The average supercapacitor has a maximum charging voltage of  between 2.5 and 2.7 V. For many applications a voltage this low isn’t particularly useful, so the common practice is to place multiple
supercapacitor in series. Unfortunately, manufactured supercapacitor may have a tolerance difference in capacitance, resistance and leakage current. These differences create an imbalance in the cell voltages of supercapacitor wired in series. It is important to keep the voltage on any single cell below its maximum recommended working voltage. Otherwise the supercapacitor could degrade from electrolyte
decomposition, gas generation, or a rising effective series resistance (ESR).

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