Learn how to make batteries safe with built-in protection circuits.
Batteries can release high power, and most packs include protection to safeguard against malfunction. 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. A third method is a solid-state switch that measures current and voltage and disconnects the circuit if either value is too high. The protection circuits of Li-ion work on this basis. (See BU-304b: Making Lithium-ion Safe.)
All switching devices have a residual resistance, which causes a slight increase in overall battery resistance and a subsequent voltage drop.
Intrinsically safe (IS) batteries contain protection circuits that prevent excessive currents that could lead to excess heat, sparks and explosion. Agencies mandate intrinsically safe batteries for two-way radios, gas detectors and other electronic instruments that operate in hazardous areas such as oil refineries, chemical plants and grain elevators.
Intrinsic safety requirements are divided into a specific hazard level and the requirements vary from country to country. North America has the Factory Mutual Research Corporation, Underwriters Laboratories (UL) and Canadian Standards Association (CSA). Europe has the ATEX directive; while in other countries follow the IECEx standards. To facilitate world trade, agencies engage in harmonization to eventually agree on a mutually approved standard.
Last Updated 2015-09-16
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