The invention relates to an energy storage comprising a plurality of series connectable energy modules connected to a string via a plurality of switches. Wherein a string controller controls which of the energy modules that are part of a current path through the string by control of the status of the switches. An energy storage monitoring system is monitoring an energy storage element operating parameter of an energy module, the energy storage monitoring system comprises: a current sensor and a plurality of energy module print. The plurality of energy module prints establishes an energy module operating parameter of the associated energy module. The current sensor establishes the current in the current path. The string controller is configured for by-passing an energy module based on information of status of the switches, the measured current in the current path and the battery operating parameter measured at the energy modules.