Series circuits and series/parallel circuits are necessary, as the low nominal cell voltage of 2.7V would otherwise not allow for higher module voltages. Cell voltages of roughly 2.5V/cell are generally recommended to prevent individual cells from overloading, which can impair service life.
There are several ways to balance the cells. Balancing currents flow in the milliampere range, in accordance with the cell capacitance and ESR, so shunt resistors can be used to balance the cell voltage. This passive balancing is suitable for smaller dynamic systems – ULTRACAP modules that are frequently charged and discharged, balancing the cell voltage during operation. As in other cases, the total load should not come close to the limits of the capacitors and the module should not contain too many cells.
It is better for static systems to be balanced actively, as the system would otherwise discharge more quickly during longer periods of inactivity (with no power supply). The shunt accelerates discharge. In such cases, active balancing is used, usually in the form of a comparator circuit like this one from Nesscap. This circuit switches off voltage balancing when a switching point is reached, leaving only the self-discharge of the cells.
Manufacturers of semi-conductors have also discovered the ULTRACAP market and they offer corresponding charging and monitoring chip sets. Linear Technologies is a pioneer here.
Large systems with particularly sensitive service lives (those with many cells, up to several hundred, and 600 to 800V rated voltage) are monitored with special, intelligent monitoring systems. Several solution strategies are available on the market for this. However, these systems are mostly developed or adapted by the users to meet their individual requirements. The requirement profiles differ greatly and therefore require intelligent controllers and flexible software solutions.