Cells
A battery cell is the actual part that contains the chemicals that store electricity. Most laptop batteries use cylindrical cells that look like regular AA batteries only about twice the size. Some batteries use square or flat cells, however the majority use cylindrical shaped cells. The battery's overall capacity is determined by the ratings of each cell and the number of cells that the battery contains. The number of cells will change the overall size of the battery along with it's capacity. For example a battery that holds 12 cells will be about twice the size of a battery that holds only 6 cells.
Voltage
A battery cell's voltage is determined by the chemistry. Each different chemistry produces a different voltage. Higher voltages are more desirable because it will allow a cell to produce a higher total power. For example: typical Alkaline cells are rated at 1.5V, NiMh are rated at 1.2V, and Li-Ion cells are rated at 3.6V. The higher voltage of Li-Ion indicates that it is capable of producing three times more power than an identical NiMh cell.
The voltage of battery cells changes between full charge and full discharge. Cell manufacturers assign a value that is in the middle of the operating range. As a general rule all cells of the same chemistry will have the same rating. Some Li-Ion cells are rated at 3.7V instead of the standard 3.6V. This slight difference can be attributed to slight differences in manufacturing processes, materials, and in some cases supplier rating methodologies.
The total voltage for a laptop battery pack is determined by the arrangement of the cells. Connecting cells in series (end to end, + to -) will add the voltage of each cell. Typical configurations have series groups of 3 or 4 cells to produce typical ratings of 10.8, 11.1V, 14.4V, 14.8V.
10.8V = 3.6V cells arranged in groups of three
11.1V = 3.7V cells arranged in groups of three
14.4V = 3.6V cells arranged in groups of four
14.8 = 3.7V cells arranged in groups of four
11.1V = 3.7V cells arranged in groups of three
14.4V = 3.6V cells arranged in groups of four
14.8 = 3.7V cells arranged in groups of four
Laptops use a smart battery system and an internal DC-DC power module. This system will allow the computer to automatically adapt to batteries with different rated voltages. It will also separate the AC adapter input from the battery so the Adapter's voltage does not directly relate to the battery's voltage.
Capacity (mAh milliamp-hours / Ah amp-hours)The total amount of energy that you can get out of a battery will change depending upon how fast it is drained. This is like a tank of water that would produce 3 gallons, 5 gallons, or 7 gallons depending upon how fast it was drained. These multiple measurements make it impossible to say exactly how many gallons that the tank can hold.
Instead of a volume measurement like gallons, batteries are rated based upon how long that they will run at a fixed drain rate. Using the water analogy it would be like saying that the tank will drain in 1 hour if you poke a 1 inch hole in the bottom. The electrical unit ampere or amp describes how fast electricity is flowing (size of the hole). Batteries combine this unit with time to produce amp-hour or milliamp-hour (1/1000th amp-hour) ratings. These ratings will describe how long it will take to drain at a given discharge rate. For example a 2 amp-hour cell will ideally produce 2 amps for 1 hour or 1 amp for 2 hours.
A laptop battery's overall capacity rating is determined by the ratings of the individual cells as well as the arrangement of the cells. Li-Ion cells are typically rated at between 2000-2900mAh with the most common rating being 2200mAh. The mAh ratings add for cells that are connected in parallel ( all + connected to each other and all - connected to each other). Laptop batteries typically have between 1-4 of these parallel groups to produce common ratings of 2200mAh, 4400mAh, 6600mAh, and 8800mAh.
Watt-Hours
This rating is only used by some manufacturers, however it is the most accurate way to describe the capabilities of a battery. This rating combines both the volt and amp-hour ratings to a single number that represents the actual amount of work that the battery is capable of performing.
Battery ratings include time in hours so the time is added to produce watt-hours. A battery's watt-hour rating is calculated by multiplying Volts x Amp-Hours. For example a battery rated at 14.8V * 2200mAh = 65120 milliwatt-hours = 65.120 watt-hours.
Many laptops support several different batteries that have different voltage and mAh ratings. Using watt-hours is the only way to compare batteries with different voltage ratings.
Battery A: 14.8 * 4000mAh = 59.2Wh
Battery B: 10.8 * 4400mAh = 47.52Wh
Battery B: 10.8 * 4400mAh = 47.52Wh
People that look at only the mAh ratings may incorrectly assume that Battery B has a higher capacity because the mAh number is higher. In this example the battery with the lower mAh rating actually has about 25% more capacity.
