PhD Thesis - Cranfield University

Chapter 2
2.6 Ultracapacitor augmentation issues
In investigating the details of ultracapacitor based energy storage systems, several circuit
implementation problems were found. The predominant limitation of ultracapacitor
technology is the single cell voltage limitation, which is currently 2.5 Volts. Because of this
limitation, multiple cells are connected in series to achieve a higher terminal voltage. Doing
so introduces a cell equalisation or balancing issue that is critical for both component failure
prevention and energy storage utilisation. On this subject, researchers investigated several
cell-balancing techniques. Linzen et al. [82] investigated four different cell-balancing
topologies. In essence, the authors of looked at both passive and active cell balancing
techniques and reported that a DC-DC converter type topology would be a practically
unattractive solution.
Barrade’s et al. [83] voltage sharing device used an inductor, which was switched between
adjacent cells via a pair of transistors. The configuration, based on buck-boost converter
topology, demonstrated the achievability of voltage balancing through some form of active
switching methods. Contrary to the conclusions of Linzen’s et al. [82], Barrade’s [83] DC-
DC based cell equaliser showed a practical feasibility. Along the same subject, Barrade in
[84] investigated the reduced energy storage capability of series connected ultracapacitor cells
if the voltage levels are not shared equally. In a recent (patent pending) design [58], Miller
and Everett [85], developed a non-dissipate charge equalisation circuit to address this voltage
sharing problem. Also based on active switching techniques, Miller’s circuit was designed for
a 15V bank of ultracapacitors specifically for the automotive industry
2.7 Alternative ultracapacitor system configurations
Typically, ultracapacitor banks have been designed as a fixed series of cells to satisfy the
terminal voltage demand. Multiple series strings can then be connected in parallel to increase
the energy storage capacity of the ultracapacitor bank. The fixed bank of ultracapacitors is
then coupled to a DC/DC converter that facilitates control of power flow. Moving away
from the fixed configuration topologies, Okamura [60] stipulated that a bank switching
topology is capable of achieving a 40% increase in usable energy of a ultracapacitor storage
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