Accelerated ageing based on floating has been used to investigate the state of health of high voltage carbon/carbon capacitors in aqueous Li2SO4 electrolyte and to determine the factors influencing their life-time. A pressure sensor connected to the cell allowed to measure gas evolution caused by overcharging of the system and to determine the maximum operating voltage limit of the cell. At voltage higher than 1.5 V, gases started to evolve, together with oxidation of the positive carbon electrode material and formation of decomposition and/or corrosion products. The positive electrode was found to entail the ageing of the entire system, being covered by twice more surface oxygenated groups than the negative one after 120 h of floating at 1.7 V. The blockage of pore entrances by oxygenated surface functionalities decreases the accessible pore volume, causing a decrease of capacitance and an increase of internal resistance during floating. A drop of capacitive current is observed at voltage higher than 1.2 V on the voltammograms already after 20 h floating at 1.7 V. From the study, it can be concluded that the AC/AC supercapacitors using aqueous Li2SO4 with stainless steel current collectors can operate safely up to 1.5 V, which is actually much more than the voltage value of 0.8–1 V reached with aqueous KOH and H2SO4 electrolytes.