Now it may be known. The batteries in Samsung's Galaxy Note 7 exploded mainly because the company was too "aggressive". And because of lack of space.
According to reports, Instrumental, a manufacturing engineering firm, tore a Note 7 down to get to the bottom of things and discovered two possible reasons why the batteries of the doomed device caught fire.
First, Instrumental believes the company failed to thoroughly test the batteries used in the Note 7. It speculated that Samsung might have tested different kinds of batteries and that the latest versions "weren't tested with the same rigor as the first samples". Normally testing a battery takes a "notoriously long time" and Samsung may have been too eager to release the device. Of course, this is just a speculation.
According to CNBC, Samsung used batteries that are made up of positive and negative layers. The positive layer is made up of lithium cobalt oxide while the negative layer is made of graphite. These layers have separators between the layers to allow energy to flow between the layers. It also prevents them from directly touching each other. Once these layers touch, the reaction may cause a spark which could lead to the battery catching fire.
According to Instrumental, this is exactly what happened with Samsung's batteries. While all modern smartphones use batteries with positive and negative layers, Samsung made the mistake of using thin separators.
The company probably decided on decreasing the thickness of the separators to streamline the size of the batteries.
During the teardown, Instrumental also found out that the lithium-ion battery did not have enough room for error. When the batteries are charged, they expand. This is why a 10 percent extra space is provided by manufacturers. The Note 7's battery filled the entire space after it expanded causing it to overheat.
There have been a number of speculations as to why the Galaxy Note 7 was catching fire and most of them pointed accusing fingers at its batteries. It was later discovered that Samsung was testing its batteries in its own lab, a practice that was uncommon.
Samsung has not issued an official statement regarding Instrumental's claims as of this writing.