Voltage and milliamp-hour are the two values used to rate notebook batteries. Volts (V) comprises of the period at which energy is drawn from the battery and Milliamp-hour (maH) constitutes the storage space of the battery. The maH rating represents the run time of the battery between charges. A battery with a high maH rating will have a greater run-time than that of a battery with a lesser maH rating.

Generally, batteries having different mAH can be used with the same notebook presuming the Voltage rating is the same, however when the chemistry make up of the battery is different (e.g switching from NiMh to LiIon) the voltage will tend to fluctuate. Otherwise the Voltage rating will need to be almost an exact match of the existing battery or as otherwise designated by the documentation. Using a battery with a different voltage configuration can seriously hurt the notebook. The maH value for the bulk of compatible/replacement batteries come higher than the original manufacturers batteries. This generally doesn't damage your notebook, in fact, it simply means that these batteries in many cases have a higher run time and can last longer than original manufacturers' batteries.

Unfortunately the maH rating is not an exact measurement of the time between charges. By this I mean just because a laptop has a maH rating of 4.5 does not mean the laptop will last 4.5 hours between charges. maH is just a standardized value. The run-time of a laptop computer battery will change depending on a number of variables. It will depend on manufacturer of the laptop computer, the types of applications being used (autocad, word, ms access) the number of times a program is archived or saved, how frequently hardware such as usb ports, internal hard drives, and cds are accessed, the internal processor of the laptop, the age of the laptop, the amount of RAM and as previously mentioned the chemistry of any given battery (NiCAD, NiMH, LiIon).

Per one of the leading resellers of laptop batteries , the life of a rechargeable battery is somewhere between 500-800 charge cycles, this is roughly 1-3 years. When your laptop battery that previously would run for 2 hours between charges now only runs for 1 hour, it is time for a replacement battery. One of the largest and environmentally conscious laptop battery stores online is www.batteryvalues.com. They are customer oriented and have a large line of Dell, Hewlett Packard, Apple, and Compaq laptop batteries and AC Adapters.