t know—something bad happened,'” he says. “Sometimes the cells explode, and there’s nothing left for me to check to find out why,” he said. In GM’s case, “it sounded to me like they were able to knock down some packages and find this flaw with enough frequency that they were like, ‘This is a real problem, and we need to do a recall,’ Less explains.

Less said the company deserves credit for expanding the recall after realizing the problem couldn’t be isolated from a batch of batteries. Plus, GM is working through the developing pains of electrification earlier than most, he says. “General kudos to GM and LG because they did a great job. Not only with withdrawals, but they make a lot of cells, and they’re leading the way.”

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