While we wait for Samsung to publish the results of its official investigation into the Galaxy Note 7 fires and subsequent recall, a new teardown from Instrumental, completed with a fire extinguisher at hand, gives us our clearest look yet at what went wrong with the Note 7. In short, Samsung engineers didn’t allow enough internal space for the handset’s large battery, leading to a pressure build-up, short-circuit, and fires.
Many of the early reports about exploding Galaxy Note 7 handsets pointed to a major issue with the battery and a breakdown in the lithium ion battery’s electrolyte-soaked separators was thought to be the most likely cause of the fire, and this seems to have been the case. While it’s still possible that the manufacturing technique or a fault in Samsung SDI’s design is still partially responsible, the teardown reveals that excessive pressure on the battery is what is most likely to have cause the positive and negative layers of the thin polymer separator to touch, causing the fire. Put simply, the engineers crammed the battery in too tightly.
The teardown reveals that the Galaxy Note 7’s battery sits within a costly CNC-machined pocket, which only leaves between 0.1 mm and 0.5 mm of space space for the cell to expand in the X and Y axes. That’s not very much, and the engineers also neglected any space for expansion in the Z axis. The battery measures 5.2 mm thick, and the pocket is only 5.2 mm deep. Ideally, Samsung should have left a 0.5 mm (10% rule of error is common in most engineering fields) ceiling for battery expansion in the XYZ planes. This is essential as all lithium-ion batteries expand slightly as they are charged and discharged overtime. Failing to accommodate for this extra space increases the pressure on the cell, putting it at a much greater risk of exploding.
What’s particularly disheartening about this revelation is that it could have easily been avoided with a less aggressive design. Samsung’s engineers ended up making a conscious decision to pursue the maximum possible battery capacity knowing the additional risks and breaking well known design principles. While ensuring that the phone’s battery life competes with rival handsets is a tough requirements, other less dangerous design compromises should have been given more of a hearing in hindsight.
Although this teardown reveals a glaring issue with the Note 7, we should also wait for Samsung to announce the results of its official investigation before drawing definitive conclusions, as it may reveal other causes. Either way, with the Galaxy Note 7 now mostly behind Samsung, the company will hopefully have learned from its mistakes in time for next year’s Galaxy S8 flagship release.