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charging NiMH batteries in a NiCd charger?

Discussion in 'Electronic Design' started by [email protected], Nov 23, 2005.

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  1. Guest

    It's that time again... my $15 cordless phone's NiCd batteries died,
    and could not be resuscitated. A replacement NiCd battery pack costs
    $12, almost as much as a new phone.

    Hacked together three NiCd AA's with electrical tape and bits of metal,
    but the connections keep getting loose - really annoying on those
    long-distance calls.

    Just bought a 3-AAA battery holder from Radio Shack that will fit in
    the phone, and now have to buy rechargeable AAAs.

    I'm considering NiMHs, because I really don't like NiCds. But, the
    phone charger is designed for NiCds.

    What will happen if I charge the phone with the NiCd phone charger?
    Will the batteries' lifetime be cut short? Will they burn my house
    down?

    SHould I just remove the 3 AAA NiMHs when the batteries get low, and
    charge them in a standard NiMH charger (which I do already have)?
     
  2. Baxter

    Baxter Guest

    Hmmm.... I have a Panasonic that is supposed to use NiCads - battery life
    is abysmal. I found a NiMH replacement - it works just fine -- very good
    battery life now. No changes required to base/charger.
     
  3. ehsjr

    ehsjr Guest

    If you are careful, you can solder the tabs from the
    old pack onto new, tabless cells. You need a high wattage
    soldering gun and sandpaper, or a file. First, roughen the
    flat surface of the tabs on the old pack, then tear the tabs
    off the old batteries. Next, roughen the + and - terminals
    on the new batteries. Then quickly tin the terminals, and
    the tabs. Then hold the tab in position with pliers, and
    quickly solder it to the battery terminal. When you have a
    tab attached to the + terminal of two cells, line up the
    batteries and solder them. Tape them, solder on the two
    wires and you're done. The key is melting the solder
    quickly, which (in my case, at least) means a 140 watt
    soldering gun. The cells will be damaged if they get too
    hot.

    Ed
     
  4. Guest

    Oh, I'm not *that* good at soldering, and the idea of soldering onto
    batteries just makes me nervous. I was able, though, to solder the
    leads from my Radio Shack AAA battery holder to the little plug that
    plugs into the phone. :)

    Plus, I want a removable solution, in case the new batteries go bad
    again. This is the second time this has happened to me. The first
    time, I gave the phone to my dad, and just bought a phone Consumer
    Reports recommends. (Alas, the batteries on the CR recommended phone
    just died!)

    My dad salvaged the old phone by attaching a 3-AA holder, but this was
    too big to fit in the battery compartment, and it sticks out like a
    sore thumb. But, it uses off-the-shelf NiCd AAs (which he has a lot
    of).

    My solution is to use AAAs... the battery holder fits neatly into the
    battery compartment. Just bought some 900 mAh NiMH AAAs today.
    They're about double the mAh rating of the cheap old NiCds that just
    died...

    I'll charge the NiMHs overnight in my NiMH battery charger, then put
    'em in the phone.

    So, is it ok to use the NiCd charger for the NiMHs? Will the NiCd
    charger overcharge the NiMHs?
     
  5. Bob Monsen

    Bob Monsen Guest

    Most NiCd chargers are designed for NiMH cells as well. They are quite
    similar, but the "end of charge" indication is slightly different with
    NiMH cells. Because of this, if your phone isn't built for NiMH cells,
    it may not detect end of charge, leading to overcharge, and overheating.
    Cells that are radically overcharged can get VERY HOT.

    I would not risk charging the NiMH cells in the unit; $12 is cheaper than
    a fire, or even buying a new phone. You might get away with it, but
    then again... If you go with a homebuilt pack, I'd advise you stick with
    NiCad cells.

    ---
    Regards,
    Bob Monsen

    A great discovery solves a great problem, but there is a grain of discovery
    in the solution of any problem. Your problem may be modest, but if it
    challenges your curiosity and brings into play your inventive faculties,
    and if you solve it by your own means, you may experience the tension and
    enjoy the triumph of discovery.
    - George Polya
     
  6. Guest

    Here is a trick that makes soldering "unsolderable" things easier.

    Take a small flat metal file, and "load" it with solder by filing it.
    You can wad up a bunch of solder in a pair of pliers and rub it into
    the file, or use bar solder if you have any. A lead sinker works too
    but it's worth the extra effort to have the same alloy you're going to
    solder with. Getting some flux in there helps too. I've had the best
    results using silver solder.

    You may want to dedicate a file for this purpose.

    Next, lightly file the surface you're going to solder with the loaded
    up file. You will be depositing bits of solder into the grooves the
    file cuts, and there will be no time for the newly exposed metal to
    oxidize.

    You should now be able to almost effortlessly solder a wire to the
    filed-down spot. The resulting joint will be nice and strong. If you
    use too much heat, you will remove all of the solder and have to start
    over, so work fairly quickly.

    To dramatically accelerate this process, you can use a dremel with a
    stone wheel. This works even on non-metallic surfaces like glass and
    ceramics.
     
  7. budgie

    budgie Guest

    A large proprtion of standalone chargers are, but IMOE budget cordless phones
    supplied with NiCd packs seem to be constant current chargers with no attempt at
    charge termination. While NiCd cells will tolerate this reasonably, it is NOT
    in the best interests of NiMH cells.
    Concur.
     
  8. GregS

    GregS Guest

    My attempts at replacing a hand vac, and a drill, NiCads with NiMH at first
    seemed OK, but after a short while the performance was terrible. I do
    realize NiCads have lower internal resistance for higher currents.

    greg
     
  9. Guest


    Ok... looks like I'll just be removing the NiMH AAAs from the phone
    when they need charging, and charge 'em in my dedicated NiMH charger.

    The phone doesn't *need* to remain in the charging cradle; I'll let
    everyone in the house know not to put it there. (Actually I'll
    probably end up hiding the charging cradle in the computer room.)

    Thanks for the advice, all.
     
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