An ultracapacitor cell can only withstand low voltages. In general, if cells are operated above their rated voltages for a long period of time, their life is reduced. This is a result of the electrolyte breaking down after exposure to high voltage. The amount of damage varies based on the voltage and the amount of time the cell is exposed to the over-voltage condition. Thus, occasional spikes above rated voltage will not immediately affect an ultracapacitor, but prolonged high-voltage uses will.

The voltage rating of Tecate's ultracapacitor cells is 2.7V or 3.0V. This higher-than-usual rating is mainly derived from the electrochemical stability of the electrolyte and electrode materials. The Tecate family of products uses an organic electrolyte. The key advantage of an organic electrolyte versus other (aqueous) electrolytes is its higher voltage stability. Yet even the higher-than-usual voltage rating of Tecate's ultracapacitor cells is not enough to meet the required voltage of most applications.

Placing multiple ultracapacitor cells in series overcomes this limitation. This means that ultracapacitors rated for higher voltages must be made of matched, series-connected individual capacitors, much like series-connected cells in higher-voltage batteries. Depending on the required energy, there could be a need to then place multiple cells in parallel. When ultracapacitor cells are placed in series or parallel, they react very similarly to conventional capacitors. Below is a summary of how key attributes respond when multiple cells are placed in series/parallel formation:

Voltage

Series connection: When placing cells in series, the overall voltage is increased directly by the number of cells in series.

Example: 4 cells (rated at 2.7V each) connected in series will have a maximum voltage of 10.8V.

Parallel connection: Placing cells in parallel will not affect the voltage.

Example: 4 cells (rated at 2.7V each) connected in parallel will have a maximum voltage of 2.7V.

Capacitance

Series connection: When placing same-value cells in series, the system capacitance is reduced by the number of cells placed in series based on the following formula: Csys = Ccell/n.

Example: 4 cells (rated at 10F each) connected in series will have a capacitance of 2.5F.

Parallel connection: Placing same-value cells in parallel will increase the overall system capacitance directly by the number of cells placed in parallel.

Example: 4 cells (rated at 10F each) connected in parallel will have a capacitance of 40F.

ESR

Series connection: When placing same-value cells in series, the overall system ESR will increase directly by the number of cells placed in series.

Example: 4 cells (DC ESR 75 mΩ each) connected in series will have a total ESR of 300 mΩ.

Parallel connection: Placing same-value cells in parallel will decrease the overall system ESR proportionally to the number of cells placed in parallel, according to the following formula: ESRsys = ESRcell/n.

Example: 4 cells (DC ESR 75 mΩ each) connected in parallel will have a total ESR of 18.75 mΩ.

Leakage Current

Series connection: Placing same-value cells in series will not affect the leakage current.*

Example: 4 cells (each with a leakage current of 0.03mA) connected in series will have a total leakage current of 0.03mA.

Parallel connection: Placing same-value cells in parallel will increase the overall leakage current directly by the number of cells placed in parallel.*

Example: 4 cells (each with a leakage current of 0.03mA) connected in parallel will have a total leakage current of 0.12mA.

*It should be noted that this does not take into account any leakage current induced as a result of cell balancing. In the case of passive balancing, the leakage current will be dominated by the bypass resistor value.