GM announced last Friday that it was recalling every Chevrolet Bolt it had ever made, including the new electric utility vehicle model that debuted this year. After a string of fires affected Bolt models, the company traced the problem to two simultaneously occurring defects in the cars’ LG Chem-made batteries.
The automaker initially discovered the problem in batteries from one of LG’s Korean plants, and it recalled cars with those cells last November. But then more Bolts caught fire, and other LG plants were ensnared in the investigation, spurring two expansions of the recall. The problem, GM said, has been traced to a torn anode tab and a folded separator.
That’s all GM has said so far. It hasn’t said how widespread the defects are, nor has it said how, exactly, the fires started. But in what little information has been released, and in the timing of GM’s recalls, there are clues. To decipher them, Ars spoke with Greg Less, technical director of the University of Michigan’s Battery Lab.
“What we’re looking at is a perfect storm,” Less said. The Bolt’s battery packs are made up of pouch-type cells, which are essentially layers of cathodes, anodes, and separators that are flooded with liquid electrolyte and encased in a flexible polymer pouch. The torn anode tab, he said, would create a projection in what should be an otherwise flat battery. The projection brings the anode closer to the cathode. “And that would probably be OK if the separator was where it was supposed to be,” he said.
But in problematic Bolt batteries, the separator wasn’t where it was supposed to be. Separators are placed between the anode and cathode to prevent the two electrodes from touching. A torn tab wouldn’t necessarily be an issue on its own because the separator would prevent any projection from bridging the anode-cathode gap. In cells with a folded separator, though, the gap would be missing from at least part of the battery. If the anode bridges the gap, Less said, “you have a short, and it’s all downhill from there.”
“It wouldn’t surprise me if both defects are caused by the same thing,” he added. “I would imagine that the separator must be folded at the edge near where the anode tab is at. What I’m guessing is that at some point during the handling of the cell, before it’s fully packaged, some part of the robot machine is catching. The tab is catching, the separator is catching—something is catching very infrequently so that it hasn’t been noticed, and it’s causing this damage.”
Infrequent defects can be hard to identify in the quality-control (QC) process. “It can’t be happening on every cell, or QC would have caught it,” Less said. Quality control programs typically operate by sampling from a factory’s output. “QC is usually pretty slow. You can’t look at every cell and keep the process cost-effective, I wouldn’t imagine,” he said. One way to inspect for proper alignment of a battery’s layers is to X-ray the cell. Another way is to manually tear the battery apart.
Inside, engineers can look for problems like torn tabs or misaligned layers. In the format Less typically works with, which at 72×110 mm is smaller than the Bolt’s cells, he said, the layers must be carefully aligned. If they’re more than 1 mm off, the cell should be scrapped.
If the manufacturing defect in the Bolt batteries occurred infrequently, it’s unlikely that GM or LG could ever know the true extent of the problem without examining every pack. Recalling every Bolt might have been the only option.
The good news, Less said, is that GM and LG were able to dissect some batteries and ascertain the cause of the problem. That’s not always possible.
“It’s not one of these mystery cell failures, where you’re like, ‘We don’t know—something bad happened,’” he said. “Sometimes the cell exploded, and there’s nothing left for me to take a look to figure out why,” he said. In GM’s case, “it sounds to me like they were able to tear down some packs and find this defect with enough frequency that they’re like, ‘This is a real problem, and we need to do a recall,” Less explained.
Less said that the company deserves some credit for expanding the recall once it realized the problem couldn’t be isolated to a batch of batteries. Plus, GM is working through the growing pains of electrification earlier than most, he said. “General kudos for both GM and LG because they are doing a good job. Not just with the recall, but they’re making a lot of cells, and they’re leading the way of doing it.”