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Electrochemical Supercapacitors for Energy Storage and Delivery
on the type. For example a typical Li-ion laptop battery normally weighing 0.4 kg can only supply the computer load (25 Wh) for ~3 hours—insufficient for a full day of work or uninterrupted auonomous operation during
a typical airplane flight. With the current state of technology, the most practical laptop power is a Li-ion battery with a rated capacity of 4 Ah. Despite
considerable advances in Li-ion battery technology, the devices still cannot
keep up with emerging notebook usage features providing digital entertainment, multimedia, and wireless connectivity. The expanded usage demands
higher power and currents with higher slew rates.
To increase the run-time of a battery-powered device, the traditional
method is to simply increase the number of cells constituting a battery pack.
However, this solution only increases the size, weight, and maintenance
needs of a portable system and is not acceptable because it reduces device
portability and market acceptance.
ESs with low equivalent series resistances (ESRs) represent emerging
technologies [5,6]. These types of capacitors do not possess the energy density of a conventional battery, but their power densities are more than 10
times higher. These features make ESs good complements for improving
the performance of battery-based applications in which the battery supplies
the energy and the ES delivers short-term power. Benefits of this combination include enhanced peak power performance, run-time extension, and
reduction of internal losses. Current ESs are available with capacitance values ranging from 1 to 5000F, power densities of 4300 W/kg, and maximum
energy storage of 8125 J. Their ESR values range from 0.3 to 130 mΩ—five
times smaller than a conventional battery ESR. Thus, combining batteries
and ESs is advantageous for supplying pulsating loads.
The coupling of a battery and an ES can be achieved in three ways. One is
directly connecting the ES to the battery terminals. This requires the voltage
rating of the ES to match the battery, so several ES cells connected in series are
needed. The second method is to connect an inductor in series with the battery
and an ES in parallel. This improves the current distribution between the capacitor and the battery. The third option is using a DC–DC converter along with
an ES to interface with the battery. This scheme has the advantage of utilizing
a lower voltage ES interfaced to a standard battery pack by a DC–DC converter.
In addition, the connection of an ES in parallel with a battery has the
effect of reducing the peak value of the current supplied by the battery. This
reduces internal losses of the system and the root mean square (rms) value
of the current supplied by the battery. The power saved by this connection
results in additional run-time for the system.
An ES can relieve a battery from the most severe load demands by meeting the peak power requirements and allowing the battery to supply average
loads. The reduction of pulsed current drawn from the battery can result in
an extended battery life. Figure 8.3 shows the expected increase in the runtime of a battery after combination with an ES [4]. The figure represents a 2 A
GSM load on a 3.5 V 600 mAh Li-ion battery. Coupling an ES with a battery
Electrochemical Supercapacitors for Energy Storage and Delivery
on the type. For example a typical Li-ion laptop battery normally weighing 0.4 kg can only supply the computer load (25 Wh) for ~3 hours—insufficient for a full day of work or uninterrupted auonomous operation during
a typical airplane flight. With the current state of technology, the most practical laptop power is a Li-ion battery with a rated capacity of 4 Ah. Despite
considerable advances in Li-ion battery technology, the devices still cannot
keep up with emerging notebook usage features providing digital entertainment, multimedia, and wireless connectivity. The expanded usage demands
higher power and currents with higher slew rates.
To increase the run-time of a battery-powered device, the traditional
method is to simply increase the number of cells constituting a battery pack.
However, this solution only increases the size, weight, and maintenance
needs of a portable system and is not acceptable because it reduces device
portability and market acceptance.
ESs with low equivalent series resistances (ESRs) represent emerging
technologies [5,6]. These types of capacitors do not possess the energy density of a conventional battery, but their power densities are more than 10
times higher. These features make ESs good complements for improving
the performance of battery-based applications in which the battery supplies
the energy and the ES delivers short-term power. Benefits of this combination include enhanced peak power performance, run-time extension, and
reduction of internal losses. Current ESs are available with capacitance values ranging from 1 to 5000F, power densities of 4300 W/kg, and maximum
energy storage of 8125 J. Their ESR values range from 0.3 to 130 mΩ—five
times smaller than a conventional battery ESR. Thus, combining batteries
and ESs is advantageous for supplying pulsating loads.
The coupling of a battery and an ES can be achieved in three ways. One is
directly connecting the ES to the battery terminals. This requires the voltage
rating of the ES to match the battery, so several ES cells connected in series are
needed. The second method is to connect an inductor in series with the battery
and an ES in parallel. This improves the current distribution between the capacitor and the battery. The third option is using a DC–DC converter along with
an ES to interface with the battery. This scheme has the advantage of utilizing
a lower voltage ES interfaced to a standard battery pack by a DC–DC converter.
In addition, the connection of an ES in parallel with a battery has the
effect of reducing the peak value of the current supplied by the battery. This
reduces internal losses of the system and the root mean square (rms) value
of the current supplied by the battery. The power saved by this connection
results in additional run-time for the system.
An ES can relieve a battery from the most severe load demands by meeting the peak power requirements and allowing the battery to supply average
loads. The reduction of pulsed current drawn from the battery can result in
an extended battery life. Figure 8.3 shows the expected increase in the runtime of a battery after combination with an ES [4]. The figure represents a 2 A
GSM load on a 3.5 V 600 mAh Li-ion battery. Coupling an ES with a battery