read battery value

Obviously you want to use high-value resistors for the potential divider, as they'll be constantly drawing power.

Or: you 'sacrifice' a pin to drive the devider (and do some calibration) to get the fourmula right... A calibration over a range may turn out that the formula is not linear anymore.

Using an extra pin - and may be an extra FET and chose to put different loads - 'practically no' load with no pin sacrificed and load over the sacrificed pin (and extra FET) - will give you more points for figuring out how well you battery still is. On the other hand, doing the measuring too often with such a setup, will add to the drain of the battery.

There is just no free lunch... that's why these specialized 'fancy' devices exist... such as accurate state of charge measurement... in this case for Li-Ion batteries.