AAA NiMH battery tester; space for a 3.3V step-up module like Pololu’s ‘U1V10F3’, an ADC input on the battery + pin, three ‘traffic light’ LEDs, a programming header, and an STM8S003F3 for the core logic. Also, space for an I2C OLED display to maybe display the read voltage.

Okay, okay, it’s hard to justify using an STM8S core these days. And there is no analog reference pin which means that the noisy output from a switching boost converter is used as a reference for reading the battery voltage, and that is not ideal. And I didn’t need to use a separate resistor for each LED - actually, one of those RGB LEDs might be better. And there is no protection diode on the input to save the circuit from a battery plugged in backwards.

But hey, it does seem to work and the code is very simple.