I can't tell you how many times I've settled into a coffee shop with the intention of having an internet sleuthing session, only to realize my notebook battery is near its end. With a little bit of care, you can maximize the battery life (i.e. the time your battery will run before it must be recharged) and lifespan of your notebook's battery.
For additional battery optimizing tips,
Energy: The Energy Saver control panel offers several settings that determine power levels for your Mac. If you change this setting to maximize performance, your battery will drain more quickly.
Brightness: Dim the screen to the lowest comfortable level to achieve maximum battery life.
AirPort Wireless: AirPort consumes power, even if you are not using its features to connect to a network. You can turn it off in its control panel to save power.
Bluetooth Wireless: Turn off Bluetooth to maximize your battery life, as it also consumes power when not in use.
The display and hard disk on your PC are the two biggest consumers of battery power. By customizing the existing power schemes in Windows XP you can extend your battery life. Windows XP provides a number of power schemes, which are predefined collections of settings designed for different environments and circumstances.
Portable/Laptop Power Scheme: This minimizes the use of power to conserve your battery, but adjusts to your processing needs so that the system speed is not sacrificed. Adjust accordingly.
Max Battery Power Scheme: This minimizes power use but does not adjust as your processing demands change. Use Max Battery only in situations that require minimal processing, such as reading documents and taking notes in a meeting.
Adjust Screen Brightness: You can conserve battery by reducing the screen brightness. Every computer is slightly different, but you can usually use a combination of keys, a function key, or a software tool to dim the screen.
Turn Off Wireless: This sucks power even when you're not using it.