Irregular sizing and manufacturing defects in the Galaxy Note 7 battery have been deemed to be the primary causes for the fires and explosions that caused the flagship device to be recalled globally in 2016. The revelations come from The Wall Street Journal, which has received advance information ahead of Monday’s public disclosure from the South Korean giant.

According to the WSJ, the report states that irregular sizing of some batteries caused them to overheat, while others suffered from undisclosed manufacturing defects. It is not yet clear if that defect is the same one described in an unpublished preliminary report from Samsung to South Korea’s Agency for Technology and Standards acquired by Bloomberg in September. For reference, here’s what Bloomberg had to say about that previous report:

Initial conclusions indicate an error in production that placed pressure on plates contained within battery cells. That in turn brought negative and positive poles into contact, triggering excessive heat.

The basic substance of that claim was echoed again recently when a Note 7 teardown conducted by Instrumental revealed an insufficient “ceiling” for expansion surrounding the Note 7 battery. With as little as a 0.1mm gap between the battery and the rigid confines of the machined pocket in which it lay, when the battery naturally swelled during normal charging and discharging, it had nowhere to go, placing pressure on the battery cell.

Batteries made by Samsung SDI (those initially blamed for the exchange and re-issue of “safe” replacement devices) were the ones found to cause overheating due to an ill-fitting battery. The affected batteries made by Amperex Technology Ltd suffered manufacturing issues “resulting from the quick ramp-up in production of replacement phones”.

Two different causes of the battery fires from two different suppliers might be the reason it took the company so long to identify exactly what lay behind the Note 7’s notorious battery problems. That simultaneous battery problems existed across two distinct suppliers – with one in part caused by the other – only throws Samsung’s quality control mechanism into further doubt.

We’ll have to wait until 8:00pm EST/5:00pm PST Sunday for the full explanation, but suffice to say that Samsung will have ramped up its quality control, especially where batteries are concerned, in a major way in the wake of this costly mistake. While there is never any guarantee that any product is entirely safe, you can rest assured that the upcoming Galaxy S8 will have been subjected to more scrutiny than any other phone in history.

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