Determination of the proper supercapacitor and number of capacitors is dependent on the intended application. For sizing the system correctly, a number of factors should be known. These factors include:
- maximum and minimum operating voltage of the application
- average current or power
- peak current or power
- operating environment temperature
- run time required for the application
- required life of the application.
Since supercapacitors are low voltage devices, the rated voltage is generally less than the application voltage required. Knowing the maximum application voltage (Vmax) will determine how many capacitor cells are required to be series-connected. The number of series-connected cells is determined by:
# series cells =
Next, the average current (I) in amps, the required run time (dt) in seconds and the minimum working voltage (Vmin), an approximate system capacitance can be calculated.
The total system capacitance is comprised of the capacitance of all the series-connected capacitors for achieving Vmax. For capacitors connected in series the capacitance of the individual cells is determined by:
For capacitors connected in parallel to achieve the required energy, the capacitance is determined by:
Note: There are many other items to consider for properly sizing the application. This includes the internal resistance of the capacitor to account for the sudden voltage drop associated with an applied current, the ambient operating temperature which affects the internal resistance and the capacitor life, and the life of the application. The supercapacitor performance requirement at end of life of the application is necessary to ensure proper initial sizing of the system