Many system applications require that capacitors be connected together, in series and/or parallel combinations, to form a "bank" with a specific voltage and capacitance rating. The most critical parameter for all capacitors, including ultracapacitors, is voltage rating. Subjecting almost any capacitor to a substantially higher voltage than it was designed to withstand usually results in an irreparably damaged, nonworking capacitor. This is especially true for ultracapacitors, so they must be protected from overvoltage conditions.
Capacitors typically have two voltage ratings. Whatever voltage the capacitor can sustain indefinitely, without damage or performance degradation, is called the continuous-working voltage. On the other hand, the voltage that a capacitor can handle for just a short period of time, like a few hundred milliseconds, is the momentary peak or surge rating.
When an ultracapacitor is subjected to more than a tolerable voltage, the organic electrolyte within the cell begins to decompose, producing a gaseous byproduct. If the overvoltage condition persists long enough, the pressure may build up until the safety vent on the ultracapacitor's package opens. Consequently, more of the electrolyte will decompose and vaporize until the ultracapacitor's effective internal resistance increases and becomes an open circuit.
In short, impressing more voltage on an ultracapacitor than it's rated to withstand usually necessitates replacement. Therefore, prevention is the most sensible practice, as it may effectively eliminate ultracapacitor repair and maintenance costs. It also eradicates other potential causes of equipment downtime.
After voltage rating, the two most significant parameters for all types of capacitors are their capacitance and equivalent series resistance (ESR). When many ultracapacitor cells are connected together in a series string, these three parameters are affected as follows:
Overall Voltage Rating Of A Series-Capacitor String: The total voltage that can be im-pressed across a string of capacitor cells connected in series equals the sum of each cell's individual voltage rating. Ultracapacitors are usually connected together in series so that they can be subjected to a higher voltage than the available individual cells are rated to withstand.
Overall Capacitance Value Of A Series-Capacitor String: The net capacitance of a string of capacitor cells is the reciprocal of the sum of the reciprocals of every cell's capacitance. This is most easily understood if all members of the string have equivalent capacitance value. Then, the capacitance of the whole string will equal the individual cell capacitance divided by the number of cells in the string. For example, connecting 100 cells, each with 1000 Farads (F) of capacitance in a series string, will produce an overall effective capacitance of 10 F.
Overall ESR Of A Series-Capacitor String: The total ESR of the string has the same cumulative characteristic as the cell voltage. In other words, it equals the sum of all individual ESR values. A 100-cell string with 5 mΩ of dc ESR each will have an overall dc ESR of 500 mΩ.
Capacitors connected in series are subject to the "weakest-link" principle. The poorest performer in the string sets the performance "pace" for the rest of the string. Therefore, five individual 500-F cells in series have 100 F of capacitance. Yet four 500-F cells in series with one 400-F cell each have only 95 F of capacitance.
Moreover, the failure of any component within the string effectively causes the unit to "fail" due to the serial connection be-tween the individual string members. In particular, an open circuit in any series-connected component effectively renders the entire string as open circuited. Plus, ultracapacitors eventually fail open circuit, so it's a significant concern when many cells are connected together in a long string. That's because the mean time between failure (MTBF) of any system is inversely proportional to the number of components in that system.
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