NiCad and NiMH charging

Discussion in 'General Electronics Chat' started by lepow, Jan 14, 2010.

  1. lepow

    Thread Starter New Member

    Jan 13, 2010
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    Im replacing my NiCad battery pack (4-AA 4.8V, 600 mah) with a NiMH battery pack (4-AA 4.8V 1800 mah). Im going to use the same wall charger that came with the device (200MA, 12 V). Voltage is a bit high for the battery pack but thats what the manf supplied. My question is exactly what factor determines the charging current for a battery pack? Will this charger charge at 200 no matter what type or configuration battery pack it is charging? What if a 1 amp charger was used. Also, as the charge builds up, will the current taper lower?
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    "guess the charger was for the NiCd.s, & probably has an internal resistance of about 30 orms or more. OK for semi-constant current charging of NiCds but not for NiMH batteries.
     
  4. hp1729

    Well-Known Member

    Nov 23, 2015
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    I know, this is another old thread.
    Re: "30 ohms..."
    So what? Then is it designed to charge at 20 mA (.6 V / 30 ohms). I can't charge my NiMHs at 20 mA?
    I am replacing 1,000 mAh NiCADs with 2,300 mAh NiMHs. So I just allow longer charging time?
    If it is designed to charge 1,000 mAh NiCADs at 1/2 C (500 mA?) then it will charge my NuMH at about 1/4 C. I can live with that.

    That being said ... The Battery University says ...

    "t is difficult, if not impossible, to slow charge a NiMH battery. At a C rate of 0.1C to 0.3C, the voltage and temperature profiles do not exhibit defined characteristics to trigger full-charge detection, and the charger must depend on a timer. Harmful overcharge can occur when charging partially or fully charged batteries, even if the battery remains cold."

    So if the battery charger does not monitor temperature, just supplies a constant current, does it really matter? Not even a timer? Pay close attention to voltage when charging. A decrease at the end of as little as 5 mV can indicate a full charge.
     
    Last edited: Mar 23, 2016
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