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Batteries can release high energies and the safety requirements for nickel- and lithium-based batteries and cells for portable applications are harmonized under IEC 62133. The standard came into effect in 2012 to reduce the global risk in transporting, storing and operating batteries.
The most basic safety device in a battery is a fuse that opens on high current. Some fuses open permanently and render the battery useless; others are more forgiving and reset. The positive thermal coefficient (PTC) is such a re-settable device that creates high resistance on excess current and reverts back to the low ON position when the condition normalizes.
Further layers of safeguards are solid-state switches that measure the current and voltage and disconnect the circuit if the values are too high. The protection circuits of Li-ion work on this on/off basis. All switching devices have a residual resistance that causes a slight increase in overall battery resistance and a subsequent voltage drop.
Safety is vitally important when using electronic devices in hazardous areas. Intrinsic safety (IS) ensures harmless operation in areas where an electric spark could ignite flammable gas or dust. Hazardous areas include oil refineries, chemical plants, grain elevators and textile mills.
All electronic devices entering a hazardous area must be intrinsically safe. This includes two-way radios, mobile phones, laptops, cameras, flashlights, gas detectors, test devices and medical instruments, even when powered with primary AA and AAA cells. Intrinsically safe devices and batteries contain protection circuits that prevent excessive currents that could lead to high heat, sparks and explosion. The hazard levels are subdivided into these four disciplines.