Handbook of Energy Storage for Transmission or ... - W2agz.com

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gas, eventually leading to dry-out and failure. Hermetically sealed packages may swell
as the pressure rises and the package can eventually rupture causing a catastrophic loss of
electrolyte and failure of the cell, usually as an open circuit. Before total failure these
conditions may cause additional voltage stress on the remaining cells and lead to unusual
performance of the series string.
Voltage de-rating, decreasing the average cell voltage in the string, is often applied as an
effective way to avoid cell overvoltages. That is, the average voltage, V ave
, on each cell in
the string must be below its maximum allowable value. This means the number of cells
connected in series need to operate at voltage V must be greater than V/V ave
. As
described, this will prevent a single cell in the string from reaching the maximum voltage
and causing problems. The resultant effect is lower power (more cells in series means
higher series resistance), less energy storage (more cells means less capacitance), and
higher cost.
The type III asymmetric are more tolerant to overvoltages. In this case the recombinant
mechanism seen in some aqueous batteries 2 helps to maintain voltage balance in a series
string. When the string is charged with a controlled current, cells that first reach overvoltage
conditions start to evolve oxygen. They do not rise in voltage while the lower
charge cells “catch up.” For healthy cells this condition continues until all of the cells
reach full voltage. Provided the rate of oxygen generation is not too high compared to
the rate of gas recombination, there is practically no loss of electrolyte. Therefore, type
III electrochemical cells have a valuable self-leveling characteristic.
Like recombinant batteries, these devices can operate at a slight overpressure and
normally release no gas. Nevertheless, commercial products have a pressure release
safety valve similar to that used on batteries. At higher over voltage conditions, there can
be gas releases with consequential loss of electrolyte, but without damage to the cells.
Because of the valve there is generally no swelling of the cells and no deterioration in
performance. If overvoltage conditions continue and lead to excessive consumption of
electrolyte, then the cell will fail as an open circuit due to electrolyte loss.
Type III capacitors differ from type II devices with respect to cell voltage de-rating. In
fact, it is undesirable to de-rate the voltage of an asymmetric aqueous capacitor. It is best
to operate series-strings of such cells with average voltage equal to their rated value,
which helps maintain cell voltage uniformity. Restated, in contrast with type I and II
electrochemical capacitors, type III capacitors should not be de-rated when series
connected to form high-voltage series strings.
Cell Balancing in a Series String
Series connecting a number of electrochemical capacitor cells usually requires an active
or passive voltage leveling system. For example attaching a parallel string of precision
resistors to help pin the voltage of each cell. Typically, resistance values are selected so
that the current flowing through the resistor string is approximately ten times the current
flowing through the capacitor string. With this ratio, and during static operation, the
2 There is a fundamental difference between aqueous batteries and symmetric electrochemical capacitors.
Such rechargeable batteries can be subjected to conditions that might lead to over voltage, but they do not
actually rise in voltage. Instead, the high voltage causes the evolution of oxygen gas at the positive
electrode of the cell. The gas travels to the negative electrode and recombines to form water. This
mechanism is used in recombinant lead-acid batteries as well as in sealed nickel cadmium and sealed nickel
metal hydride batteries.
Electrochemical Capacitors 18