Batteries were the root cause for Galaxy Note 7 failures according to Samsung who recently held their press conference to discuss their findings on what caused the incidents to occur. Samsung launched a fairly comprehensive investigation into what caused the issues with the Galaxy Note 7, looking at various aspects of the device to find out where the problems initially occurred. This included looking at software as well as hardware, and they conducted tests that were designed to replicate the results that people were experiencing. Samsung states that they subjected the device to a 4,000 volt electrostatic discharge to see if the issues were caused by USB Type-C. They also looked at fast charging and conducted tests with the fast charging on and off, as well as charging the device with and without the back cover to see if these factors had any affect on the phone overheating.
Samsung had 700 engineers conducting tests over the span of a few months, and Galaxy Note 7 devices were constantly being charged and discharged to attempt and replicate the incidents. Based on Samsung's observations there was more than one root cause for the failure. The initial batch of devices that were part of the first recall resulted in failures due to the unintended damage to the negative electrode windings in the battery cells. Samsung mentions that the damaged negative electrode windings were consistently in the corner of the cell that was closest to the negative tab of the cell, and that the cause of the damage was because the cell pouch design that was used did not have enough room for the electrode assembly.
The second failure was due to a completely different set of factors that Samsung says were not present in the first battery cell investigation, and these batteries were from the second wave of devices that were sent out as replacements for the first set of phones that were recalled. The root cause for the failure in these cells was due to internal cell faulting, which Samsung explains happened for a couple of different reasons, one of which was because of a short circuit in the separator that sits between the positive and negative electrodes, while the other was due to lithium plating and damage to the coating on the outside of the negative electrode. Samsung also states that some cells in this second batch of incident device batteries were assembled without the protective insulation tape that is supposed to be placed over the positive tab, which only increased the chances that the battery would fail and overheat. Although Samsung had determined that the root cause of the issue was the battery in both sets of devices, they checked to see if any of the electronics had played a part in the failures, and they mention that the battery was the sole reason for the incidents. In addition to their own investigation, Samsung also had investigations done by industry experts, which included TÜV Rheinland, Exponent, and UL, all of which came to the same conclusions as Samsung.