Supercap balancing

Ultracap balancing

Ultracap-Knowledge | Rainer Hake | reading time: 6 minutes

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.

Passive balancing

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.

Passive Ultracap Balancing

Active balancing

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.

Ultracap active balancing

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.


Block diagram of CVTS circuit in the UMU

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.

Leakage current in supercaps: strong dependence on temperature

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).

Diffusion and leakage currents in a double-cell supercapacitor with 2 cells in series

Leakage current: Courtesy of CAP-XX Ltd, Australia. Translation and editorial adaptation: Rainer Hake


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Why are components like ultracaps called different names?

The variety of terms is explained by the English-language terms commonly used in the electronics industry, which are often mixed with German terms or used synonymously. In some cases, manufacturers have also introduced artificial terms to better distinguish themselves from competitors. Here are the most important examples:

Double layer capacitors (DSK) are referred to synonymously as:

  • EDLC (Electric Double Layer Capacitor)
  • Supercapacitors = Supercaps
  • Ultracapacitors = Ultracaps
  • Goldcap™ [Panasonic]
  • Boostcap™  [Maxwell]
  • Greencap™  [Samwha]
  • PURIXEL™  [Pureechem] for supercap cells
  • PURETRON™  [Pureechem] for supercap modules

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