The second example, using the basic current measurement circuit shown earlier, keeps track of the accumulated current over time that is put into the battery. Since the charge Q is the integral of current with respect to time, when measuring the average current over specific intervals of time and keeping track of the accumulated result, it is possible to maintain a running total of the present state of charge for the battery. However, just knowing the state of charge does not give the ability to predict the remaining run-time for a given application, because the effective available capacity depends on the load rate. If a good estimate of the battery’s internal resistance is not known, it is not possible to predict the variation in usable capacity based on load rate.