Samsung says two different battery flaws were to blame for the fires that plagued its flagship Galaxy Note7 smartphone throughout the fall, leading to two separate recalls and, ultimately, the permanent withdrawal of the model from the market. The details are being released after an internal investigation, following weeks of speculation by reporters and analysts about what the company’s report would conclude.

Consumer Reports and several other outlets were briefed on the findings in separate meetings on Thursday. An article published by the Wall Street Journal on Friday and widely cited by other news organizations said the report found that a number of the fires occurred because “some of the batteries were irregularly sized.” That conflicts with details provided to Consumer Reports, and a Samsung representative said the information “did not come from Samsung.”

It has been known since early on that the phones’ lithium-ion batteries caused the explosions and fires, which began to occur around the globe shortly after the model was launched to consumers on Aug. 19, 2016. The Note7 fires became a staple of news in the fall, leaving in their wake damage including a destroyed Jeep, some injuries, and the evacuation of a Southwest Airlines flight.

According to Samsung, the new investigation shows that problems were isolated to the batteries, and were unrelated to other hardware components or the software that manages energy use in the device. However, some experts say that Samsung’s drive to make thinner phones with longer battery life may have contributed to the problem.

In addition to its own investigation, Samsung contracted with independent groups, including Underwriters Laboratories, to look into the causes of the fires.

The company used batteries from two suppliers in its Note7 phones—a company division called Samsung SDI and a separate company, Amperex Technology Ltd., that makes batteries for many smartphone brands. Samsung officials said the batteries from the two companies were designed and manufactured differently, and failed for separate reasons.

That distinction is important, because it helps explain the unusual sequence of events during the recalls in the fall. Samsung first said it was stopping sales of the phone on Sept. 2, and announced a formal recall on Sept. 15 in cooperation with the Consumer Product Safety Commission. But, with permission from the CPSC, Samsung began distributing replacement Note7 phones just a few days later.

Soon these, too, started catching fire and a second recall was announced on Oct. 13.

Now, Samsung is saying that its investigation shows there was no reason to believe the new phones would malfunction. “It was a very tough period, and we are sorry,” DJ Koh, Samsung’s global president of mobile communications, said during the briefing. “The most important thing for us is customer safety,” and, ultimately, “earning back customers’ trust.”

How Defects Led to Short Circuits

Like other Li-ion phone batteries, the Note7 batteries were made of sheets of material that served as positive and negative electrodes, with a separator between them. The electrodes and separator were folded up into a “jelly roll,” and then inserted into a pouch or casing.

Chemical reactions in a battery’s electrodes allow an electrical charge to flow between them, generating a current that flows through a circuit and powers the device. Normally, the electrodes don’t touch; if they do, a “short circuit” is created, and that can spark a fire.

During the presentation, executives showed a CT scan of a Note7 from before the first recall, in which one corner of the battery’s pouch impinged on the jelly roll, deforming the negative electrode. In some consumers’ phones, the positive and negative electrodes touched, causing a short circuit.

Samsung officials said that the company found that flaw before Sept. 15, and could see that wasn’t present in batteries made by the second supplier. At the time, the company did not disclose those details to the public.

We “were very comfortable” switching all battery production to the second manufacturer, marketing senior VP Justin Denison said.

Fires in the second group of batteries were caused by a manufacturing defect, Denison said. A sharp edge, or burr, was created by inconsistent welds in one section of the battery, the “positive tab.” (See illustration, above.) In some cases the burr pierced insulating material and the separator between the two electrodes, causing a short circuit.

Thin Design a Contributing Factor

Samsung says its investigation involved 200,000 handsets, 30,000 standalone batteries, and 700 engineers working in a dedicated facility. Phones were tested with the back cases both on and off (see photo at the top of the article) and with various types of software running. They were evaluated during both rapid and standard charging. According to the company, batteries tested on their own failed at about the same rate as complete phones did.

This may show that no hardware beyond the batteries was at fault, but others in the battery industry say that smartphone design trends are making flaws more likely.

Qichao Hu is the CEO of SolidEnergy, a developer of high-energy-density batteries used in mobile devices, high-altitude drones, and clean-energy vehicles. He thinks Samsung’s battery problems were partly a result of the company’s attempt to maximize battery life while cramming components into ever-thinner phone cases. High-end phones typically use custom-designed batteries.

“Samsung is really pushing the boundaries, making the separators really thin,” Hu says. “Making the separator thinner makes the battery much easier to short because it’s already a porous membrane and that makes it easier for pin holes to form in the separator.”

According to Samsung, the Underwriters Laboratories investigation concluded that thin separators could have contributed to the risk of short circuits in both Note7 batteries. UL would not comment for this article.

If you want safer batteries, according to Hu, you’ll probably be looking at a bulkier phone with a removable battery. “They have a protective plastic case and may even have a gas relief valve to diffuse volatile conditions,” he says. “Non-removable batteries don’t have a case—just aluminum foil wrapping.”

The other option for phone makers, according to Hu, is to create a very thin phone with a shorter battery life.

Successful Recall

The saga of the Galaxy Note7 is ending with a singular accomplishment: one of the most successful recalls in history, as measured by the number of products that were recovered from consumers. As of mid-January, Samsung says 97 percent of all Galaxy Note7 smartphones have been returned in the United States, and 96 percent worldwide.

“A 97 percent recall rate is almost unheard of,” says Pamela Gilbert, a former executive director of the CPSC who is now a partner at Cuneo Gilbert & LaDuca, LLP. The historic figure is often less than 50 percent, she says, even when auto safety is concerned. “People have an amazing ability to avoid doing things in their best interest if it’s inconvenient.”

Like Consumer Reports and some other organizations, Gilbert was critical of Samsung in September for not involving the CPSC quickly enough. During the Consumer Reports briefing, executives said that the company notified the agency of problems on Sept. 2. (Officials at the Consumer Product Safety Commission declined to comment for this story.) But in the end, Gilbert says, “I think [Samsung] took this very seriously and wanted to get these phones back.”

One reason for the success of the recall was the steady beat of publicity, both from news outlets and other sources. For instance the Federal Aviation Administration banned Note7 phones from flights, and until Jan. 10 the agency required airline personnel to make announcements telling passengers not to bring the phones on planes. Samsung dispatched customer service staff to the country’s 22 largest airports to help facilitate returns.

Tim Baxter, the president of Samsung America, said the biggest reason for the high success rate was that “the subject of the recall was actually the communication tool in reaching consumers.”

Once the recall was put in place, 23 million text messages were sent to Note7 owners telling them to return the phones, Baxter said. When people plugged the phones in to charge, a message appeared advising them of the recall, and through a series of firmware updates, the company and its cellular partners reduced the functionality of the phones, until they were finally rendered inoperable.

Samsung officials said that they have instituted a new, eight-point program of quality assurance measures to help prevent future problems. And they plan to share what they’ve learned with others in the battery and mobile-phone industries.

Related update: CPSC: Risk of Fire Is Real, And We Really Need To Modernize The Standards For Lithium-Ion Batteries

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