Here's a tip how you can use the Powercfg in Windows 7 command to identify a laptop battery that might be failing. Simply run the following command in a command prompt window:
Doing this will generate a report that can help you identify any issues with regard to your computer's power management settings. Typical output will include a Battery section that might look something like this:
Battery ID 00860 2009/02/13Hewlett-PackardPrimary
Serial Number 00860 2009/02/13
Long Term 1
Design Capacity 73440
Last Full Charge 53480
Look at the last two lines of this section of the command output. If the Last Full Charge value is significantly less than the Design Capacity value then your battery isn't holding enough of a charge anymore and may be starting to fail.