How Things Work: Lithium-ion batteries

Lithium-ion batteries come in all shapes and sizes and power our electronics. Shown here is a laptop battery (the larger battery) and two smaller cell phone batteries. (credit: Celia Ludwinski/Photo Editor) Lithium-ion batteries come in all shapes and sizes and power our electronics. Shown here is a laptop battery (the larger battery) and two smaller cell phone batteries. (credit: Celia Ludwinski/Photo Editor)

A ubiquitous and essential appliance in the lives of college students, the laptop has become a modern, mobile workstation and also a source of endless diversions. But what if it did not exist? If something unfortunate were to happen to the precious battery that keeps a laptop alive, it would become but an inert hunk of unresponsive metal. Thus, it is important to know how the lithium-ion battery that powers a laptop works and become knowledgeable about how to prolong its life.

The battery inside a laptop is lithium-ion, which means that its electrodes (the conductors in a circuit that connect non-metal parts) are made of lithium and carbon. The positive cathode is made of lithium cobalt oxide, which emits positive lithium ions; the negative anode is made of carbon. The electrodes are placed in a gel electrolyte, a material that conducts electricity, and separated by a sheet of perforated plastic. Ions are free to move throughout the gel, but there is an initial charge difference between the electrodes, with the positive ions concentrated on the cathode.

When recharging a laptop battery, the lithium ions move from the cathode to the anode and create an electric potential difference (a voltage of about 3.7 volts), according to When the battery is discharging, the lithium ions move back from the anode to the cathode. Total discharge occurs when the return of ions to the cathode decreases the voltage to zero. A lithium-ion battery that is completely discharged will be unusable, though, so these batteries have a failsafe mechanism that shuts them down before they discharge too much. This system, which itself is a small computer attached to the battery pack, monitors the charging levels and temperature of the battery to prevent dangerous usage. This monitor uses power, which is one of the reasons that a battery not in use will still lose charge and experience a decrease in voltage over time. This is how batteries can become ruined even without use.

To prolong battery life, do not allow your laptop battery to drain all the way to 10 percent or lower without subsequently charging it. Contrary to popular belief, there is no need to completely discharge, or “deep discharge,” a laptop battery or any other kind of battery, including those in cell phones and iPods, each time it is used. In addition, lithium-ion batteries can handle hundreds of repeated cycles of charging and discharging. The only possible issue, therefore, is not the process of charging, but the maintenance of a minimum voltage. When a battery goes to zero voltage, it will no longer be able to recharge, and the only solution is to buy a new one.

From the moment lithium-ion batteries are manufactured, they constantly degrade and lose charge, so a laptop battery will only last two to three years, regardless of usage. Make sure to check the manufacturing date when you buy a new battery. An unused battery that has been sitting around is just as degraded as one that has been in use for the same amount of time. Heat will also degrade lithium-ion batteries faster than normal. According to Popular Science magazine, critical temperatures for a laptop battery occur around 70 degrees Fahrenheit. Storing a lithium-ion battery at temperatures at or above this level will degrade it more quickly.

Lithium is a very reactive element, especially compared to lead and nickel, which are what car batteries (lead-acid batteries) and electronic toy batteries (nickel-cadmium) typically contain. Hence, lithium-ion batteries can store a lot more energy than typical batteries — about 150 watt-hours per kilogram of battery, according to However, since lithium-ion batteries have so much energy, they also warm up quickly from the heat radiated during discharge. When the battery becomes too hot, it releases the polymer gel, which can be ignited by heat or a spark. This ignition is what causes some laptop batteries to catch fire.

Besides laptops, lithium-ion batteries power many modern handheld electronics, including digital cameras and cell phones.