Here the essential concept of how Impedance Track Gauging works is illustrated. The red curves in the graph on the left shows the discharge, relaxation, partial recharge, and relaxation of a Li-Ion cell. Note that the cell is neither fully discharged or fully charged, but used and charged intermittently as it might be in a real-life application. The graph on the right is similar, but shows partial discharge, followed by an idle period, and then a second partial discharge. So the basic point is that if the cell is at rest, at two different points, P1 and P2, the state of charge at these two points can be determined using the OCV – SOC correlation as seen in the previous page. The left graph can be used as an example. If using the numbers shown in the previous page then P1 at 3.9V corresponds to about 70% state of charge. At this point, the battery is connected to a charger for a little less than a half hour. It does not get fully charged, but the OCV at P2 is now just under 4.0V. This corresponds to about 95% state of charge. If using the information from the coulomb counter, it is possible to know how many milliamp hours were applied to the battery during this partial charge. In this example the battery is charged 1A for 20 minutes. This is 1A x (1/3) Hour or 333mAH. Therefore it took 333mAH to take this particular battery from 70% to 95%. From this, it can be inferred that the total capacity of this battery is 333/0.25 or 1333mAH. The impedance track method can work in either direction, as shown in the graph on the right. This would just require two different OCV points corresponding to two different SOC conditions, and also knowing how much charge was passed to get from one point to the next. It is not necessary to do a full charge/discharge cycle to learn the actual capacity of the cell, which is a benefit compared to the basic coulomb counting alone. In practice, it is best to have some separation between the OCV points. While a full learning cycle is not needed, the estimate becomes more accurate when the points are at least about 40% apart in state of charge.