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Battery Charging

Discussion in 'Power Electronics' started by 8characters, Nov 3, 2012.

  1. 8characters

    8characters

    11
    0
    Jul 16, 2012
    Hi fellas,

    I'm attempting to charge a 12v NiMH 1800mAh battery (bank of 10 AA) but the supplied charger doesn't seem to work. When the battery has 7v remaining and connected the charger will deliver 12.9v at a feeble 5-10mA, this can't be right... right? An all night charge hardly effects it at all, it will show on the multimeter the 12.9v but under 30mA load drains within minutes.

    When I connect the battery to a power supply it takes 15v to get the current draw up to 200mA. The power supply is capable of delivering 30A. This charges the battery fine.

    Is this just a case of the voltage of the charger being too low (12.9v) to charge a 12v battery? the chargers are listed as 12v-100mA but as said, only deliver 12.9v @ 5-10mA into a 7v drained battery.

    Just trying to figure if the chargers are junk as I suspect. Is there a voltage these should be charged at? Is it physically possible to deliver a higher current @ the 12.9v these charges give?

    In theory will the 12.9v @ 5mA (when drawn into 12v drained to 7v) charge the battery compketely if left long enough? like... a month.
     
    Last edited: Nov 3, 2012
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    9,219
    1,857
    Nov 17, 2011
    This is possibly a combination of a battery in bad shape (having a rather large internal resistance) and an unsuitable charger.
    The first assumption is backed by the experiment with the 15 V supply.
    The second assumption is backed by the typical characteristics of a NiMh cell. The voltage of a fully charged NiMh cell is at least 1.41 V (explanation here), so a pack of 10 has a minimum charge voltage of 14.1V. Your charger is obviously not up to this.

    To answer your questions about the charger we need more information, like the model of the charger. Was it supplied with the battery? Is it rated for charging NiMh cells at all?


    Regards,
    Harald
     
  3. 8characters

    8characters

    11
    0
    Jul 16, 2012
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    9,219
    1,857
    Nov 17, 2011
    My advice: get a good charger. This one is obviously not up to the task.

    Charging a 1300mAh pack at 200mA takes approx. 8 hours (you charge ~1.4 times the capacity), A good charger can fill this pack in 1-2 hours.
     
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