Any Super capacitor/ultra capacitor will discharge through an internal shunt resistance in the case of electrification. This discharge current is called leakage current, which affects the self-discharge of the Super capacitor/ultra capacitor unit. Similar to some secondary battery technologies, the voltage of Super capacitor/ultra capacitor also needs to be balanced when used in series, because the leakage current exists in the super capacitor, and the magnitude of the internal shunt resistance determines the voltage distribution on the series Super capacitor/ultra capacitor unit. When the voltage of ultracapacitor is stable, the voltage on each cell will change with the leakage current, not with the capacitance. The larger the leakage current, the smaller the rated voltage. On the contrary, the smaller the leakage current, the higher the rated voltage. This is because the leakage current will cause the discharge of Super capacitor/ultra capacitor cells and reduce the voltage, which will then affect the voltage of other cells connected in series. It is assumed that these cells in series are powered by the same constant voltage.
In order to compensate for leakage current changes, a common method is to connect a resistor in parallel beside each unit to control the leakage current of the whole unit. This method effectively reduces the change of the corresponding shunt resistance among the units.
Another recommended method is the active cell balance method, in which each cell is actively monitored and balanced when there is a voltage change. This method can reduce any additional load on the unit and greatly improve the work efficiency.
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