Most batteries are vulnerable to being overdischarged; many are also vulnerable to overcharging. In lithium–sulfur, while overcharging is not seen as a “safety” issue , the high charge state “shuttle” phenomenon is associated with capacity fade . Consequently, it is important to avoid extremes of charge state. As illustrated in Figure 9.1, our ability to access the full “usable capacity” depends on the ability to reliably estimate the state of charge at these extremes: uncertainty in state measurements translates directly into a reduced window of operation. This underlines the importance of reliable state estimation techniques: the effective capacity of a battery depends not only on the chemistry but also on the accuracy of the state estimation algorithms. Without these, an otherwise good battery technology can be rendered useless.