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Supplemental Cell Phone Power Source

Discussion in 'Electronic Design' started by Ignoramus31975, Mar 30, 2007.

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  1. I have a Motorola V557 cell phone.

    My problem is that its battery and charging system is very lousy,
    batteries take a long time to change and discharge quickly if I use it
    with my Linux laptop as a cell modem (pppd).

    Note that its AC wall wart charger is rated for 5VDC, 700 mA.

    So, what I was thinking of doing is as follows.

    1. Buy an extra AC charger (cost about $5). Cut off its wall wart part
    and throw it to garbage. Keep the cell phone plug.

    2. Make a auxiliary power source, using

    - DC plug (which can be called receptacle) from the AC charger for output,
    - four AA battery holder,
    - linear regulator to achieve 5 VDC output.

    Since every charged battery holds about 1.62 volts, four would give me
    about 6.48 volts fully charged, and as long as they are over 1.25
    volts each, I could still be getting 5 volts DC as output.

    I think that four AA rechargeable batteries could give my cell phone a
    decent run time, and there is plenty of nice four AA battery chargers
    available at minimal cost (I have one at home anyway).

    I could use LM2825 regulator with 5V output option. See

    This one is highly energy efficient. I could also use it with 9v
    batteries for smaller foot print. It is, however, very expensive, so
    maybe I will opt with a lower efficiency linear regulator.

    Any thoughts?

  2. ehsjr

    ehsjr Guest

    What kind of chemistry gives 1.62 volts per cell? If you're
    talking NiMh or NiCd, it's ~1.43 per cell immediately after
    charging - and that drops rapidly to 1.2 volts per cell,

    If you don't mind the size/weight, go for a 6V gel cell and
    a diode or 2 in series to drop the voltage.

    The regulator chip you specified won't work in either
    case. According to the datasheet, it needs at least 2
    volts headroom - so at least 7 volts in for 5 volts out.
    I suspect, but do not *know*, that you don't need a
    regulated 5V DC. If that is true, then you could use
    4 NiCd or NiMh cells with no regulator chip.

  3. jasen

    jasen Guest

    what sort of battery are these?
  4. It sounds like you are charging the phone over the USB connection. If
    so, your laptop USB port doesn't do a great job because it doesn't have
    sufficient capacity to rapid charge the phone, so the USB power
    handshaking between the phone and the port steps the current draw down.

    If this is the case, you might speed charging up by putting a powered
    USB hub between the laptop and phone.
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