# Help identify the discrete component in this battery pack

I am rebuilding a NiMH battery pack for a RF remote control device which can be recharged by plugging the device in.

The battery pack is connected with three wires:

• black to the negative
• red to the positive
• BLUE through a mysterious discrete component to the negative

The component in line with the blue wire looks a bit like a glass-body diode but my multimeter cannot measure a diode voltage. (It measures infinity, not zero, if this helps.)

However, I can measure about 11.5 kOhm across the component in both directions.

Can anybody enlighten me, what this component is likely to be? I would like to verify that the component is still functional because it is unclear whether or not the device still charges properly.

Its a thermistor like this (photo from internet, not spam related):

This kind of resistor depends on the temperature of both batteries.

EDIT: In this photo, is a Negative Temperature Coefficient. There are also Positive Temperature Coefficient resistors. The main difference between them is how the resistor decreases or increases when the temperature grows, respectively.

That's a cheap component and a cheaper way to determine when both batteries are charged up.

When a NiMH is almost charged, its temperature starts to grow. If you measure the temperature based on a voltage divider into a DAC, you can measure the temperature and, therefore, activate the charge or deactivate it.

EDIT: The thermistor must be very close to the batteries to read a correct value, so that's why this discrete component is located close to the batteries.

The temperature based graph to read when stop charging the battery is like this:

If you can measure the temperature, you can check when it's charged up.

• +1 your answer is much better than mine. – dim Nov 19 '18 at 10:12

It is a thermistor. This senses the battery temperature, so the charger can know when something goes wrong.

Measuring it likely won't tell you if the battery pack is still working correctly, though.

• Of course, that makes perfect sense. I should have thought of that. Easy to verify and not very likely to break. - Many thanks! – ARF Nov 19 '18 at 10:11