Recharging Ni-MH cells and batteries...

Discussion in 'Power Electronics' started by Externet, Sep 16, 2016.

  1. Externet

    Thread Starter AAC Fanatic!

    Nov 29, 2005
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    Hi. Please check any wrongs here :

    - For a cell to be left permanently on trickle charge, do not exceed 0.05 C current
    - For a cell to be left permanently on trickle charge, do not exceed 1.40 Volts
    - For a series of many cells (battery) to be left permanently on trickle charge (until needed), do not exceed 0.05 C current.
    - For a series of many cells (battery) to be left permanently on trickle charge, no cell should go beyond 1.40V, and no cell should be under 1.00 V after one hour of recharging.
    - A cell that goes open in a battery does not harm the others during charging.
    - A cell that goes shorted in a battery during recharge will harm many or all others in the series.
    - NiMH cells are to be recharged on constant current with voltage limiting.
    - Cells do not need to be charged to the maximum of their capabilities/capacity.
    - Undercharged cells do not go bad.
    - Discharged cells do not go bad.
    - Cells exposed to reverse voltage do go bad.
     
  2. AlbertHall

    Well-Known Member

    Jun 4, 2014
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  3. Externet

    Thread Starter AAC Fanatic!

    Nov 29, 2005
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    Yes, I went there before posting.
     
  4. NCSailor

    New Member

    Jun 15, 2013
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    Then I guess you missed the part where it said never leave NiMH batteries (cells) on a charger for more than a day or two.

    Your voltage and current references make sense, however, I think you will find that some (older) cells will not reach 1.4V.
    If a cell goes open it will not harm the other cells, EXCEPT the other cells that are in series will not be charged.
    A cell that goes short (not likely during charging but a definite possibility if left uncharged for a period.) will also not harm the other cells during charging as long as the current is regulated as recommended. BUT, the shorted cell could still have enough internal resistance to develop some heat.
    Undercharged cells when left to continue their fairly high self discharge will become discharged and they do go bad if the voltage falls much below 1.0V /cell.
    Cells exposed to reverse polarity are usually not recoverable or if recovered, they have very much reduced capacity.

    The fact is that while NiMH batteries offer some advantages over NiCads and other battery chemistries, they are a poor choice for standby power systems and their charging requirements are much more finicky than NiCad, Li-Ion, and Lead Acid batteries. They are also not as good as NiCads for high discharge rates. Discharge rates should generally be less than C/2 unless short life is acceptable.
     
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  5. Externet

    Thread Starter AAC Fanatic!

    Nov 29, 2005
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    Thank you, NCSailor.
    I may have missed that part. I was trying to discern if a battery that has already finished its proper charging process, and exhibiting a stable 'X' Volts after a day or two out of the charger, could be connected to a source of exactly that 'X' Volts with zero charging current indefinitely. (Thinking just on keeping up with any self-discharge effect)
     
  6. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    If it keeps up with self-discharge then the charging current won't be zero.
    Further the battery voltage will vary with temperature changes.
     
  7. ian field

    Distinguished Member

    Oct 27, 2012
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    Nickel chemistry cells will eventually die if left permanently on trickle charge - they like to be fully discharged occasionally, the cadmium types particularly thrive on healthy exercise.

    Terminal voltage is a bit of a moving target - normally, the terminal voltage rises slightly just before full charge then drops back (more subtle on Ni-Mh). They have a marked tempco and IIRC: the terminal voltage drop at full charge is caused by the temperature rise at that point.

    Settled down to float charging and at room temperature - 1.37V/cell may be more like it.
     
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