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UPS Backup Time Guesstimate

Discussion in 'Electronic Design' started by Len Krauss, Oct 15, 2005.

  1. Len Krauss

    Len Krauss Guest

    I'm considering a UPS to power a light load. It's a combo cordless
    phone/answering machine powered by AC to DC converter block. Input: 120v,
    60hz, 8.5w. Output: 9vdc, 500ma.

    A low cost refurb UPS (APC BK280) has specs as follows:

    Output power capacity 280 VA, 180 Watts
    Replacement battery cartridge RBC2
    Typical backup time at half load 18.2 minutes
    Typical backup time at full load 6.7 minutes

    On the assumption that the cordless phone's batteries are fully charged when
    a long outage begins, I'm thinking that even less than the indicated power
    would be drawn to keep the answering base alive until the phone's batteries
    get weak, a which point more power would be drawn for charging. The phone
    would not be used during an outage, so its NiCad batteries would discharge
    just from being idle.

    What's your guesstimate of backup time running on UPS described above???

  2. Ian Stirling

    Ian Stirling Guest

    What's your guesstimate of backup time running on UPS described above???

    You can't get there from here.
    The PSU essentially zero load.
    The UPS will probably have 10-20% of its maximum load as waste power.

    So, take the uptime as 5-10 times the 'full load' value. and add on 20%
    as the battery will be more efficient.
    However, I wouldn't bother.
    I'd take 6 D cells, which will output 9V, then add a diode in series with
    both the PSU output and the D cells.
    The batteries will automatically take over when the PSU fails.
    (depending on the exact voltage of the PSU, the batteries may drain a little.
    However, you should still get well over 10 hours of uptime, more likely
  3. Pooh Bear

    Pooh Bear Guest

    Just over an hour maybe ?

  4. Pooh Bear

    Pooh Bear Guest

    Hmmm... that's the rating of the wall wart - *NOT* the phone !

    In which case the UPS will likely run about as long as it does with no load at
    all. Whatever that is.

  5. Guest

    Same as the one I gave on Tuesday:

    Since this involves rechargeable batteries, all "marketing" numbers can
    be halved, making the half load number something like 9 minutes. The
    phone's power supply takes 8.5 W, so you'd expect something like (90 W /
    8.5 W) * 9 or about 90 minutes. In reality it will be a bit longer than
    this. Make sure nothing else gets plugged into the battery-backed
    outlets on the UPS.

    Matt Roberds
  6. ehsjr

    ehsjr Guest

    You need to re-think this:

    "The phone would not be used during an outage". So why do you need
    a backup?
    Imagined response: "Well, the phone might ring, in which case it
    would be used."

    Charging the handset is simply not a factor - unless, as I suspect
    you will find if you get a long outage, that the "non-use" is
    bullshit. I think what actually may happen is that the phone will
    get more use during an extended outage than usual.

    Just buy a corded phone or use Ian's idea.

  7. Len Krauss

    Len Krauss Guest


    One other possibility, and the only one I'm actually trying to deal with,
    viz., calling in from elsewhere to retrieve messages on answering machine.
    Phones, as you know, often operate in a power outgage. An additional issue
    with this machine is that its message day and time stamp become inaccurate
    after even a brief power blip.

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