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How to replace AC converter with battery ?

Discussion in 'Electronic Basics' started by Jacquelin Hardy, Aug 23, 2005.

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  1. Hello group,

    I have a Bluetooth serial plug, working with an AC converter that outputs 5V
    800mA.

    I would like to use the device without the AC supply by using batteries. A
    Google search didn't get me nowhere. Can someone help me with this ?
    Ideally, I would use the smallest possible rechargeable battery with an
    autonomy of 10 hours.

    Many thanks

    Jacquelin Hardy
     
  2. Rodney

    Rodney Guest

    Hi,
    If you connect four 1.2 volt NiCd or NiMH batteries in series(end to
    end) you will have 4.8 volts which will probably work. If the rating per
    battery is 600mA hr( you can get better ones) it will theoretically run for
    one hour with 600mA drain or a bit less with 800 mA drain. To get 10 hours
    of runtime at 800 mA you will need a battery that will output 800 mA for 10
    Hrs. That works out to 8000 mA. For that you will need an 8 amp hour
    battery. You will have a problem finding such a thing in a small package.
    Maybe someone else has a suggestion.

    Rod
     
  3. mike

    mike Guest

    It's tough to get 800ma for one hour out of a 800mAH battery. They're
    rated for longer discharge times where they are more efficient.
    I'd double your numbers to allow for some degradation over time.
    Fully charged batteries can be 6.4V, although I've seen more.
    Fully discharged will be closer to 4.0.
    I think I'd do some regulation.

    Another issue is that the average power drain of the device may be a LOT
    lower than the marketing/regulatory number written on the supply.
    You have to measure it.
    If it's really drawing that much current, you should look into
    alternative solutions to your problem...or get a wheeled cart to lug
    the batteries around.
    mike

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  4. ehsjr

    ehsjr Guest

    The Bluetooth won't draw 800 mA - you can't use the
    power rating of the AC adapter to figure out the battery
    size needed. According to the datasheet here:
    http://www.free2move.se/pdf/Datasheet_F2M01C1.pdf
    it should draw ~200 mA, peak - at least for the particular
    device shown at the site. Without knowing the specifics
    of her device, 200 mA peak is a reasonable figure

    Assuming 200 mA for 10 hours, a pack made up of 4
    2100 mah AA cells seems like it might work.

    Ed
     
  5. Jasen Betts

    Jasen Betts Guest

    The easiest way is to steal 5v from a keyboard, joystick, mouse, or USB socket.
    for batteries you're going to need want something like a 7.2V battery and a
    5V regulator.
    it's probable that the device doesn't use all of the 800ma
    that's available from the converter. so it's be hard to pick the right
    battery capacity without measuing it's power consumption first.

    Bye.
    Jasen
     
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