Cell phones, like other types of portable electronic devices, typically use lithium-ion batteries. These batteries are very popular for several reasons. First, they are very lightweight and compact compared to other types of rechargeable batteries, which makes them ideal for small, portable devices. They also have excellent recharge rates, and can usually reclaim all of their energy at every recharge, although their efficiency does slowly decrease over several years. They also suffer much less from memory effects that can sap the power from other types of batteries.
The core of the battery is a lithium-ion cell, which is constructed like a traditional battery but from different materials. They can be cylindrical or square-shaped and they are covered in a protective metal casing so that the chemical components cannot mix or leak out. Inside the cell is a positive electrode, a negative electrode, and a separator to divide them. The electrodes are submerged in an electrolyte that is injected into the cell to carry the charge from one electrode to the other. The positive electrode is made from lithium cobalt oxide, from which the battery gets its name. The negative electrode is made of pure carbon and the electrolyte is a gel polymer. All three components are tightly sealed in the battery cell. The battery case often has a hole that can release the build-up of heat in case the battery gets too hot.
Cell-phone batteries are typically made of one square lithium-ion cell. This cell is connected to a computer unit that monitors the flow of electricity. Temperature sensors are sometimes connected to the cell so that computer can tell if the battery is overheating. A voltage converter or regulator circuit is connected to the power outlet to monitor the flow of electricity and adjust the voltage to match the cell phone's requirement. These devices are very important, since without the proper modulation the electrical current would overload the circuits in the cell phone and ruin them. The battery may also be equipped with various types of monitors that help it recharge more effectively.
Because of the delicate voltage requirements of lithium-ion batteries, restrictions are placed on which batteries can be used in which cell phones. This is why it is best to buy a battery from the correct manufacturer, since a slightly different battery can produce a different current and endanger the phone's circuits.