The most basic protection is a fuse that opens if excessively high current is drawn. Some
fuses open permanently and render the battery useless once the filament is broken; other
fuses are based on a Polyswitch? which resembles a resettable fuse. On excess current, the
Polyswitch?creates a high resistance, inhibiting the current flow. When the condition
normalizes, the resistance of the switch reverts to the low ON position, allowing normal
operation to resume. Solid-state switches are also used to disrupt the current. Both solid
-state switches and the Polyswitch?have a residual resistance to the ON position during
normal operation, causing a slight increase in internal battery resistance. A more complex protection circuit is found in intrinsically safe batteries. These batteries are mandated for two-way radios, gas detectors
and other electronic instruments that operate in a hazardous area such as oil refineries
and grain elevators. Intrinsically safe batteries prevent explosion, should the electronic
devices malfunction while operating in areas that contain explosive gases or high dust
concentration. The protection circuit prevents excessive current, which could lead to high
heat and electric spark. There are several levels of intrinsic safety, each serving a specific hazard level. The
requirement for intrinsic safety varies from country to country. The purchase cost of an
intrinsically safe battery is two or three times
that of a regular battery. more info:www.good-battery.com
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