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Microsoft KB Archive/105241

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Knowledge Base

Windows NT May Shut Down Immediately if Power Fails with UPS

Article ID: 105241

Article Last Modified on 2/20/2007


  • Microsoft Windows 2000 Server
  • Microsoft Windows 2000 Advanced Server
  • Microsoft Windows 2000 Professional Edition
  • Microsoft Windows NT Advanced Server 3.1
  • Microsoft Windows NT Workstation 3.1
  • Microsoft Windows NT Advanced Server 3.1
  • Microsoft Windows NT Workstation 3.5
  • Microsoft Windows NT Workstation 3.51
  • Microsoft Windows NT Workstation 4.0 Developer Edition
  • Microsoft Windows NT Server 3.5
  • Microsoft Windows NT Server 3.51
  • Microsoft Windows NT Server 4.0 Standard Edition

This article was previously published under Q105241


If you use an uninterruptible power supply (UPS) that does not have the ability to send a low battery signal to the computer, Windows NT may shut down immediately if a power failure occurs.


If a UPS is used that is not capable of sending a signal to the computer to inform it of a low battery condition, Windows NT instead relies on information provided by the user to determine the length of time that the system can be run on battery power. When the battery power reaches two minutes of life remaining, Windows NT begins a shutdown of the system.

When you boot Windows NT, it starts under the assumption that the battery is completely drained. This is a safety feature which assumes that a computer has just been turned on and that the UPS battery is not charged yet.

Windows NT then computes the amount of life it expects the battery to have based on the "Expected battery life" and "Battery recharge time per minute of run time" settings in the UPS Service setup screen. If, for example, the battery recharge time is set to 100 minutes, then the system must run 100 minutes for Windows NT to believe that the battery has a one minute charge. It must run 200 minutes for Windows NT to calculate a two minute charge.

Because Windows NT will begin a shutdown immediately if the power fails when the battery life is calculated to be 2 minutes or less, Windows NT may shut down immediately if the power fails and the calculated battery life is two minutes or less.

Thus, Windows NT may shut down sooner than expected after a power failure if the computer was rebooted recently, even though the UPS may be fully charged.

This behavior is expected because Windows NT assumes worst-case conditions for unattended UPS monitoring.

Additional query words: prodnt

Keywords: kbhardware KB105241