A method comprising: enabling (403,703) an operative connection of energy storage devices (20,21) 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 (20,21); setting (405,705) a common reference current (Iref) at the second terminals of each of the converters; determining (407,707), by fuel gauges connected to the energy storage devices, an energy storage fraction for each of the energy storage devices (20,21); 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 (409,709) individually with multiple variable rates of energy storage (P charge) through the respective connections, or energy from the energy storage devices is depleted (411,711) individually with multiple variable energy depletion rates (P discharge) through the respective connections; wherein the energy storage or depletion is controlled (413,713) so that the energy storage devices (20,21) 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 (403,703) an operative connection of energy storage devices (20,21) 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 (20,21); setting (405,705) a common reference current (Iref) at the second terminals of each of the converters; determining (407,707), by fuel gauges connected to the energy storage devices, an energy storage fraction for each of the energy storage devices (20,21); 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 (409,709) individually with multiple variable rates of energy storage (P charge) through the respective connections, or energy from the energy storage devices is depleted (411,711) individually with multiple variable energy depletion rates (P discharge) through the respective connections; wherein the energy storage or depletion is controlled (413,713) so that the energy storage devices (20,21) 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.