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charging mobile battery with another battery

Discussion in 'Misc Electronics' started by sam, Dec 8, 2004.

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  1. sam

    sam Guest

    Hi folks,

    I'm just after some advice/thoughts. A friend and I have been thinking
    and talking about renewable energy and such. We're thinking that we
    could use an exercise bike to charge some sort of battery, and then use
    that battery to charge our mobile phones.

    Firstly, is it feasible to charge the phone directly from the bike
    (using an alternator and glue circuitry of course)? I'm not sure if
    these calculations are right, so please critique as necessary :) Let's
    say that the phone charger provides 3.7V at .337A and takes three hours
    to charge. So that should mean that a full charge of the phone is
    about 0.003 kWh. Assuming we can generate 0.0001 kWh per minute (we
    got that figure from
    that would mean that about 30 minutes of pedalling should charge the
    phone. Is all that possible?

    OR (and this is what we're thinking instead since it is more useful
    anyway): what about if we charged a battery and then used that battery
    to charge the phone? How would we go about doing that? What sort of
    battery would we use to charge a phone? I assume any (rechargeable)
    battery would do, as long as we could bring down the voltage and
    provide the correct current. Do phone chargers (nokia 3310, or the
    3xxx series, specifically) have any fancy electronics in them?

    If this is all crazy stupid let me know ;) It's just some
    experimentation we'd like to do; nothing serious and we certainly don't
    want to power our homes from an exercise bike :)

  2. CWatters

    CWatters Guest

    Set it up so that the bike charges a 12V battery (lead acid from a car, or
    Nicad etc). Then you can recharge almost anything using chargers intended
    for use in cars (eg Laptop charger, mobile phone charger etc.). Not as
    efficient perhaps but it makes the electronics easier.
  3. Tube2IC

    Tube2IC Guest

    Sure you can charge one battery from another. You just need to ensure
    the following:
    1. Charging battery should be at higher potential than the battery
    being charged. i.e. if you are charging a 3V Battery, the charging
    battery should be at least a few volts higher. This will allow the
    charging current to flow from the charging battery to the battery under
    2. Beware of Battery chemistry. Some batteries drop their potential as
    they discharge. If the potential drops below that of the battery under
    charge, then the charging will reverse. [This can be prevented by using
    a diaode in series. You will need to compensate for the forward drop of
    the diode which is usually 0.7V for a silicon diode]
    3. It is probably best if you use NiMH high capacity cells to build a
    6V or 7.5V battery and construct a harness to plug it into your cell
    phone's charger jack (Nokia's are 5.2V to 5.6V and it would probably be
    OK to feed it 6VDC)
  4. CWatters

    CWatters Guest

    Hi Tube..

    Do you know for sure that current limiting is built into the phone these
    days? Othewise it's not a good idea to feed it with a voltage source.

  5. Tube2IC

    Tube2IC Guest

    What I am saying here is to hook the external battery of 6V to the
    cellphone's charger port. The Cellphone charger port (on Nokia) is 5.4V
    to 5.6V depending on model. [(6-5.6)/6]*100 = 6.6% That should be well
    within the 10% tolerance usually designed into the charger ports. Your
    car charger probably varies about as much.

  6. Tube2IC

    Tube2IC Guest

    All valid points if a direct charge of the battery is being planned.
    Since the Cellphone's charging circuitry is used to control the rate of
    charge etc. there is no danger of expolding batteries. The cell phone
    battery will charge as designed by its manufacturer.
    What must be ensured is that the specifications of the charger port are
    not exceeded. i.e. Do not connect a 12V car battery to a 5.4V charger
  7. CWatters

    CWatters Guest

    Yes that's ok if the phone is designed to use a charger that is a pure
    _voltage source_ (eg the battery charging current limiting is built into the
    phone). Perhaps the charger spec is 5.5V nominal but with a built in 300mA
    current limit. eg when connected to a Li battery the voltage falls and it
    switches into current limiting mode? If the charge current limiting circuit
    is built into the charger like this it isn't present when you connect the 6V
    battery in it's place.
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