Detecting negative delta V for NiMh termination

Discussion in 'Embedded Systems and Microcontrollers' started by bignick270, Mar 16, 2010.

  1. bignick270

    Thread Starter Active Member

    Mar 25, 2009
    How should I go about terminating a Nimh charge using the Negative Delta V approach using a microcontroller in C?

    I am having trouble with false terminations using the negative delta V approach. I do not have the luxury of a temperature sensor to do a delta T test(wish I did).

    Any suggestions?

    In what kind of time frame should I retest the voltages?
  2. russ_hensel

    Distinguished Member

    Jan 11, 2009
    Average values for less noise in the data.
    Adjust the amount of delta V required to terminate charge to find one that triggers at the right time.
    I would measure the voltage about once a second, I would look for a delta v that last at least 10 seconds.
  3. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    Be careful - the voltage on NiMH does not always 'droop' at full charge, it simply stops rising.

    The voltage on NiCD cells drops which is why NiCD chargers are no supposed to be used with NiMH as the charger never sees a voltage drop so never terminates the charge.

    Just had a quick look, there is a good article on charging NiMH here:

    The timescale you test over depends how fast you are charging; if you are doing a '1 hour' charge the voltage will be changing fairly quickly, but if you are trickle charging the voltage change will be slower.

    I'd do some pen and paper notes with a multimeter on a cell while it's charging (from dead flat) to see exactly how the voltage changes over time.

    You may also need to amplify and offset the cell or battery voltage so full scale on the ADC covers the practical voltage range you will see on the cell/battery.
    If you don't do that, the ADC steps may be too coarse to properly measure the critical, fine voltage changes as the cell reaches full charge.