Batteries in a Portable World 2^nd Ed.
       A Handbook on Rechargeable Batteries for Non-Engineers

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3.6 Protection Circuits

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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