Connectivity check between cells and control wiring electronics with only one switch A connectivity check for a rechargeable energy storage system comprising a string of rechargeable energy storage cell modules in series is described. Each cell module comprises an individual rechargeable energy storage cell or plurality of rechargeable energy storage cells, a switch for use in a connection check, a monitoring and control circuit adapted for measuring a cell module voltage, a single common connection between a terminal of a rechargeable energy storage cell and the monitoring and control circuit within each module. To perform the connectivity check the switch is opened in one module, a cell voltage is measured in the module to give a first measured value, the switch is closed. The cell voltage is measured again to provide a second measured value, and a determination is made if there is a difference between the first and second measured values which would indicate a bad connection.