Battery pack thermistor

[[email protected]]

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°c
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°c. How do I make sense of this and find one that
will meet these requirements?

 

[Arfa Daily]

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

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°c
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°c. How do I make sense of this and find one that
will meet these requirements?