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 devices open permanently and render the battery useless; others are more forgiving and reset. The Polyswitch™ is such a re-settable device. It creates a 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 the current and disconnects on excessive load conditions. All switching devices have a residual resistance during normal operation, which causes a slight increase in overall battery resistance and a subsequent voltage drop.
Intrinsically Safe Batteries
Intrinsically safe (IS) batteries contain protection circuits that prevent the formation of high currents, which could lead to excess heat, sparks and explosion. Authorities mandate IS batteries for two-way radios, gas detectors and other electronic instruments operating in hazardous areas such as oil refineries, chemical plants and grain elevators. There are several levels of intrinsic safety, each serving a specific hazard level, and the requirements vary from country to country. The provisions are in addition to the protection circuit for lithium-ion, and the approval standards are rigorous. This results in a high price for the battery.
Making Lithium-ion Safe
Battery packs for laptops and other portable devices contain many levels of protection to assure safety under (almost) all circumstances when in the hands of the public. The safety begins with the battery cell, which includes:  a built-in temperature switch called PTC that protects against high current surges,  a circuit interrupt device (CID) that opens the electrical path if an over-charge raises the internal cell pressure to 1000 kPa (145psi), and a safety vent that releases gas in the event of a rapid increase in cell pressure.
In addition to these internal safeguards, an external electronic protection circuit prevents the charge voltage of any cell from exceeding 4.30V. Furthermore, a fuse cuts the current if the skin temperature of any cell approaches 90°C (194°F). To prevent the battery from over-discharging, a control circuit cuts off the current path at about 2.20V/cell.
Each cell in a string needs independent voltage monitoring. The higher the cell count, the more complex the protection circuit becomes. Four cells in series had been the practical limit for consumer applications. Today, new chips accommodate 5–7, 7–10 and 13 cells in series. For specialty applications, such as the hybrid or electric vehicle delivering several hundred volts, specialty protection circuits are made, which sharply increases the overall cost of the battery. Monitoring two or more cells in parallel to get higher current is less critical than controlling voltages in a string configuration.