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Trickle-charge NiMH

Discussion in 'Electronic Design' started by Robert Scott, May 9, 2010.

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  1. Robert Scott

    Robert Scott Guest

    I would like to design a product that uses a NiMH battery (3.6v, 700mAH). But I
    do not want to include the proper NiMH fast-charging circuitry, partly because
    of cost, but mostly because of board space. I am willing to give up fast and
    medium speed charging in order to be able to use a simple series resistor. The
    question is, what charging rate is safe for a full-time trickle charge? I was
    thinking that about C/15.
  2. You should refer to the battery handbook from your selected
    manufacturer for this information. Sanyo has some pretty good info.
    The good news is that you're unlikely to cause the battery to go into
    thermal runaway spraying caustic all over the place with C/15. ;-)

    Best regards,
    Spehro Pefhany
  3. Nico Coesel

    Nico Coesel Guest

    AFAIK NiMH batteries like to be trickle charged in pulses. Panasonic
    has some excellent information on charging NiMH.
  4. Robert Scott

    Robert Scott Guest

    Even if it does not go into thermal runaway, is it likely to shorten the life of
    the battery to use full-time C/15?
  5. ehsjr

    ehsjr Guest

    According to Powerstream, full-time (i.e. standby) charging should be
    around .03 to .05 C, so C/15 is a little high.

    I have charged _NiCd_ successfully at .031 C. It takes a lot
    of time, but it works. That does not mean it is the best way
    to charge them, but they do eventually come up to terminal
    voltage which is 1.43 volts per cell. That said, I have not
    done that with NiMh. So, try it and see if it works for you.
    If you can get a good charge at between .03 and .05 C,
    you can leave your cells on the charger full time per Powerstream.

  6. IIRC, neither Sanyo nor Panasonic recommend it, period. Panasonic

    The overcharging of nickel-metal hydride
    batteries, even by trickle charging, causes a
    deterioration in the characteristics of the
    batteries. To prevent overcharging by trickle
    charging or any other charging method, the
    provision of a timer to regulate the total charging
    time is recommended.

    Best regards,
    Spehro Pefhany
  7. Robert Scott

    Robert Scott Guest

    OK, I'll see what I can do with a timer. Thanks.
  8. Martin Brown

    Martin Brown Guest

    I know that is what they say, but the shelf life of charged NiMH is not
    exactly stellar and my camera is a bit tetchy about batteries that are
    even slightly off their peak fully charged voltage. I have been giving
    some thought to having a solar powered trickle charger that keeps a set
    of 4 NiMH cells properly charged countering their comparatively high
    self discharge rate. The recommended method of low duty cycle high
    current top up is not so attractive on a solar powered device.

    ISTR you can leave them almost forever on a C/300 charge rate.
    (though the manufacturers do not approve)

    Martin Brown
  9. Why? Can't you use a capacitor to supply the peak current?

    Best regards,
    Spehro Pefhany
  10. JosephKK

    JosephKK Guest

    Do the calc's. Q, delta Q, Delta V for batt and cap.
  11. JosephKK

    JosephKK Guest

    Hmmm. Makes me wonder how they would respond to a C/10 "burp" 1 second
    discharge once every 5 minutes with a C/30 trickle charge. Might be an
    interesting experiment.
  12. There's no numbers provided, but assuming a 286us pulse (bq2002) and
    C/2 on a 2.2Ah per cell pack, and 1V delta V, you'd need 315uF. So a
    470uF cap would be more than enough. No big deal. What "cal's" did you
    have in mind?

    Best regards,
    Spehro Pefhany
  13. JosephKK

    JosephKK Guest

    OP specified 3.6 V, 700 mAh and asked about C/15 charge rate. It is
    still there.

    For low duty cycle C/2 pulse float charging from solar source, would you
    fill in this table? Please assume the battery is at half charge.

    Batt. Cap.
    Q initial

    Q final

    Delta Q

    Delta V
  14. Why don't you do it for me, based on the 286us pulse width I found..
  15. JosephKK

    JosephKK Guest

    Cause i don't think that short of pulse will do much charging, won't be
    anywhere near square enough (i), and i have no idea of what rep rate you
    expect to achieve. Nor what final charge rate you expect.
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