Apparatus for monitoring relative capacity and state of charge between at least two cells, or cell modules, A and B of a plurality of Lithium Sulfur cells arranged in series, during charging of the cells at a constant current in which each of the plurality of Lithium Sulfur cells exhibits a two-stage charging mechanism, wherein during a first stage of charging in which lithium sulphide in the cells is oxidised to lithium polysulphide the cell voltage remains relatively constant at a plateau, and wherein, when the cells transition to a second stage of charge in which the lithium polysulphide is further oxidised to lithium and sulphur, the rate of change of voltage measurably increases, the apparatus comprising: a timer a voltage monitoring module configured to monitor a voltage drop across each of the Lithium Sulfur cells or cell modules arranged in series based on signals received from a voltage monitoring circuit; and a cell monitoring module coupled to the timer and the voltage monitoring module and characterised in being configured to, during a charging cycle in which the cells are charged at the constant current: record a time stamp T1(Cell A) at which the monitored voltage of the first cell, cell A, leading the charging reaches a chosen first voltage V1(cell A), set to be in the second stage of charge; record a time stamp T1(Cell B) at which the monitored voltage of the cell B following the charging reaches the chosen first voltage V1(Cell A); record a time stamp T2(Cell A) at which the monitored voltage of the leading cell A reaches a chosen second voltage V2(Cell A) set to be substantially at a deemed top of charge; record a monitored voltage V2(Cell B) of the following cell B at T2(Cell A); and determine, based on at least T1(Cell A), T1(Cell B), V2(Cell A) and V2(Cell B), a metric indicative of a relative capacity difference between cell A and cell B. a timer a voltage monitoring module configured to monitor a voltage drop across each of the Lithium Sulfur cells or cell modules arranged in series based on signals received from a voltage monitoring circuit; and a cell monitoring module coupled to the timer and the voltage monitoring module and characterised in being configured to, during a charging cycle in which the cells are charged at the constant current: record a time stamp T1(Cell A) at which the monitored voltage of the first cell, cell A, leading the charging reaches a chosen first voltage V1(cell A), set to be in the second stage of charge; record a time stamp T1(Cell B) at which the monitored voltage of the cell B following the charging reaches the chosen first voltage V1(Cell A); record a time stamp T2(Cell A) at which the monitored voltage of the leading cell A reaches a chosen second voltage V2(Cell A) set to be substantially at a deemed top of charge; record a monitored voltage V2(Cell B) of the following cell B at T2(Cell A); and determine, based on at least T1(Cell A), T1(Cell B), V2(Cell A) and V2(Cell B), a metric indicative of a relative capacity difference between cell A and cell B. record a time stamp T1(Cell A) at which the monitored voltage of the first cell, cell A, leading the charging reaches a chosen first voltage V1(cell A), set to be in the second stage of charge; record a time stamp T1(Cell B) at which the monitored voltage of the cell B following the charging reaches the chosen first voltage V1(Cell A); record a time stamp T2(Cell A) at which the monitored voltage of the leading cell A reaches a chosen second voltage V2(Cell A) set to be substantially at a deemed top of charge; record a monitored voltage V2(Cell B) of the following cell B at T2(Cell A); and determine, based on at least T1(Cell A), T1(Cell B), V2(Cell A) and V2(Cell B), a metric indicative of a relative capacity difference between cell A and cell B.