Technically speaking, a battery is an electrochemical device that converts stored chemical energy into electrical energy. When connected to an external electric load it can produce current. Batteries are used in a wide variety of applications, from small hand-held devices to large UPS systems. Under normal conditions when AC power is present, the AC-DC adapter (laptop power supply) maintains the laptop battery charge and provides power to the circuits. The charged battery powers the circuits when DC voltage from the adapter is not available.

There are three main battery types: NiCd, NiMH and Li-Ion.

Nickel-Cadmium (NiCd)

batteries were the original ones. They could supply power for about two hours between charges, suffered from what is casually called memory effect and could explode when overcharged.

Nickel-Metal Hydride (NiMH)

batteries represent the second generation of the energy storage. They generally have 10-25% greater capacity than NiCds, are environmentally friendlier, but still suffer from the "memory effect", although to a lesser degree.

In reality, the batteries have no "memory". In both types of Nickel-based devices crystals build up occurs over time that prevents you from charging them completely and would require full discharge to restore the capacity. A typical charger provides fast charge to 80% of the pack capacity, then switches to trickle charging for another two hours to fully charge.

Lithium Ion (Li-Ion)

batteries currently represent the latest energy storage technology. Li-Ion's are used in a broad range of devices from hybrid vehicles and aircrafts to whole house power backup systems. They can deliver typically 40% more capacity than comparable NiCd packs, are lighter and thinner, have low self-discharge, better cycle life, and do not suffer from memory effect. However, their charging is more complex. Unlike other types, Li-Ions should not be trickle charged or float. Prolong overcharging can damage them, although normally they won't explode if their rated voltage is not exceeded. Li Ions can still catch fire when overheated and can release flammable electrolytes. Newer Li-ion phosphate cells are said to be inherently safer.
Today's standard batteries for notebooks may have up to 4-6 hours run time with the life from 400 to 1200 charge cycles. Note that battery's capacity is a variable quantity- it depends on the how fast you discharge it (see this analysis of A*h vs. discharge rate). LiIons are also more convenient since they do not need full charges and discharges for peak performance. There is a modification of a Li Ion cell called Li-Ion polymer. The difference between these two types is the latter one has a solid ion conductive polymer instead of a liquid electrolyte.

A Li Ion pack consists of a number of the series/parallel connected cells (typically from 4 to 16) to provide the required operating voltage and current levels. Nominal voltage of each cell is typically 3.6-3.7 V, the charging voltage is about 4.2-4.3 V per cell. The Li-ion phosphate cells have 2.9V nominal and 3.6V charging voltage. Note that because of the cell's relatively high voltage, many low-power portable products can operate from a single cell. Depending on the cell connection scheme, the nominal Lithium Ion battery voltage for different laptop computer brands can vary from 7.2 V to 14.8 V. Various Apple MacBooks may have 7.4V (two-cell), 11V (three-cell), or 14.5V (four-cell) battery packs. iPads are using 3.7 V Li-Ion polymer. Note that in general, the rated output voltage of a power supply is not necessarily the same as nominal laptop battery voltage- an adapter rating may be higher.
iPads and most tablets can be charged from 5V bus coming out of compatible USB ports. This allows them to do both charging and communication with the same connector. The voltages on USB data lines D+ and D- instructs the device on how much current it can draw from a port. Different brands require different voltage levels on these lines for fast charge.

Over time any battery looses its capacity whether or not it is being used. For longest LiIon battery life, it should be stored at the nominal voltage at about 40F. The fastest permanent capacity loss occurs at elevated temperatures with the pack maintained around 4.2 V per cell (which is a fully charged state). In most cases the packs are actually used in the worst conditions for two reasons: they are inside a laptop that drives them hot and they are usually connected to the charger that is keeping them fully charged all the time. The more time the cell remains at high float voltage, the shorter its lifetime. On the other hand, frequent full discharges should also be avoided when possible. To increase the battery life try to use partial-discharge and partial-charge cycles instead of keeping it always fully charged or going through full discharge and full charge cycles.

Any system may eventually require a replacement battery. Consider replacing it when you notice substantial decrease in its run time. If you can't get the exact part number, a substitute with a slight VDC difference may be OK since the pack's volts normally vary by 15% depending on its charge level and the load. However, it is always advised to check with your brand's manufacturer. To boost the running time by 4 to 10 hours you may use a universal external laptop battery from a third-party manufacturer. You just need to verify its compatibility with your brand.

The resources below provide additional information.



How to prolong lithium-based product life


Toshiba laptop battery replacement- Illustrated how to repair the pack

Fixing laptop power supply