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Battery pack thermistor

I'm rebuilding a 14.4v and 18v Milwaukee battery pack and need to replace the thermistor. I could read 6K8J on it. I understand this to be 6800ohm 5%. It was a small disc about 1/4" in diameter with two leads. It must be NTC because when my body heat is applied the resistance drops. It seems that most battery packs are cut off at 70=B0c to prevent over-heating the nicd/nimh cells. I see digikey has several that fit this general discription but the benchmark for most thermistors is 25=B0c. How do I make sense of this and find one that will meet these requirements?

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jonathanwschmidt
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The 25 deg specification normally just represents the expected resistance at an ambient temperature (of 25 deg ...) so that will probably be around about what your original is - 6k8 at 25 deg. The fact that the charger cuts off if the temperature exceeds say 70 deg, is nothing to do with the thermistor as such. It is the charger electronics that determine when to cut the charge off, as a result of what the thermistor is saying, if you see what I mean. So, if the resistance of the thermistor dropped to say 2k at 70 deg, then that would be the resistance that the circuitry in the charger would be 'looking' for to determine that 70 deg had been reached.

If the original thermistor is still working (if it's a 'rebuild' job, why not just re-use it?) then hot it up to 70 deg in a plastic bag in hot water, and measure it. Then use this figure to compare to the curves of the ones that Digikey keep, to find the closest match. If necessary, the value of the resistor(s) back in the charger can be tweaked to get you back to the 70 deg figure, if the replacement thermistor is a little different.

Arfa

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Arfa Daily

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