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Laptop batteries look plain and solid from the outside. They come in all shapes and sizes, but for the most part they all look the same inside. Laptop batteries usually consist of 6 to 8 cylindrical lithium-ion 18650 cells with a PC board which communicates with the laptop and offers over charging (voltage) protection.
The outside of a laptop battery is just the casing. It holds all the insides together and protects your hands when you are holding it.
You'll notice most Laptop batteries come with a lot of print on one of the sides. It tells you how that particular battery should and should not be handled. They all say something along the lines of:
Li-ion stands for lithium-ion. It is the most commonly used chemistry for Laptop batteries because of its high charge density and compact size. However, these perks come with great safety concerns. If lithium comes in contact with air, it will burn violently.
If you were to crack open a Laptop battery pack (something we DO NOT recommend because of the possibility of shorting out a battery and starting a fire) you would find the following.
This is a picture of a typical Li-Ion laptop battery with its casing removed. A common misunderstanding is thinking these batteries are made from one solid cell: False. Laptop batteries are made from multiple battery cells that are arranged in specific ways to attain the desired voltage and amperage.
Lithium-ion Cells (Source: HowStuffWorks.com)
As with most batteries you have an outer case made of metal. The use of metal is particularly important here because the battery is pressurized. This metal case has some kind of pressure-sensitive vent hole. If the battery ever gets so hot that it risks exploding from over-pressure, this vent will release the extra pressure. The battery will probably be useless afterwards, so this is something to avoid. The vent is strictly there as a safety measure. So is the Positive Temperature Coefficient (PTC) switch, a device that is supposed to keep the battery from overheating.
This metal case holds a long spiral comprising three thin sheets pressed together:
Inside the case these sheets are submerged in an organic solvent that acts as the electrolyte. Ether is one common solvent. The separator is a very thin sheet of micro perforated plastic. As the name implies, it separates the positive and negative electrodes while allowing ions to pass through. The positive electrode is made of Lithium cobalt oxide, or LiCoO2. The negative electrode is made of carbon. When the battery charges, ions of lithium move through the electrolyte from the positive electrode to the negative electrode and attach to the carbon. During discharge, the lithium ions move back to the LiCoO2 from the carbon.
These cells are controlled by a circuit board. It is usually located against one of the interior walls of the casing. The circuit board regulates how the cells work for both the process of discharging and recharging energy. It makes sure that your battery is not overcharged when charging, and it makes sure that each cell drains the same amount so you can have a longer run time.