Discharging Ni-MH batteries

Discussion in 'The Projects Forum' started by MMH, Mar 22, 2013.

  1. MMH

    Thread Starter Member

    Feb 8, 2013
    143
    4
    Hello there everybody!!

    I just read that Ni-MH batteries should not be discharged below 1.0 volts. While discharging(a single 1.2v cell) with a DC motor, the voltage across the terminals went down to 0.4 volts. BUT. after leaving it alone for around an hour, the open circuit voltage measured 1.13v. Is this alright, or should I stop discharging it when the "voltage across the terminals(while discharging)" is 1.0v??
     
  2. richard.cs

    Member

    Mar 3, 2012
    162
    31
    I would imagine that's 1 volt when under load however deeply discharging a single cell doesn't tend to do that much harm. As soon as you've got several in series you start to get problems where the weakest cell is pushed into reverse by the stronger ones - that's a lot nastier.

    On the other hand there's very little usable energy left below 1 Volt so why not stop there?
     
  3. MMH

    Thread Starter Member

    Feb 8, 2013
    143
    4
    Thanks for answering!!

    I am aware of the fact of reverse polarity. I did not stop discharging at 1v since I was calibrating it. Can you be sure of that as when I connected the batt to the motor(after discharging it to 0.4v and leaving it for an hour) the voltage Slowly got down to 1.2, then 1.19 and so on.
     
  4. richard.cs

    Member

    Mar 3, 2012
    162
    31
    What is the capacity of the cell and what current were you drawing during the test?
     
  5. MMH

    Thread Starter Member

    Feb 8, 2013
    143
    4
    Thanks for answering!!

    The capacity of the cell is 2700mAh and I don't know the current the load draws . The amperage of the circuit is 20mA.
     
    Last edited: Mar 23, 2013
  6. richard.cs

    Member

    Mar 3, 2012
    162
    31
    The current drawn by the load is important in answering your question. A cell that size under a 1 mA load really should be stopped at 1 Volt. At a 3 amp load then you can go a lot lower.

    As I said there's not much energy left below one volt anyway - Ni-mh cells have quite a flat discharge curve and they've really fallen off the end by that point.
     
    MMH likes this.
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