Over the last couple of weeks many users complained about a supposed battery status report bug in Windows 7, but after investigating the issue Microsoft claims Windows 7 is not at fault and that it's really the battery that is in need of replacement. The reason why so many people get this message after upgrading to Windows 7 is because this operating system has a new battery health status feature that isn't part of Windows XP or Vista.
Windows 7 makes use of built-in features of batteries that allow the battery health status to be read. "This is reported in absolute terms as Watt-hours (W-hr) power capacity," Sinofsky explains, "Windows 7 then does a simple calculation to determine a percentage of degradation from the original design capacity. In Windows 7 we set a threshold of 60% degradation and in reading this Windows 7 reports the status to you."
The key point here is this: this functionality was not part of Windows XP or Windows Vista. As such, people who think Windows 7 is causing the batteries to fail have it backwards; the battery was already in need of replacement, but XP nor Vista had the ability to report that to you.