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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

battery pack

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.

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2 Answers 2

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

Photo from internet

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:

Photo from internet

Related: Voltage input for charging NiMH Batteries

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

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    \$\begingroup\$ +1 your answer is much better than mine. \$\endgroup\$
    – dim
    Commented Nov 19, 2018 at 10:12
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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.

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    \$\begingroup\$ Of course, that makes perfect sense. I should have thought of that. Easy to verify and not very likely to break. - Many thanks! \$\endgroup\$
    – ARF
    Commented Nov 19, 2018 at 10:11

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