PTC Thermistor for nicad battery pack

: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.

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.

Sounds like a polyswitch perhaps?

Ron

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.

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.

On Thu, 06 Mar 2008 06:36:34 GMT, "A. Kay" put finger to keyboard and composed:

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

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Please remove one 'i' from my address when replying by email.

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

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

There are other manufacturers of similar devices which have been around for a long time. ie Pepi

Electronic Goldmine sells a pack of 3 for $1 each

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?

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

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.

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.