–
+
Current
sensor 2
–
Pulse load
Ultracap
i Load
i Conv
Controller
Duty
In1
In2
_
Converter
Current sensor 1
Conv_avg
H
D
L
+
T
Battery
Simulink block
253
Coupling with Batteries and Fuel Cells
FIGURE 6.3
Representation of actively controlled hybrid battery/supercapacitor system. (Source: Gao, L., R.
A. Dougal, Member et al. 2005. 20, 236–243. With permission.)
With this method, the safety limit of the battery is not exceeded. A parallel
charging of the supercapacitor can occur at this time as well. At high load
currents, both the battery and supercapacitor work simultaneously to supply
power; however, the controller maintains the steady discharge of the battery
while the supercapacitor supplements the remaining high current.
A comparison of passive and active control using this topology was conducted by Gao, Dougal, and Liu [4] with investigation of the power capacity
and discharge cycle time of each HESS. Their results (Table 6.2) were gathered from a simulation performed in a virtual test bed environment with
pre-validated models of a Sony US 18650 Li-ion battery and a Maxwell PC100
supercapacitor, and further validated with a hardware prototype [4].
This coupling of battery and supercapacitor demonstrated some successful improvements in the power capacity of a passive hybrid that measured
2.3 times that of a standalone battery [4]. A further 4 times increase in the
power capacity was realizable by incorporation of a controlled boost DC–
DC converter. However, the benefits of implementing active control for
enhanced power capacity and reduction of battery loss are offset by the
cost. Furthermore, the discharge time could also be reduced (~14.4 min) for
a battery to go from complete charge to discharge in comparison to passive
control. Therefore, reduced operating time and added energy loss are both
attributed to the power loss through operation of the converter, and supercapacitor loss from its operation at high currents. Figure  6.4 describes an
evaluation of the energy loss as a function of duty cycle for both passive and
active hybrid ESS circuits.