Originally Posted by trilerian

The Nano's ADC is extremely linear. As said, its resolution being 10bit isn't good, but you can oversample it. The ADC on the Nano is ~10000 samples / sec. For the Minima, I can create a simple circuit to read voltage and use bench supply to output different voltages from 0V-5V and see how the Minima reads it. I have one that I haven't really messed with. Time is not on my side though.

As to the voltage reading of the battery, what I am suggesting is to remove the voltage sense from the ground plane and measure the potential difference through a differential amplifier. You can still filter the inputs with the op-amp by creating a 2nd order filter, or you can filter the output of the op-amps with a 1st order RC filter like you are doing now. But separating the voltage sense from the ground plane allows you to measure the true voltage at the point where you are measuring it without worrying about how the current is affecting the reading. Like I said, this may be overboard for a dyno, and if you are just breadboarding this it would be difficult to achieve anyway. Speaking of, do you plan on building your own pcb and break out the mcu from the dev board at some point? If so, that is a consideration for the mcu choice.