What May Have Failed Inside the Phone
Federal regulations give a plain starting point for what "defect" means. In the Consumer Product Safety Commission's reporting rules, 16 C.F.R. § 1115.4 says a defect includes "a fault, flaw, or irregularity that causes weakness, failure, or inadequacy in form or function." It describes defects that come from a manufacturing or production error, from a product's design and materials, and from a product's "contents, construction, finish, packaging, warnings, and/or instructions." That regulation governs what companies report to the agency; state law decides what counts as a defect in a lawsuit. Phone battery failures tend to fall into the same three groups.
Manufacturing defects happen when one unit is not built the way it was designed. Examples include contamination inside the cell during production and damage to the separator, the thin barrier between the battery's positive and negative electrodes. If the separator is punctured or fails, the electrodes can touch, causing an internal short circuit that can start thermal runaway.
Design defects concern the plan for every unit, where the design itself is unsafe. Examples include inadequate thermal cutoff, a BMS that does not stop charging at dangerous temperatures, and a battery shape or housing that leaves no room for swelling, so that a swelling cell is pressed and damaged.
Warning defects concern what the user was told: a failure to warn of known risks or to give adequate instructions. Examples include missing or unclear warnings about incompatible chargers, third-party replacement batteries, or charging in enclosed spaces.
Sorting a failure into one of these groups matters to someone who was hurt, because the type of failure points toward which company in the chain may be responsible and what evidence is relevant. That sorting is not done by appearance. An incident report records that a fire happened, not what caused it; finding the failure mechanism usually takes technical analysis of the physical battery and phone. For that reason, what remains of the phone and charger often carries more weight than any description of the fire, and whether it has been kept intact is one of the first things an attorney may ask about.