9V NiMH battery charger question

Discussion in 'General Electronics Chat' started by davidGG, Dec 19, 2013.

  1. davidGG

    Thread Starter Member

    Dec 22, 2012
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    0
    Hello.
    I am charging my NiMH 9V battery right now using a constant current source. The batteries capacity is 175mA and I am charging it at 17mA (C/10).
    From what I understand the source VOLTAGE does not matter as long as the current is constant.
    My batteries are currently charging at 17mA. However, when I put a voltmemter across the batteries terminals I am reading 9.05V. Is this how it is supposed to be? When I remove the battery, attach a motor to it and let the motor run for about 2 seconds and then put a voltmeter across the battery I read 8.58V.

    Why does this happen? Do I need to charge my battery for a longer amount of time even though the battery reaches above 9V when charging?

    So my question is...can i just disregard the voltage at the battery when it is being charged and just focus on the current being provided and the time that has passed?
     
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,394
    497
    You are measuring voltage across the battery while it being charged and read 9.05 volts? That is not battery voltage, that is charger voltage.

    Then you use the battery to power a motor and partially discharge the battery.

    I don't see any point in your procedure.
     
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    It is not really a 9V battery: NI cells have a voltage of 1.4V fully charged (per cell). The "9v" batteries are six or seven cells, so full charge voltage is either 8.4 or 9.8V depending on number of cells.

    Ni-Mh batteries don't like a C/10 constant charge. Ni-Cads can handle it no problem, Ni-Mh shouldn't exceed about C/30.

    I use a CC/CV charger which is to say current limit until it reaches set voltage and then tapers off to hold a fixed voltage.
     
  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
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  5. davidGG

    Thread Starter Member

    Dec 22, 2012
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    When I am charging the battery and I put a voltmeter across the battery I read 9.05 and when I remove the charger and just measure the battery I read the same thing.

    When I remove the battery from the charger, place a motor across the battery, let the motor spin for two seconds, remove the motor, place a voltmeter across the battery, the battery voltage goes from 9.05 to somewhere near 8.70.
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,055
    That's completely normal. "Surface charge" or some such term is used to describe the effect. If the cell is allowed to rest, some voltage will return.

    To drive a current into the battery, there must be a driving force. The charger voltage must exceed the resting voltage of the battery. Connecting a charger to a battery will always raise the voltage at the terminal, if any current is flowing. Where the terminal voltage goes after disconnecting the charger is another story.
     
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