We present the thermocharge and discharge characteristics and long-term cycling performance of a thermochargeable galvanic cell with carbon fiber electrodes, a ferro/ferricyanide ion redox couple incorporating a cation-exchange membrane, and an extra pair of electrodes on each side of the membrane. This cell is a rechargeable concentration cell. No external charger is needed, and the cell can be used while it is being charged. It also functions as a thermoenergy conversion system, which can store energy. The ratio of discharge capacity to thermocharge capacity was more than 90% when there was a temperature difference between the main electrodes during thermocharging and discharging. When the temperature difference was removed after thermocharging and the cell was discharged, the ratio remained above 80% when the difference during thermocharging was greater than 35 degrees C. Thermocharge and discharge capacities were almost constant for over 40 cycles. The cell retained the advantage of the conventional thermocell with a ferro/ferricyanide ion redox couple; its voltage was constant as long as constant thermal energy was supplied. The cell was discharged from the main electrodes at 0.5 mA/cm(2) and constant voltages of the main and extra electrodes for over 1000 h while simultaneously thermocharging from the extra electrodes at the same current density. As a thermoenergy conversion system, our cell showed nearly the same thermal efficiency and power density as the conventional thermocell.