UPS backup question.

Discussion in 'macOS' started by Alan S, Apr 15, 2009.

  1. Alan S macrumors member

    Mar 21, 2009
    I've got a question on how UPS backup works.

    I've got a APC UPS backup for my iMac at work. Recently we had a power outage at work, and even though the iMac was connected into the "battery backup", the machine was powered off when I got back to it.

    The sys-admin at work says that these UPS will briefly cut power to the computer to signal when power has been restored. This seems counter intuitive to me. Is that true?

    Since then, I've now connected the iMac to the UPS via a UBS connection, and configured the Mac OS-X settings found under "System Preference", "Energy Saver" on the "UPS" and "Options" tabs.

    Does anyone knowledge able in this area have advice for good settings.

    I've got the following
    Shutdown the computer when it has been on UPS for: 5 min.
    Shutdown the computer when the time left on UPS is: 2 min.
    Shutdown the computer when the UPS power level drops to: 25%.

    On the "Options" tab all te "Other Options are set including:
    Restart automatically after a power failure.
  2. Makosuke macrumors 603

    Aug 15, 2001
    The Cool Part of CA, USA
    Unless it's true-online (which it's highly unlikely an office UPS is), there's going to be a small power glitch both when the UPS kicks in and back out, but that's not why your iMac had shut down--it shouldn't be bothered by the small flicker, nor is it able to somehow detect it.

    If I'm reading right that you didn't have the USB cable connecting the iMac to the UPS when this happened, and assuming that the power was out for more than a couple of minutes, it's pretty simple: The battery ran down, at which point the power was cut to the iMac.

    With no communication between the UPS and iMac, the iMac had no way to know that the UPS's battery was running out (or even that it was running on a battery at all), and the UPS had no way of telling it this, so it chugged along until the battery was drained and it was forced to cut power.

    Now that the iMac is connected to and communicating with the UPS, it should automatically shut itself down, which is an improvement over a sudden power off though you may still lose unsaved work if something is open and you're not there to manually save and shut down.

    Now, if the outage wasn't long enough to run down the battery, it's possible that either the UPS is too small to handle the iMac's load, in which case it'd just shut down immediately, or that its battery is going (they last a few years), and so while it was once able to handle the load it isn't now.

    Easy enough to test--just yank the cord out of the wall and see what happens. If the iMac stays on and warns you, happy days. If it dies immediately and/or the UPS starts complaining, you're going to need to either replace the batteries or get a bigger UPS.
  3. Alan S thread starter macrumors member

    Mar 21, 2009
    Yes this is the case. Although I was under the impression that the power outage was not very long.

    I also had an LCD monitor (24") hooked up to the battery back-up. Not sure how much power it draws, but I removed it after reading this.

    I'll do this tomorrow. Thanks...
  4. Consultant macrumors G5


    Jun 27, 2007
    Sounds like the UPS is too under capacity for the computer. Need larger UPS that can handle more load.

    No. Not if your UPS has enough capacity.

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