The calculated discharge curve method is based on thermodynamically reversible work: the product of the open-circuit voltage, initial current, and time, i.e., the sum of useful energy and energy losses. A calculated discharge curve is based on the constant step change of the battery voltage in correspondence with a cardinal number set. The essential solution is the transformation of the discharge data: voltage vs. time into time vs. voltage, using Basic Equation (three-point operators: power of internal resistance and time), which are valid for all battery electrochemical systems, battery designs and discharge conditions. The mono and multi-cell battery operating conditions are: (1) the four discharge modes (constant loads: resistor, current, voltage, and power), (2) two load regimes: self-driving and device-driving (galvanostat, potentiostat) or battery connection (serial, parallel, combine), and (3) continual and intermittent discharge. The next battery characteristics are introduced: discharge time as time intervals and subintervals, standard and average characteristcs, real and virtual discharges and real and virtal cells and batteries.