A battery is one or more voltaic cells that store chemical energy that can be converted to electrical energy or electricity. The two basic classifications of battery are the primary battery and the secondary battery. Primary batteries cannot easily be recharged so once their energy is expended the battery is exhausted and cannot be used. Secondary batteries can be recharged and have their chemical reactions reversed to restore the battery to a charged state. Laptop batteries, as well as all other rechargeable batteries, are secondary batteries.
Laptop batteries are rated by voltage (V) and milliamp-hours (mAh). Voltage is the measure of potential electrical difference across the battery's terminals and is the rate at which energy is drawn from the battery. A properly designed battery will display nearly the same voltage during the battery's discharge. Voltages vary depending on the chemistry of the battery. Alkaline and carbon-zinc cells typically have a voltage rating of 1.5 volts; NiCD and NiMH batteries generally exhibit a rating of 1.2 volts; and lithium celled batteries are generally rated at 3 or more volts. Laptop battery voltages are higher and are increased over the individual cell ratings by connecting them in series.
Milliamp-hours corresponds to the storage capacity of the battery and is directly related to the size of the cells. Simply put, a larger cell of the same battery chemistry and design will have a larger electrical capacity. This mAh rating corresponds to the amount of time that it takes a battery to discharge.
Batteries with different Milliamp-hours can be used on the same laptop or notebook computer as long as the voltage specification is met. In fact, OEM laptop computer vendors offer upgraded batteries for their notebooks to extend the operating time of the computer. However, laptop manufacturers charge premium prices for direct replacement laptop batteries or higher capacity laptop batteries. So when shopping for a replacement battery, look for one that meets the voltage specs of your laptop or notebook computer's OEM battery but look to get one with a larger capacity.