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PTC Thermistor for nicad battery pack

Discussion in 'Electronic Repair' started by A. Kay, Mar 6, 2008.

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  1. A. Kay

    A. Kay Guest

    I am looking for a PTC thermistor, epoxy coated with approximately these
    characteristics:
    6.8k ohm at 45 degrees celsius

    I have an old nicad battery pack with a thermistor that failed during
    charging and one of the battery cells split along with the thermistor. The
    thermistor is inside the battery pack and connected in series with the
    charging circuit. The numbers on the thermistor are 6K8K. After searching
    the internet I deduced that the numbers meant 6.8K ohm with 10 percent
    tolerance. The 45 degrees celsius was the upper recommended
    charging/operating temperature for the batter pack stated in the manual.

    I took the battery pack to a Batteries Plus to rebuild it and their
    suppliers could not find a replacement. I figured that this thermistor isn't
    that unusual.

    Maybe someone can suggest a replacement, at least an electronics store that
    I can buy only one or two and not a hundred.
     
  2. Ross Herbert

    Ross Herbert Guest

    :I am looking for a PTC thermistor, epoxy coated with approximately these
    :characteristics:
    :6.8k ohm at 45 degrees celsius
    :
    :I have an old nicad battery pack with a thermistor that failed during
    :charging and one of the battery cells split along with the thermistor. The
    :thermistor is inside the battery pack and connected in series with the
    :charging circuit. The numbers on the thermistor are 6K8K. After searching
    :the internet I deduced that the numbers meant 6.8K ohm with 10 percent
    :tolerance. The 45 degrees celsius was the upper recommended
    :charging/operating temperature for the batter pack stated in the manual.
    :
    :I took the battery pack to a Batteries Plus to rebuild it and their
    :suppliers could not find a replacement. I figured that this thermistor isn't
    :that unusual.
    :
    :Maybe someone can suggest a replacement, at least an electronics store that
    :I can buy only one or two and not a hundred.
    :

    Most battery packs I have opened don't use a thermistor, they use a thermal
    protector switch.

    http://www.sensata.com/files/battery-4mm.pdf

    If it looks like the ones in the pdf then it isn't a thermistor. Find a supplier
    for the Klixon/Sensata device and use that.
     
  3. A. Kay

    A. Kay Guest

    Thanks for the advice. My device looks nothing like the thermal protector
    in the pdf. Mine looks similar to an epoxy coated tantalum capacitor. So,
    this thermal switch might be a viable replacement. Do you know of a
    supplier?
     
  4. Ron(UK)

    Ron(UK) Guest

    Sounds like a polyswitch perhaps?

    Ron
     
  5. I'm curious as to what this pack powers. A classic Vivitar electronic flash,
    perhaps?

    I still have a 283 that runs only from a wall wart. But it's a nice flash,
    if only because it has a remote sensor with _continuous_ settings.
     
  6. A. Kay

    A. Kay Guest

    The battery pack is a 13.2 volt sub C pack for a Freud cordless drill. The
    protective case on one of the cells was actually pealed away and one leg of
    the thermistor was welded (not soldered) to the cell casing.
     
  7. Franc Zabkar

    Franc Zabkar Guest

    If the device is in series with the battery, then it must be a thermal
    and/or current limiting protector of some kind, possibly a polyswitch.
    Elsewhere you state that the application is a cordless drill, so I
    would expect a charging current of up to 1A or more.

    Alternatively, if your battery pack has a third terminal, then your
    mystery device could be a temperature sensing thermistor.

    - Franc Zabkar
     
  8. Ross Herbert

    Ross Herbert Guest

    For the Sensata Klixon device I don't know of a reseller so if you specifically
    want that item you should enquire from Sensata. Sensata say that theie USA/Canda
    distributor is Component Concepts http://www.componentconcepts.com/ but to see
    whatthey have you have to click on the Thermal products tab and select Texas
    Instruments from the menu. Apparently, Klixon came unddr the TI company at one
    time and CC haven't updated their catalog yet. One product which has been around
    in their range is the 7AM and this is also made by a number of companies ie
    Thermtrol http://www.thermtrol.com/

    There are other manufacturers of similar devices which have been around for a
    long time. ie Pepi http://www.pepiusa.com/modn.html
    Electronic Goldmine sells a pack of 3 for $1 each
    http://www.goldmine-elec-products.com/prodinfo.asp?number=G14366

    Hope this helps.



    :Thanks for the advice. My device looks nothing like the thermal protector
    :in the pdf. Mine looks similar to an epoxy coated tantalum capacitor. So,
    :this thermal switch might be a viable replacement. Do you know of a
    :supplier?
     
  9. Jimmie D

    Jimmie D Guest

    I have seen some batery packs with thermistors. They look like a two wirres
    with a tiny drop of epoxy on the end, smaller than a mtch head. These were
    used in the battery packs of some aviation radios when they used lead acid
    batteries. Since they switched to using nicad the thermistor is no longer
    used.

    Jimmie
     
  10. A. Kay

    A. Kay Guest

    My batteries are definitely nicads. Do you think if I used a thermal
    protector switch instead of a thermistor (which I think I have) will it
    damage anything? I could check the resistivity of the thermistor out of the
    circuit and if it is zero at room temperature then I think it will probably
    work since a thermal protector switch is basically on or off.
     
  11. A. Kay

    A. Kay Guest

    The battery pack does have a third contact, hence I suspect it is a
    thermistor. I am considering using a thermal protector switch to replace it
    because when disconnect the thermistor from the pack, the charger does not
    charge. I am guessing the thermistor is zero or close to zero resistance at
    room temperature. Hence, the thought that I could replace it with a thermal
    protector switch which is basically on or off.
     
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