Windows 7 has the ability to tell you when your battery is no longer holding a full charge. When your battery gets down to 40 percent of its original capacity (that is, it can only hold 40 percent or less of its designed capacity after it has been charged), you'll see a notification that says "Consider replacing your battery." Windows reports this information directly from the battery—you will only see this notification when the battery is reporting it's unable to hold more than 40% of a full charge.
This notification is new in Windows 7—earlier versions of Windows can't notify you when your battery is going bad. If you've recently upgraded to Windows 7 and are receiving this notification, it probably means that you need to replace your battery. If you don't want to receive this notification, you can turn it off by clearing the check box on the battery meter.
If you receive the "Consider replacing your battery" notification, and aren't sure if you should replace your battery, here are some things to consider:
Laptop batteries have different capacities—the amount of charge that they're designed to hold. Over time, laptop batteries will lose their ability to hold a charge, as is the case for all rechargeable batteries. This means that even though you're fully charging your battery, the actual amount of power that the battery can hold is going down. You can find information about the capacity of your laptop battery by looking at the information printed on the battery, or reading the information that came with the laptop or battery.
Even relatively new batteries can fail or become damaged—exposure to extreme heat can damage a battery—and quickly go from holding a charge to not holding a charge.
If you plan to replace your battery, you should contact your computer manufacturer, or buy a battery that is made by your computer manufacturer, since it is designed to work best with your computer.
The manufacturer of the battery is responsible for the condition and quality of the battery. Batteries do sometimes fail, so it's a good idea to only buy batteries that come with a warranty.
The battery meter in Windows might become less accurate as the battery begins to hold less capacity. Your laptop might go into hibernation or shut down sooner than indicated. For more information, see Battery meter: frequently asked questions.
For information about solutions to other battery problems, see Solutions to common battery problems. For information about preserving the life of your battery, see Taking care of your laptop's battery.