Starts small:  Defects that cause thermal runaway may be caused by: manufacturing defects in the cell, discharging at high rates to supply current to power hungry applications, the stress caused by fast charging, and over-charging. Actually many types of unintended abuse like dropping a device or sitting on it can also initiate safety problems.
 

Easily initiated: The most common cause of thermal runaway in cells is an internal short circuit. Shorting may be caused under some charging conditions when sharp whiskers, called dendrites, of lithium metal penetrate the separator to bridge between the electrodes. The flow of current through the short generates heat which decomposes the flammable electrolyte and releases oxygen from the cathode to form an explosive mixture ignited by the generated heat. A short can also occur due to a tear in the separator, metal impurities introduced during production, or contact between electrodes which become misaligned during use or in manufacturing.
 

Hidden threat: The development of the defect leading to a short before it causes thermal runaway can take weeks without significantly affecting the performance of the cell. Detecting these early precursors especially in their benign early stage of development is very difficult.
 

Growing risk: 20 years after their major commercialization lithium ion batteries are becoming more hazardous. Their energy has doubled but they are now being made with less expensive materials and stressed more by faster charging and higher power uses. This promotes the creation of shorts.
 

Old Solutions: Conventional methods detect the hazardous defects when they have already begun to induce irreversible failure in the cell, near the threshold to thermal runaway. At this point it is too late to prevent an explosion. Also, these methods are expensive, have low sensitivity and measure a single parameter which lowers their reliability. In short, they cost a lot and are too little and too late to stop thermal runaway.