Re-calibrating Prime's Battery?

[DrDarius]

At present there are chips available that measure the charge flow, and, on top of it, have a battery model built in so they can estimate SOC with reasonable error immediately after the circuit is powered for the first time.
Those are very common and not expensive chips. Maxim makes them, for instance (MAX17048/49, MAX17205, etc.). I used them in many designs with very good results.
Disclaimer: I am in no way affiliated with Maxim. I don't own their shares. I'm just a user of their products.
Measuring voltage of the Lithium Ion family of batteries ans trying to derive SOC out of it is a very inaccurate method because of the shape of the voltage curve.
If you are interested in details, look for "gas gauge". This is a colloquial name for this feature.
Given the price, availability, and accuracy of measurement of those chips, I am surprised that they were not used in Prime. (That said, I have not met a calculator yet that would have precise battery SOC system in place...)

Darius