Laptop Battery use and storage guidelines.

One of my laptop's batteries had died a little while ago and couldn't figure out why, so I've been reading around the subject to try to find out what can be done about it and what I can do to prevent it in future.

First of all, I've learnt that it is possible (for the more technologically ept of us) to replace the Lithium Ion batteries inside of laptop batteries. However I wouldn't recommend this unless you are an expert at wielding a soldering iron.

To avoid this and an expensive new battery I decided to simply buy one cheaply second hand from Ebay, this was not necessarily the best approach however as laptop batteries (which I will refer to as Li-ion batteries from now on) have a definite shelf life which affects the storage capacity of the battery (and therefore how long it keeps your laptop powered). This shelf-life can be be prolonged by various means.

The best guidelines I found was at the excellent Battery University , mostly in an article on Li-ion battery life which I found extremely informative and I am now citing here. 

Guidelines for prolonging Li-ion battery life

• Unlike Ni-Cd batteries, lithium-ion batteries should be charged early and often. However, if they are not used for a longer time, they should be brought to a charge level of around 40%. Lithium-ion batteries should never be "deep-cycled" like Ni-Cd batteries.
• Li-ion batteries should be kept cool. Ideally they are stored in a refrigerator. Aging will take its toll much faster at high temperatures. The high temperatures found in cars cause lithium-ion batteries to degrade rapidly.
• Lithium-ion batteries should never be depleted to empty (0%) and then stored.
• Li-ion batteries should be bought only when needed, because the aging process begins as soon as the battery is manufactured.
• When using a notebook computer running from fixed line power over extended periods, the battery can be removed and stored in a cool place so that it is not affected by the heat produced by the computer.

Storage temperature and charge

Storing a Li-ion battery at the correct temperature and charge makes all the difference in maintaining its storage capacity. The following table shows the amount of permanent capacity loss that will occur after storage at a given charge level and temperature.

It is significantly beneficial to avoid storing a lithium-ion battery at full charge. A Li-ion battery stored at 40% charge will last many times longer than one stored at 100% charge, particularly at higher temperatures.

If a Li-ion battery is stored with too low a charge, there is a risk of allowing the charge to drop below the battery's low-voltage threshold, resulting in an unrecoverably dead battery. Once the charge has dropped to this level, recharging it can be dangerous. An internal safety circuit will therefore open to prevent charging, and the battery will be for all practical purposes dead.

In circumstances where a second Li-ion battery is available for a given device, it is recommended that the unused battery be discharged to 40% and placed in the refrigerator to prolong its shelf life. Batteries should be allowed to completely warm to room temperature over up to 24 hours before any discharge or charge.