most of times it depends upon what type of battery you are using the circuit. the state of charge of different batteries varies depending on their chemical composition. it would be better if you you study some of the battery chemistry such as lead acid, nicd or nimh batteries. some batteries provide prower proportional to the time where as some batteries are designed to provide a costant power till some minimum time. each type of battery will have different sense electronics.
hope this would help you.