what is the laptop battery?

A laptop battery contains of three main parts

1)Cells
2)Circuit Plate
3)Shell

Number 1 i.e. Cells are the most expensive of the three that is why batteries are sometime called cells. So generally speaking the price of laptop battery depends on how many cells in the battery and which kind of cells are in the battery.
Some manufacturers use re-cycled cells that are removed from used laptop batteries. So the shell of battery will look the same but inside the cells will be re-cycled causing the battery to discharge unsteadily and also cause it to have less charge discharge cycles.

Battery parameters

1)Voltage
2)Capacity mAh

Voltage of a replacement battery may not be identical to the original battery but the voltages must be within a reasonable range. In order to rate the voltage of the cell, a voltage value between 3.0 and 4.2 must be picked. The voltage that is picked is called the “nominal” voltage, which means that it is for naming purposes only, whereas the actual voltage of the cell depends on the state of charge. Historically some manufacturers picked 3.6V while others picked 3.7V to name the cell. The functionality and performance of either cell is identical and cannot be differentiated by the device.

Capacity mAh is a very important parameter of the battery and many people do not realise the importance because one single laptop will accept 3800 mAh 4400mAh and also accept 6600 mAH batteries.
So what is the difference. Simply saying the higher the capacity the longer the battery run time will be. So you may buy 1 battery for 25 pounds and another one exactly the same is 35 pounds but in reality they are not the same one is 3800 mAh and the other is 6600 mAh.

Chemistry of batteries

we can split the chemistry into 3 sections

1)NiCad (Nickel Cadmium)
2)NiMH (Nickel Metal Hydride)
3)Li-ion (Lithium-ion)

The main difference between NiCad and NiMH, is that NiMH batteries are (the newer of the two technologies) they offer higher energy densities than NiCad batteries. NiMH delivers more capacity than its NiCad counterpart. Therefore the NiMH have increased run-time with no additional bulk to weigh down your portable device. NiMH batteries also offer another major advantage, they are less prone to develop the memory effect and thus require less maintenance and care. NiMH batteries are also more environmentally friendly than their NiCad counterparts, since they do not contain heavy metals.

Li-Ion (Lithium ion) battery has quickly become the emerging standard for portable power in consumer devices. Li-Ion batteries produce the same energy as NiMH batteries but weigh approximately 35% less. This is crucial in applications such as camcorders or notebook computers, where the battery makes up a significant portion of the device’s weight. Another reason Li-Ion batteries have become so popular is that they do not suffer from the memory effect at all. They are also better for the environment because they do not contain toxic materials such as Cadmium or Mercury.