Energy storage system

1. An energy storage system, the system comprising:

one or more energy storage units, each energy storage unit comprising at least two energy storage modules arranged in series, each energy storage module comprising a plurality of electrochemical energy storage devices connected in series;

a DC switching device in series with and disposed between the at least two energy storage modules; wherein the DC switching device comprises a semiconductor device and a rectifying unit in parallel with the semiconductor device;

wherein the DC switching device further comprises a current limiter in series with the semiconductor device and rectifying unit; and

wherein the current limiter comprises an inductance operable to reduce a rate of increase of a DC current at the DC switching device.

2. The system according to claim 1 , wherein the DC switching device further comprises a further semiconductor device and parallel rectifying unit, in series with the current limiter, the further semiconductor device and parallel rectifying unit being inverted with respect to the semiconductor device and parallel rectifying unit.

3. The system according to claim 1 , wherein the semiconductor device comprises a transistor, an insulated gate bipolar transistor, or a relay.

4. The system according to claim 1 , wherein the rectifying unit comprises one of a diode, bridge rectifier, relay, or thyristor.

5. The system according to claim 1 , wherein the energy storage system comprises two or more energy storage units connected in parallel.

6. The system according to claim 1 , wherein the energy storage unit further comprises an isolator between the energy storage modules and a load to provide galvanic isolation.

7. The system according to claim 1 , wherein the energy storage unit further comprises a DC link capacitor in parallel with the energy storage modules.

8. The system according to claim 1 , wherein the DC switching device is located between two energy storage modules at approximately a midpoint of the at least two energy storage modules.

9. A method of controlling charging and discharging of two or more energy storage units of the system according to claim 1 , the method comprising:

switching the DC switching device on for a predetermined period of time to allow a charge to flow into or out of each energy storage unit;

switching the DC switching device off at the end of the predetermined time period; and repeating the switching on and switching off, to control the average current to or from that unit.

10. The method according to claim 9 , wherein a duty ratio of the DC switching device, equal to time ON divided by (time ON+time OFF), is chosen according to requirements of the system.