Obtaining and loading a battery chart
As mentioned already, there are a lot of factors which affect the voltage/current/state-of-charge including temperature and temperature cycling, age, number of charge-discharge cycles, storage.
As far as I know, most state-of-charge monitors use some initial set points and generic values and then 'learn' the battery system they are connected to.
Some good information is available from HomePower magazine here.
I have also produced a basic info sheet on generic battery questions.