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NiCd and NiMH charging (and Dewalt)

Discussion in 'Electronic Design' started by Neil Kalo, Feb 19, 2008.

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  1. Neil Kalo

    Neil Kalo Guest

    Is there much difference in a circuit designed to charge NiMH versus
    NiCD batteries? How does the circuit tell what is attached? In
    particular Dewalt brought out a completely new range of NiMH/NiCd
    chargers when they introduced NiMH batteries. Was there any real need
    to do this?
     
  2. Tim Wescott

    Tim Wescott Guest

    Yes, they did. NiMH batteries are more touchy about charging than NiCd
    batteries. If you charge a NiCd battery using the optimal NiMH procedure
    you'll get a slightly less than optimal charge; do it the other way
    around and you'll get a damaged battery and possibly a fire.

    --
    Tim Wescott
    Control systems and communications consulting
    http://www.wescottdesign.com

    Need to learn how to apply control theory in your embedded system?
    "Applied Control Theory for Embedded Systems" by Tim Wescott
    Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
     
  3. I think this is an urban legend.

    BTW, the Lacrosse BC700 battery charger does not distinguish NiCd and
    NiMH. It is a very good charger indeed.

    http://www.discovergadgets.com/product.asp?itmky=492702


    Vladimir Vassilevsky
    DSP and Mixed Signal Design Consultant
    http://www.abvolt.com
     
  4. Tim Wescott

    Tim Wescott Guest

    (shrug)

    It may be an urban legend, but it was splattered all over the design
    rags, with endorsement and encouragement from the NiMH battery
    manufacturers, when NiMH batteries were first coming out.

    As I stated in my post, there's no reason that you couldn't make a
    battery charger that would do fine with both types, but a peak charger
    for a NiCd wouldn't necessarily work well for a NiMH. It looks like
    Lacrosse did just that.

    I don't have a gross of each type of batteries, or the will, to pull out
    an old NiCd charger and prove it one way or another.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Do you need to implement control loops in software?
    "Applied Control Theory for Embedded Systems" gives you just what it says.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  5. Fred Bloggs

    Fred Bloggs Guest

    If it's for a tool then you're talking about fast chargers. The NiMH is
    more susceptible to lifetime reduction due to overcharge than the NiCd,
    and sophisticated algorithms using the terminal voltage inflexion point
    rather than deltaV/deltaT(ime) have been developed to overcome this. The
    terminal voltage inflexion is where the slope of the slope of voltage
    with time turns negative, this also requires a temperature rise
    qualification and an oversampling onboard DSP. An older NiCd charger
    won't do this, and will almost certainly overcharge the NiMHs, resulting
    in reduced lifetime, and enough of a reduction to justify using a
    charger specifically designed for MiMH.
     
  6. Fred Bloggs

    Fred Bloggs Guest

    Is there such a thing as a charger advertised to fast charge both NiCd
    and NiMH? I recently surveyed the literature to include manufacturers
    application notes, patents, and peer reviewed research literature on
    charging the NiMH, and no such animal was forthcoming in fast charge
    form factor...Link us to the Dewalt charger specs if you can.
     
  7. Neil Kalo

    Neil Kalo Guest

    The DW9117 charges NiCd and NiMH in 15 mins (allegedly):

    http://www.dewalt.com/us/products/attachment_detail.asp?productID=1889

    But on the UK site it says NiCD only!:

    http://dewalt.co.uk/attachments/productdetails/catno/DE9117/

    The UK DE9130 is a 30 min charger:

    http://dewalt.co.uk/attachments/productdetails/catno/DE9130/

    I thought that all the Dewalt chargers used the inflexion point
    method. I know cyclists often modify Dewalt chargers to use with their
    lighting packs. In the UK all the NiCD only chargers are black and the
    NiMH capable ones are yellow. Even the NiCD chargers have a third
    contact for a temp sensor, but I've only seen NiMH packs with a
    matching contact.

    How do they tell the difference in the chemistries in practice?
     
  8. James Beck

    James Beck Guest

    Well, I can tell you from experience that (depending on how aggressive
    the charger is) setting a universal quick charger to NiCD and plugging
    in NiMH can sure pop them. I have a charger that you can select the
    delta V in 4 different ranges. 2 for NiCD and 2 for NiMH. I had it set
    for the wrong cell and popped off an 8 cell 2700mAH A size NiMH pack. I
    think what really happened was the cells got hot enough to destroy the
    individual cell covering and the cells just shorted together and off it
    went, but that's just my best guess because the unit was destroyed and
    there were flying cell cans going everywhere.

    Jim
     
  9. Fred Bloggs

    Fred Bloggs Guest

    Okay- thanks for links, I'll check up on them. Now that you mention it,
    I do recall one or two papers on battery chemistry identification. That
    15 minutes is fast as hell.
     
  10. Agreed. Also, the peak charger for NiMH wouldn't necessarily work well
    for NiCd.
    The results can be the opposite: setting the universal 1 hour charger to
    NiCd and plugging in the NiMH results in the ~50% charging of NiMH. The
    charger is Duracell CEF80NC. Perhaps it has the different limits on the
    max. charge amp hours for NiMH and NiCd.

    There is also considerable variation in the charge/discharge curves
    between the different makes of the MH batteries and even from one cell
    to another if both are from the same make.

    Vladimir Vassilevsky
    DSP and Mixed Signal Design Consultant
    http://www.abvolt.com
     
  11. Fred Bloggs

    Fred Bloggs Guest

    That's not even close-all the NiMH are at 90% capacity at inflexion
    which is well below the peak before negative dV/dt. You can't use
    absolute voltage thresholds at high charge rates above 1C or more, it's
    too variable, and well above the intrinsic 1.4V/cell.
     
  12. Mike

    Mike Guest

    As a standalone universal charger then Ansmann do quite a few.

    http://www.ansmann.de/cms/products/charging-technology/fast-chargers.html

    How they detect each battery chemistry is not stated, but in practice
    they accurately charge with negligible cell heating.

    Bosch power tool chargers are also capable of charging NiCd and NiMH I
    haven't taken one apart but as they have 5 or 6 terminals I suspect
    they use the additional contacts with sense components.


    --
     
  13. Fred Bloggs

    Fred Bloggs Guest

    There are lots of products advertising this capability and they are
    inferior. This for example:
    http://focus.ti.com/docs/prod/folders/print/bq2005.html
    is an inferior methodolgy for fast charging NiMH despite what TI claims.
    I'm not sure about NiCd, but for NiMH there is a rock solid
    correlation between state of charge and terminal voltage inflection. If
    this is the case with NiCd also, then the charger can switch from fast
    to something like a 0.5C rate for a timed interval of 2x(1-p) hours,
    where p is percentage capacity at inflection, e.g. p=0.9 for NiMH makes
    for 12 minutes additional charge time and is acceptable for fast charge.
    The life reduction of NiMH is exponentially dependent on rate of charge
    in overcharge and 0.5C may put them in the negligible region. There
    could be other correlates such as magnitude of the temperature
    temperature gradient. Most of the cheap plug-in chargers are fixed time
    duration at 0.2C for 8 hours which is dead-on the 67% charge efficiency
    for this chemistry.
     
  14. Neil Kalo

    Neil Kalo Guest

    I did a quick and dirty test: discharging and charging an old Dewalt
    12V NiCd battery pack. I would like to understand more what is going
    on in the graph:

    http://neilkalo.googlepages.com/batterycharging

    During discharge the sudden dives are presumably individual cells
    giving up. Similarly during charge. I stopped the charge (at about
    18:20) when the NiCd started getting hot. I was expecting an S-shaped
    "flat spot" near the end of the charge cycle, but don't really see it.
     
  15. What was your charge and discharge rate?
     
  16. Neil Kalo

    Neil Kalo Guest

    Discharge was a 5W bulb, the current was actually about 315mA, charge
    was 2A. The pack is 2Ah
     
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