The original, simplest battery monitoring circuits just measured the voltage of the cell. While a full battery does have a higher voltage than an empty battery, it is quite difficult to get an accurate estimate of capacity beyond the simple “full or empty” type of level indication using a voltage measurement alone. As seen before, this is because with varying load current levels, the instantaneous voltage of a cell can move up and down for a given state of charge. For example if a Li-Ion cell had a large load current applied to it, a reading of 3.8V might represent nearly full, such as 80% capacity. But if the load is very small, for example when a battery-powered device is in standby mode, 3.8V might be just barely above 50%. So a voltage-based estimate of a cell’s state of charge can be highly inaccurate. The next evolution in battery gauging circuits came with the “coulomb counting” method. As seen before, if a small sense resistor is used in combination with a highly accurate measurement circuit and timer / counter, it can keep track of milliamp-hours in and out of the cell. If starting at a known point, such as 100% full or completely empty, and going from one end to the other while counting mAH in and out of the battery, it is possible to get an accurate model of how much capacity the cell actually can deliver under real-life operating conditions. From there, the gauge circuit can keep track of the state of charge as the battery continues to be charged and discharged. However, there are some practical considerations that reduce the accuracy of this more advanced method of gauging. First, even when a battery is not being used, such as during long periods of storage, the cell may lose capacity due to self-discharge. A coulomb-counting gauge cannot account for this. Furthermore, if the range of possible load currents is highly variable, the current sensing circuit may not be able to accurately measure both very high and very low load current levels. TI’s advanced Impedance Track gauging method combines the best features of voltage-based and current-based gauge methods. When the current level is too low to be measured, the voltage level of a cell can be used to get an accurate indication of the cell’s state of charge. When load current is flowing, and the voltage measurement may not accurately indicate the battery’s state of charge, the coulomb counting method can be used to determine the capacity added or subtracted from the battery. By combining these two methods, and using the appropriate information based on actual operating conditions, the Impedance Track gauge can determine the effective internal resistance of the cell under actual operating conditions. This also takes into account the variation in internal resistance due to cell aging as the battery is repeatedly charged and discharged over time.