A method of battery monitoring including the steps of: acquiring a battery voltage measurement from the battery; and acquiring a time measurement associated with the battery voltage measurement. A scaled voltage value is determined by scaling the battery voltage measurement with respect to a start voltage and/or predetermined end voltage. A scaled time value is determined from the scaled voltage value in accordance with a predetermined battery characteristic. An absolute reserve time value is then obtained from the scaled time value, the reserve time being indicative of the difference between the time measurement and an end time associated with the predetermined end voltage.