NOVELTY - An electrochemical system comprises an electrochemical device with an anode, a cathode, and an electrolyte, where the electrochemical device is connectable to a fuel delivery system, and to an oxygen delivery system; and a control system comprising a sensor, and a controller that receives the sensor signal from the sensor.    USE - Used as electrochemical system.    ADVANTAGE - The novel electrochemical device can be switched between battery mode and fuel cell mode. It can continue to generate electricity, while operating in battery mode, thus eliminating the need for an external battery back-up. When the fuel supply is replenished, the device in battery mode can switch back to fuel cell mode if desired. The anode is chemically rechargeable. The moldability of the liquid anode allows the interconnect, to have much thinner dimensions than many prior art interconnects. The interconnect provides reduction in weight by at least 20, preferably at least 40% compared to prior art interconnects; elimination of intricate, machined gas flow patterns, which reduces the cost and manufacture time; and rapid and repeated start-up heating of the stack, due to the minimal thermal and mechanical stresses resulting from contact with adjacent anode and cathode materials.    DETAILED DESCRIPTION - An electrochemical system comprises an electrochemical device (32) with an anode (34), a cathode, and an electrolyte, where the electrochemical device is connectable to a fuel delivery system (301) that supplies fuel (41) to the electrochemical device, and to an oxygen delivery system that supplies oxygen to the electrochemical device; and a control system comprising a sensor to measure a property of the electrochemical device and produce a sensor signal representing the property, and a controller that receives the sensor signal from the sensor, correlate the sensor signal to a concentration of an anode species in the anode, and provide a control signal to fuel delivery system or oxygen delivery system based, at least in part, on the concentration of the anode species in the anode.    An INDEPENDENT CLAIM is also included for a method of controlling an electrochemical system as above, by measuring a property of the electrochemical system; correlating the measured property to a concentration of an anode species in an anode of an electrochemical device associated with the electrochemical system; and controlling the introduction of fuel or oxygen into the electrochemical device.    DESCRIPTION OF DRAWING(S) - The figure is a schematic diagram of the above electrochemical system.    Electrochemical device (32)    Anode (34)    Feed conduit (40)    Fuel (41)    Fuel delivery system (301)    Anti-coking agent (318)    Hot zone (322)    Heating device (324)