NiCad and NiMH charging

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

  1. lepow

    Thread Starter New Member

    Jan 13, 2010
    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


    Apr 5, 2008
  3. Bernard

    AAC Fanatic!

    Aug 7, 2008
    "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
    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