4 V battery as a charger

Discussion in 'General Electronics Chat' started by naveed, Jan 7, 2011.

  1. naveed

    Thread Starter Active Member

    Mar 12, 2008
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    0
    Why is this 4V battery not charging my mobile phone directly?
     
  2. tom66

    Senior Member

    May 9, 2009
    2,613
    214
    :eek::eek::eek::eek:

    Do NOT try and recharge a lithium polymer ("mobile phone") battery with a 4V lead acid! You WILL blow up something and start a fire. At the very least, you will damage the charging battery.

    There is a reason they put those scary warnings on batteries. See here.

    Besides, you need 4.2V (+/-0.05V) to charge a LiPo. 4V will not charge them.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Actually, a 4v lead-acid battery should measure 4.23v-4.28v when fully charged.
    However, it would be dangerous to attempt charging a Li-Po battery directly from a lead-acid battery with no current limiting or terminal voltage regulation.
     
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    Whatever kind of battery technology the OP has in his phone, this can't be a good idea. Maybe he has avoided having an accident because the lead/acid battery was not too well charged?

    I also note that the rated output voltages on my own phone chargers are all greater than their battery voltages, even allowing for a reasonable rise of voltage on charge. Does this imply that the phones have something between the charging input socket and the batteries, like for instance a diode to protect against discharge if the input were shorted?
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The charger circuit that limits the voltage to 4.2V is inside the phone, it is not inside the 5V wall-wart power supply. Maybe the charger circuit will not work when its input is less than 5.0VDC.
     
  6. #12

    Expert

    Nov 30, 2010
    16,276
    6,788
    It's a trick question. Look at the picture closely. That battery isn't charging the cell phone because it is not connected to it!
     
  7. tom66

    Senior Member

    May 9, 2009
    2,613
    214
    It's usually a buck regulator which steps voltage down. Also, they integrate temperature monitoring, SoC sense and current monitoring - almost like a life-support machine for the battery. Plugging 4V straight into the cell will not work; it could even be dangerous.
     
  8. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    A charger IC is used inside the phone to charge its lithium battery.
     
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