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Most battery packs include some type of protection
to safeguard battery and equipment, should a malfunction occur.
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.
Commercial Li-ion packs contain one
of the most exact protection circuits in the battery industry.
These circuits assure safety under all circumstances when
in the hands of the public. Typically, a Field Effect Transistor
(FET) opens if the charge voltage of any cell reaches 4.30V
and a fuse activates if the cell temperature approaches 90°C (194°F).
In addition, a disconnect switch in each cell permanently
interrupts the charge current if a safe pressure threshold
of 1034 kPa (150 psi) is exceeded. To prevent the
battery from over-discharging, the control circuit cuts off
the current path at low voltage, which is typically 2.50V/cell.
The Li-ion is typically discharged
to 3V/cell. The lowest ‘low-voltage’ power cut-off is 2.5V/cell.
During prolonged storage, however, a discharge below that
cut-off level is possible. Manufacturers recommend a ‘trickle’
charge to raise such a battery gradually back up into the
acceptable voltage window.
Not all chargers are designed to apply a
charge once a Li-ion battery has dipped below 2.5V/cell.
A ‘wake-up’ boost will be needed to first engage the electronic
circuit, after which a gentle charge is applied to re-energize
the battery. Caution must be applied not to boost lithium-based
batteries back to life, which have dwelled at a very low voltage
for a prolonged time.
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