Power supply stops charging battery until battery is completely drained. Laptop doesn't run off power supply during this time. Power supply is working, I've checked it with volt meter.  Rebooting doesn't help.

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Information:
With the rise in laptop sales it is essential for laptop buyers to make sure they use their laptop batteries to the fullest and also make sure they can achieve the highest performance with the available battery. Generally most laptops claim to have 4-6 hours of battery life but in real it comes to 50% of the promised backup time because of many factors like heavy laptop usage while on battery backup, running USB Based gadgets, CD/DVD ROMs etc. You also need to make sure about some battery life saving tips because the laptop batteries generally have a life of around 1000 recharge cycles. Here is a list of some essential tips which can almost double your laptop battery life.
Screen brightness:
Almost all laptops come with the ability to dim the screen brightness. Some even come with ways to modify CPU and cooling performance. The display system takes 50% of your available battery and lower brightness would ensure you more battery life.
Power Cord:
Recharge your laptop using the power cord/adapter provided by the laptop dealer and don’t use any other local cord because they can decrease the life of your battery.
Remove your batteries When not in Use:
If you don’t use your laptop for a continuous period its recommended to remove your batteries completely from the laptop and add it back when you plan to use it. This is because laptop batteries have a limited lifetime and its generally 1000 charge cycles after which they stop offering complete backups. Even if it is not in use, the battery still drains, so you should completely remove it and store in a cool place and at a 40% charge rather than 100% charge.
Background Processes:
When you are running your laptop on batteries, avoid heavy multitasking, because processor and ram will then need more power thus making your battery drain further.
Wi-Fi:
Wi-Fi should be turned off when not in use because it drains your laptop battery in search of signals and wireless router. You can use the hardware switch available on the laptop to disable your wifi or disable it from the Network Connections option available under your Control Panel.
Disable Aero Glass:
Disable Aero Glass Feature which comes preinstalled in Windows Vista. You need to right click on your Desktop > Preferances > View Colour > Appearance > Classic Appearance and Windows Basic graphical interface > Ok , this way you can get the classic appearance which has less resource using features and helps you use less battery power.
Heat Control:
Your laptop operates more efficiently when it’s cooler. Clean out your air vents with a cloth or keyboard cleaner.
Optimize Power Options:
Go to ‘Power Options’ in your windows Control Panel and set it up so that power usage is optimized (Select the ‘max battery’ for maximum effect).
Avoid Multitasking:
Do one thing at a time when you’re on battery. Rather than working on a spreadsheet, letting your email client run in the background and listening to your latest set of MP3’s, set your mind to one thing only. It is important to note that it is fine running an idle program at the background but if the program is constantly doing some work like downloading or extracting a Zip file, this is when the battery drains out.
External Drives:
USB devices (including your mouse) drain down your laptop battery. Remove or shut them down when not in use. It goes without saying that charging other devices (like your iPod) with your laptop when on battery, is a like putting a wood on fire.
Battery Recharge:
Do not leave a charged battery dormant for long periods of time. Once charged, you should at least use the battery once every two to three weeks(2 weeks is recommended). Also, do not let a Li-On battery completely discharge.
Hibernate Not Standby:
Although placing a laptop in standby mode saves some power and you can instantly resume where you left off, it doesn’t save anywhere as much power as the hibernate function does. Hibernating a PC will actually save your PC’s state as it is, and completely shut itself down. Settings up Automatic Hibernate feature to make sure the laptop automatically saves all your running documents and applications, and hibernates when you are not using the laptop for a given amount of time is very important.
Clean Battery:
Clean your battery’s metal contacts every couple of months with a cloth moistened with rubbing alcohol. This keeps the transfer of power from your battery more efficient.