NOVELTY - The method (701) involves storing (709) of energy in energy storage devices with variable energy storage rates so that energy storage in the energy storage devices is complete at a common charging time responsive to a common voltage or current reference, using power converters. The energy from the energy storage devices are depleted (711) with variable energy depletion rates so that the energy storage devices reach a depleted state at a common discharging time responsive to the common voltage or current reference. A battery charge fraction is determined respectively for each battery.    USE - Method for storing and depleting electrical energy e.g. in bank of multiple batteries from electrical power sources and power delivery e.g. from bank of multiple batteries to electrical network for use in different energy sources such as wind turbines, hydro turbines, fuel cells, mechanical energy storage such as flywheel, gas pressure, spring compression, pumping water and/or lifting mass against gravity and chemical energy storage such as battery and fuel cell.    ADVANTAGE - The common end time is achieved through mechanisms such as using a common reference such as a digital and/or analog reference. The energy storage and energy depletion is balanced to prevent some of the energy storage devices to be empty when other energy storage devices still contain energy. The situation that a battery will be empty before another battery or that a battery is full or empty before all the other batteries are discharged or charged is avoided. The cost of producing AC power is relatively inexpensive.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a system for storing energy.    DESCRIPTION OF DRAWING(S) - The drawing shows a flow chart illustrating method for storing energy.    Method for storing energy (701)    Step for setting voltage or current at respective connections (705)    Step for determining energy storage function for each energy storage device (707)    Step for storing energy with variable energy storage rates through respective connections (709)    Step for depleting energy from energy storage devices with variable energy depletion rates through respective connections (711)