A method comprising: enabling an operative connection of energy storage devices to respective converters , wherein each of the converters have a pair of first terminals and a pair of second terminals , wherein each pair of first terminals is connected to a respective energy storage device ; setting a common reference current (Iref) at the second terminals of each of the converters ; determining , by fuel gauges connected to the energy storage devices, an energy storage fraction for each of the energy storage devices ; and based on the determined energy storage fraction for each of the energy storage devices, controlling individually a voltage conversion ratio of the respective converters ; wherein energy is stored individually with multiple variable rates of energy storage (P charge ) through the respective connections, or energy from the energy storage devices is depleted individually with multiple variable energy depletion rates (P discharge ) through the respective connections; wherein the energy storage or depletion is controlled so that the energy storage devices are substantially complete at substantially the same time responsive to the common reference current; wherein the second terminals are connected to an inverter ; wherein the converters are serially connected to each other via the second terminals forming a serial string , wherein the serial string is connected in parallel across DC terminals (W,X) of the inverter 200, wherein the reference current (IRef) is a common reference current through the converters , wherein the reference current (IRef) is set to be constant by the inverter . enabling an operative connection of energy storage devices to respective converters , wherein each of the converters have a pair of first terminals and a pair of second terminals , wherein each pair of first terminals is connected to a respective energy storage device ; setting a common reference current (Iref) at the second terminals of each of the converters ; determining , by fuel gauges connected to the energy storage devices, an energy storage fraction for each of the energy storage devices ; and based on the determined energy storage fraction for each of the energy storage devices, controlling individually a voltage conversion ratio of the respective converters ; wherein energy is stored individually with multiple variable rates of energy storage (P charge ) through the respective connections, or energy from the energy storage devices is depleted individually with multiple variable energy depletion rates (P discharge ) through the respective connections; wherein the energy storage or depletion is controlled so that the energy storage devices are substantially complete at substantially the same time responsive to the common reference current; wherein the second terminals are connected to an inverter ; wherein the converters are serially connected to each other via the second terminals forming a serial string , wherein the serial string is connected in parallel across DC terminals (W,X) of the inverter 200, wherein the reference current (IRef) is a common reference current through the converters , wherein the reference current (IRef) is set to be constant by the inverter . <EOS>