Abstract: A method for monitoring and diagnosing the status of a battery in an electrical system, more particularly, a method for determining a battery's reserve time based on comparison of measurements made during a battery discharge to a universal normalized discharge curve. Specifically, a battery discharge is initiated and the battery's voltage and discharge time is continuously measured. The measured battery voltage is normalized by dividing it by either the peak voltage or foothill voltage, as monitored during the discharge. By comparing this normalized voltage measurement to a universal normalized discharge curve, the percent discharge level of the battery is determined. Using the percent discharge level and the discharge time, the reserve time and total capacity of the battery can be determined.