An apparatus for determining the occurrence of a leakage current between a series connected electrochemical battery cells, comprising: a first cell connection terminal for connection to a first cell's first terminal via first filter circuitry; a second cell connection terminal for connection, via second filter circuitry, to a connection between the first cell's second terminal and a second cell's first terminal, the first and second cell adjacent in the series arrangement; a first cell balancing terminal for connection to the first cell's first terminal bypassing the first filter circuitry; a second cell balancing terminal for connection to the connection between the first cell's second terminal and the second cell's first cell terminal; balancing circuitry for providing a connection between the cell balancing terminals; the apparatus configured to provide for identification of a leakage current based at least on a voltage between the cell connection terminals and the cell balancing terminals.