221
Electrochemical Supercapacitor Design, Fabrication, and Operation
For example, a simulation by Barrade et al. [16] tested an ES stack containing four ESs of 800 F and a fifth 1000 F capacitor connected in series
and charged to 12.5 V. In the stack, five 0.1 Ω parallel resistors were used to
bypass each cell for voltage balancing. It was found that an extended time of
400 seconds was necessary to charge an equalized voltage across the entire
stack. Furthermore, the total energy needed to charge the stack was about
120 kJ, but the stored energy was only about 15 kJ, so the charging efficiency
was only 12.5%. Therefore, more efficient balancing methods are necessary.
5.5.1.2 Zener Diode Balancing
This balancing method uses Zener diodes instead of resistors as the bypassing elements to maintain a threshold voltage across each capacitor equal to
the threshold potential of the diode(s). The current passes through a connected capacitor and can be effectively limited upon reaching a defined
potential. This method results in an energy efficiency as high as 92%. For
example, if 16.3 kJ are used to charge an ES stack, 15 kJ are stored [1]. Zener
diodes can also dissipate the power beyond the local threshold voltage.
5.5.2 Active Balancing
As discussed above, passive balance schemes utilize shunts (resistors and
diodes) to dissipate energy to regulate cell voltages. The resulting circuits are
mainly suited for applications requiring low power or low current charging
and discharging rates. High charging and discharging rates within a short
time require a minimal loss in efficiency (non-dissipative equalization) to
optimize performance. Some active balancing methods have been developed
to achieve this.
Figure 5.12 shows an active balancing circuit scheme. In the figure, buck–
boost transistors or diodes can provide an equalizing current I eq for charging
current I. The direction and magnitude of the equalizing currents depend on
the local voltage across the corresponding single cell. For example, if a local
voltage exceeds the limiting voltage, a reverse current can be applied until
the voltage is balanced. This process actively proceeds along the entire stack
until equivalent voltages are established. Using this active balancing circuit,
the energy efficiency may be as high as 97%.
5.6 Cell Aging and Voltage Decay
An ES has an unlimited shelf life when stored in a discharged state, but
aging does occur and typically decreases the capacitance and increases the
resistance. The life of an ES specified by industry standards is a 20% decrease
Electrochemical Supercapacitor Design, Fabrication, and Operation
For example, a simulation by Barrade et al. [16] tested an ES stack containing four ESs of 800 F and a fifth 1000 F capacitor connected in series
and charged to 12.5 V. In the stack, five 0.1 Ω parallel resistors were used to
bypass each cell for voltage balancing. It was found that an extended time of
400 seconds was necessary to charge an equalized voltage across the entire
stack. Furthermore, the total energy needed to charge the stack was about
120 kJ, but the stored energy was only about 15 kJ, so the charging efficiency
was only 12.5%. Therefore, more efficient balancing methods are necessary.
5.5.1.2 Zener Diode Balancing
This balancing method uses Zener diodes instead of resistors as the bypassing elements to maintain a threshold voltage across each capacitor equal to
the threshold potential of the diode(s). The current passes through a connected capacitor and can be effectively limited upon reaching a defined
potential. This method results in an energy efficiency as high as 92%. For
example, if 16.3 kJ are used to charge an ES stack, 15 kJ are stored [1]. Zener
diodes can also dissipate the power beyond the local threshold voltage.
5.5.2 Active Balancing
As discussed above, passive balance schemes utilize shunts (resistors and
diodes) to dissipate energy to regulate cell voltages. The resulting circuits are
mainly suited for applications requiring low power or low current charging
and discharging rates. High charging and discharging rates within a short
time require a minimal loss in efficiency (non-dissipative equalization) to
optimize performance. Some active balancing methods have been developed
to achieve this.
Figure 5.12 shows an active balancing circuit scheme. In the figure, buck–
boost transistors or diodes can provide an equalizing current I eq for charging
current I. The direction and magnitude of the equalizing currents depend on
the local voltage across the corresponding single cell. For example, if a local
voltage exceeds the limiting voltage, a reverse current can be applied until
the voltage is balanced. This process actively proceeds along the entire stack
until equivalent voltages are established. Using this active balancing circuit,
the energy efficiency may be as high as 97%.
5.6 Cell Aging and Voltage Decay
An ES has an unlimited shelf life when stored in a discharged state, but
aging does occur and typically decreases the capacitance and increases the
resistance. The life of an ES specified by industry standards is a 20% decrease