The conclusion is said to be two fold: 1) that batteries used were irregularly sized, which led to overheating; and 2) some just had manufacturing problems. The batteries used for the Note 7 included those from Samsung SDI Co, another affiliate, or Amperex Technology, according to the WSJ. Initially, Samsung thought the battery problems came from their Samsung SDI affiliate, which may have been correct. The batteries there were found to ship with an irregular size that caused overheating. However, as they killed their own battery maker and switched to Chinese battery makers after the initial recall, the ramp-up in production caused manufacturing issues that led to the second recall.