Steven Sinofsky, president of the Windows and Windows Live Division, has posted a response on the Engineering Windows 7 blog.
Several press articles this past week have drawn attention to blog and forum postings by users claiming Windows 7 is warning them to "consider replacing your battery" in systems which appeared to be operating satisfactorily before upgrading to Windows 7. These articles described posts in the support forums indicating that Windows 7 is not just warning users of failing batteries - as we designed Windows 7 to do this - but also implying Windows 7 is falsely reporting this situation or even worse, causing these batteries to fail. To the very best of the collective ecosystem knowledge, Windows 7 is correctly warning batteries that are in fact failing and Windows 7 is neither incorrectly reporting on battery status nor in any way whatsoever causing batteries to reach this state. In every case we have been able to identify the battery being reported on was in fact in need of recommended replacement. Sinofsky goes on to explain that PC batteries inherently degrade in their ability to hold a charge and provide power, and ultimately batteries must be replaced to restore an acceptable battery life (batteries usually have a warranty of 12 months). Windows 7 taps into a feature of modern laptop batteries which have circuitry and firmware that can report the overall health of the battery in Watt-hours power capacity. Windows 7 then calculates the percentage of degradation from the original design capacity; the threshold is set at 60 percent degradation, so if the battery is performing at 40 percent of its designed capacity then users will see Windows 7 report that it might be time to change the battery.
He also said that Windows 7's new "Consider replacing your battery" message does not exist in Windows XP and Windows Vista, so many users would probably not have been aware of their batteries degrading.