To verify the safety of a new cell, a manufacturer may release 1 million samples into a workforce on observation. The cell is approved for the use of critical missions, such as medical, if no failure occurs in one year that could compromise safety. Similar field-testing is also common with pharmaceutical products.
Li-ion using conventional metal oxides is nearing its theoretical limit on specific energy. Rather than optimizing capacity, battery makers are improving manufacturing methods to enhance safety and increase calendar life. The real problem lies when on rare occasions an electrical short develops inside the cell. The external protection peripherals are ineffective to stop a thermal runaway once in progress. The batteries recalled in 2006 had passed the UL safety requirements — yet they failed under normal use with appropriate protection circuits.
There are two basic types of battery failures. One occurs at a predictable interval-per-million and is connected with a design flaw involving the electrode, separator, electrolyte or processes. These defects often involve a recall to correct a discovered flaw. The more difficult failures are random events that do not point to a design flaw. It may be a stress event like charging at sub-freezing temperature, vibration, or a fluke incident that is comparable to being hit by a meteor.
Li-ion batteries that have been exposed to stresses may function normally but they become more sensitive to mechanical abuse. The liability for a failed battery goes to the manufacturer even if the fault may have been caused by improper use and handling. This worries the battery manufacturers and they go the extra mile to make their products safe. Treat the battery as if it were a living organism by preventing excess stress.
