For those who want to get a bit nerdy, here’s what Samsung says was wrong with each battery. For the first battery, Samsung says a design flaw in the upper right corner of the battery made the electrodes prone to bend and, in some cases, led to a breakdown in the separation between positive and negative tabs, causing a short circuit.
With the second battery, which came from a separate supplier, Samsung believes there was nothing wrong with the design itself, but says a manufacturing issue led to a welding defect that prompted that battery to also shortcircuit and ignite.
Whoops.