How Cell Phone Batteries Work

All batteries, regardless of type, have an anode, or positive terminal, and a cathode, or negative terminal. Inside the battery, a paste called an electrolyte causes a chemical reaction when you connect the terminals to a circuit. In a lithium-ion cell phone battery, the anode is commonly made of graphite, a pure form of carbon. A compound called lithium cobalt oxide forms the cathode, and the electrolyte consists of salts such as lithium perchlorate. When you use your cell phone, the chemicals in the battery react to produce an electric current. Unlike household alkaline batteries, which have a cell voltage of 1.5 volts, lithium-ion batteries have a cell voltage of 3.7 volts. Batteries are connected in series to achieve multiples of 3.7 volts for devices that need more voltage.

Tiny electronic circuits inside the battery monitor its voltage and perform other important tasks. Your cell phone receives information from the monitor and displays a battery status icon on its screen. The monitor prevents the battery from being overcharged and disconnects the battery if its voltage drops too low; if the voltage falls below about 2.5 volts, you can't recharge it.

The charge indicator on your cell phone's screen shows you when to recharge the battery. The charger is a device which plugs into a standard 115-volt AC electrical outlet. Circuits inside the charger convert alternating current to direct current and reduce the voltage to a level appropriate for the phone. When you plug the phone into the charger, its electricity reverses the chemical reactions in the battery, restoring it to a fully-charged state.

Under normal use, you can expect between two to three years of service or 300 to 500 charge-discharge cycles from a cell phone battery, whichever comes first. After this point, the battery still works but cannot power your phone as long as a new one. Some cell phones have user-replaceable battery packs; others, such as the iPhone 4 and 5 models, need a qualified technician to change the battery. To prolong the battery life, recharge it before it becomes severely depleted. When you store a cell phone battery for long periods, first charge it to 50 percent and check it every few weeks. Because batteries self-discharge a few percent a month, they will eventually become useless if not checked at least every six months. Store batteries in a cool place, as elevated temperatures cause them to self-discharge more quickly.