Environmental Engineering Reference
In-Depth Information
according to a preset algorithm, perhaps a neuro-fuzzy state machine that made
decisions on when to switch to parallel balancing based on time and usage.
Models of ultra-capacitors will be treated later in this chapter.
10.2.5.3 Electrochemical double layer capacitor specification
and test
Just as it is essential to have standardized specifications and testing procedures
for batteries, the same applies to capacitor storage systems. At the present time,
the electrochemical double layer (ultra-capacitor) capacitor industry has not
reached standardization of packages, terminations or testing procedures. A grass
roots consortium of interested parties consisting of ultra-capacitor manufacturers,
electrode assembly material suppliers and applications users are beginning to
address this need [41]. This body will coordinate the communization of cell
package sizes, standardized terminations, common testing processes, standardized
specifications and test requirements to meet UN shipping requirements. This latter
part of standardization is necessary for devices containing AN and other active
ingredients in the electrolyte.
Electrochemical double layer capacitors utilize the electrolyte solvent AN as
one means to reduce electrolyte resistivity at cold temperatures. When the capacitor
is charged, the electrolyte becomes depleted of ions because the cations and anions
accumulate near the electrodes of opposite charge. Then as temperature is reduced
the internal resistance increases somewhat. The solvent AN tends to confine the
electrolyte resistance change to about a factor of 2 at
40
C. The popular elec-
trolyte component PC results in a resistance change of nearly 12-fold when the
temperature is reduced to -40
C. Because AN is more flammable and toxic than
PC, there are proposals within the international community to control its handling
and shipment. However, as a component of electrolyte in an ultra-capacitor it
remains dilute and contained, but testing requirements will persist. A typical
organic electrolyte based on AN solvent will have approximately 75% AN content
in a 1 molar solution with a salt. The Maxwell 3,000 F ultra-capacitor cell, for
example, contains 160 ml of electrolyte of which 120 ml is AN, and this in turn is
mostly consumed to solvate the salt molecules.
The most common testing and characterization procedure for capacitors is
constant current charging until the voltage reaches the rated value followed by
constant voltage float charge until the slower time constant charging is complete.
The capacitor available energy is then determined by constant current discharge.
From this measured available energy, the capacitance is validated as well as the
unit's equivalent series resistance (ESR). This procedure is illustrated by reference
to Figure 10.39
When constant current of magnitude
รพ
I
is applied to the capacitor, the voltage
across the terminals will step to a value given by the product of this injected current
and the capacitor ESR. The voltage will then increase linearly so long as the
capacitance is not voltage sensitive, up to the rated voltage of the cell or pack,
U
co
.
Following the constant current charge, the characterization equipment subjects the
capacitor to constant voltage at rated value and trickle current to replenish the fast
