Help identify the discrete component in this battery pack
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:



Related: Voltage input for charging NiMH Batteries

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