Eight commercial 10F electrochemical double-layer capacitors (EDLCs) were connected together and placed in a container filled with mineral oil. The whole system was placed into a Dewar container. Temperature variation and heat exchanged between the test EDLC and the environment during its charging, discharging, and "self-discharge" were measured, together with voltage U changes. Charge separation during charging was equivalent to a transition into a more ordered system, which results in entropy decrease, while discharging caused entropy increase (the Peltier-Seebeck effect). Consequently, a number of charging/discharging cycles led to a corresponding series of entropy and temperature changes. The final shape of temperature versus time curve during charging/discharging cycles was due to overlapping of irreversible Joule-Lenz and reversible Peltier heats. When charged EDLC was kept under the open-circuit condition, measured heat flow was negligible in comparison to energy loss calculated from potential drop, assuming that energy E accumulated is proportional at any time to voltage to the second power (i.e., E similar to U (2)). The result was interpreted assuming that the EDLC "self-discharge" phenomenon is not associated with energy loss by the device, but rather with charge redistribution between EDLC particles characterized by different time constants.