The term “Open Circuit Voltage” or OCV refers to the natural, unloaded voltage of the cell when there is no load applied. Moving from left to right in the graph shown here correlates to going from 100% state of charge to 0% state of charge. The vertical axis is the cell voltage. The red OCV curve represents the voltage that would correspond to a given state of charge if the battery has no load on it. The lower curve in blue is the actual voltage profile of the battery for some given constant current load. Because the battery has some non-zero internal resistance, the blue curve is displaced from the red curve by some amount. If a battery is discharged at very light loads, the internal resistance has negligible effect and the total charge “Q” that can be delivered approaches the theoretical maximum “Qmax.” However, for a heavy load current level, some loss occurs due to the internal resistance of the battery. So, in a real application, some lesser amount of charge, “Quse” is delivered by the time the battery reaches its minimum operating voltage.