Eugene's observations

The battery type of most laptops is lithium-ion. Here are some facts:

• Certain kinds of mistreatment may cause Li-ion batteries to deteriorate rapidly or in a worst case scenario, explode, so don't let excessive heat, chemicals or physical motion interfere with it.
• They do not suffer from memory effect - it is not necessary to completely discharge them before recharge.
• Battery capacity deteriorates with time - irrespective of use. This is about 20% a year at 20 decrees, and nearly 40% at 40 degrees, the effect is greatly exacerbated with increased heat - such as is typical when a laptop is in use. So if you are running from mains electricity, disconnecting the battery will prolong its life. Failing that, consider using a cooling pad.
• After about 300 - 500 charge cycles the storage capacity will have dwindled to a near unusable amount.
• 6 cell batteries give about 3 hours continuous use when new, 9 cell models extend this to about 4 hours assuming normal use.

Assuming a well manufactured battery (not a cheap second hand one) internal circuitry protects from damage caused by draining whilst at low charge or overcharging.

To reduce the heat of the running laptop and thereby prolong battery life and enable a faster CPU, try using a cooling pad that plugs into a USB port such as one from Antec at about £20.

Laptop batteries have a small chip inside that controls/monitors the charging process and also monitors the number of charge/recharge cycles.

This chip is factory programmed with information how this sort of battery typically degrades over time.

It also can derive information from the charging cycle itself: The time it takes to reach full-charge at a given voltage/current changes when the battery gets worn out.

(Voltage drop during discharge is not reliable as it depends a lot on the amount of current drawn while discharging, so it varies with the use-pattern of the laptop.)

Windows communicates with this chip to get information about the battery health.