Laptop owners upgrading their Windows XP and Windows Vista machines to Windows 7 are complaining that Microsoft's new OS has severely reduced their available battery life.

One user tells The Reg that after upgrading his circa 2007 HP notebook from Vista to Windows 7, the machine's battery life dropped from two hours to a half hour, and countless others are voicing similar complaints on Microsoft's TechNet forums.

The problem is caused by a new tool designed to alert users when their battery needs replacing. In some cases, according to TechNet posts, this alert appears time and again - even as a battery continues to run as it always did - falsely warning that the machine may suddenly shutdown. But in other cases, the machine does shutdown prematurely. It would seem this occurs well before the battery has run out of juice.

In a statement sent to The Reg, Microsoft has acknowledged the issue, saying it's related to the way Windows 7 reads system firmware. "We are investigating this issue in conjunction with our hardware partners," the statement reads. "The warning received in Windows 7 uses firmware information to determine if battery replacement is needed. We are working with our partners to determine the root cause and will update the [Technet] forum with information and guidance as it becomes available."

According to some users posting to Technet, the issue dates back several months, to the Windows 7 beta. But others say they didn't see a problem until the final "release to manufacturing" (RTM) build. "I was having excellent battery life until I installed the RTM of Windows 7 - all RC and beta builds that I installed I had nearly three hours of battery. Now, I have about 20 minutes and Win7 shuts down my laptop," writes one user.

"This change in behavior happened when I went from an RC to RTM. Instant behavior change. Needless to say, I - like you that have posted here - [am] not happy."

Microsoft confirms that its "consider replacing your battery" warning is new to Windows 7. But the company also says that in some cases, the new tool may be working properly. In other words, if it says your battery needs replacing, it may need replacing. But for so many laptops owners posting to TechNet, this is clearly not the case.

One poster says the problem has occurred on the same machine with two different sets of batteries. "At first, I was thinking the laptop is a couple years old, and sat on a shelf gathering dust, not being stored properly," he writes. "I went to replace the batteries, and realized that HP had recalled them. So i replaced them with brand new batts from the factory. And wala! same issue."

According to posts, the issue occurs across many multiple laptop makes and models, and Microsoft's statement seems to confirm this is the case. ®