How does balancing & ultracap protection work?
The cell voltages in series circuits can vary, because manufacturing processes can create different capacitance values and internal resistances within the permitted tolerance ranges. Accordingly, there are corresponding balancing currents. A similar process occurs in batteries.
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 (Maxwell branded). 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.
UMU Ultracap monitoring units
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.
Leakage current (residual current, self discharge)
The calculation of leakage current for a supercapacitor cell is a complex function of voltage, time and temperature. Supercapacitors with multiple cells in series require a balancing circuit to ensure all cells have approximately equal voltage. This is because the leakage current of different supercapacitor cells will differ over time, temperature and voltage. Furthermore, even if different cells could be matched by leakage current during production, there is no guarantee that the cells will age identically, so their leakage current functions will diverge over time, or, in operation, one cell may be at a different operating temperature to the other (e.g. closer to a heat source such as a power amplifier).
Leakage current is strongly dependent on temperature
Since leakage current is very sensitive to temperature, the cells will then have different leakage currents irrespective of how well matched they are. Fig 1 illustrates the variation in leakage current of a population of GS103 cells at room temperature. Note how over the first 40hrs, the leakage current is much greater than the equilibrium value that the cells will eventually reach. This early phase is known as diffusion current, when even though the cell has reached its final voltage, it is still taking charge which is used in migrating ions further into the pores of the activated carbon. It is for this reason that the Y axis of Fig 1 is labelled “Input Current” rather than “Leakage Current”. Supercap manufacturers as CAP-XX perform a 100% production testing of all parts for capacitance, ESR and leakage current, so all parts shipped have leakage current that meets specification.
Example: Diffusion- and leakage current in a
dual cell supercapacitor with 2 cells in series
Consider a dual cell supercapacitor with 2 cells in series. Since they are in series their diffusion + leakage currents with no balancing circuit must settle to the same value. In order to achieve this equilibrium condition, the cell voltages will adjust so their leakage currents become equal. Without a balancing circuit, this may mean that one cell will be subjected to over-voltage and become damaged. This is illustrated in Fig 2.
In Fig 2, the magenta and cyan curves represent the voltage vs leakage current curves for 2 cells in series. If the 2 cells have the same C, and are rapidly charged, they will initially have the same voltage. However, since the two cells are in series, they must settle to have the same leakage current. The cell with the lower leakage current (cyan curve) increases its voltage while the cell with the higher leakage current (magenta curve) decreases voltage until their leakage currents are the same.
The cell with the cyan curve is now at risk of going over voltage and ageing prematurely with increased ESR and C loss. In order to prevent the scenario shown in Fig 2, a balancing circuit is required. The balancing circuit will source or sink current from the midpoint between the 2 cells so the current flowing in each cell is equal. This application white paper will now consider some different balancing circuits.
If you are interested in further examples of different compensation circuits, feel free to ask. Use our inquiry form for this (please with reference to: Leakage current equalization circuits).
Leakage current: Courtesy of CAP-XX Ltd, Australia. Translation and editorial adaptation: Rainer Hake