PhD Thesis - Cranfield University

Chapter 3
In section 3.11, it was shown that the estimation of the batteries using Peukert’s equation is
acceptable as long as the batteries are not operated at high power rates. In fact, Peukert’s
equation was developed to estimate battery capacity under constant current discharge [110].
However, for a strategic power and energy management system that limits the power
delivery of the batteries, the use of this estimation technique is justifiable but by no means
ideal.
From the understanding on battery behaviour and operating limitations stems the knowledge
that an increase in both the energy efficiency and battery life can be expected if the battery
ideal operating constraints are followed. This translates to operating the battery system
within some operating boundaries and mitigating peak powers and high power discharges to
the ultracapacitors. For the battery system, parameters that need to be considered are the
battery discharging power limit, the charging power limit, the rate of discharge and the rate
of charging.
The simulation of hybridising batteries and ultracapacitors showed that the mitigation of
peak powers to the ultracapacitors results in a weight reduction in comparison to a battery –
only system. Although the simulation only divides the power demand into two segments
with no active power blending performed, it does demonstrate and support the grounds of a
battery-ultracapacitor system for loads having large peak to average power ratios. The next
chapter discusses the power and energy demands of a typical electric vehicle and follows to
presents a further case study of hybridising batteries and ultracapacitors.
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