PhD Thesis - Cranfield University

Chapter 8
From the results of the experiment, practical reference parameters can be extracted as the
PMS battery operating constraints. Figure 8.8 presents the measured current and voltage
waveforms for Segment 1 as projections of the desired voltage boundaries to the
corresponding current levels. In the voltage plot, the lower battery voltage is first defined to
be approximately 45V. The point x1 marks an instance where the battery voltage crosses
beyond this lower limit. The projection of x1 on the time scale to the battery current
waveform gives the corresponding current value, which is marked by intersection point x2.
From the current plot, the maximum battery current is found to be 150A, thus specifying
that the battery power limit (V x I) is to be set at a value of 6.75kW. Similarly, the upper
voltage boundary specifies the charging power (or regenerative power) limits. The
intersection values and projections using marking points x3 and x4 gives a maximum
charging current of 25A . As was discussed previously, charging the battery this way is not
efficient and ideally the minimum battery power should be zero with all regenerative braking
power handled by the ultracapacitor instead.
Battery Voltage (V)
Battery Current (A)
Desired Battery
working voltage boundary
Defined Battery
working current boundary
x2
x1
x3
x4
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Time (s)
Time (s)
Figure 8.8 Determining battery operating limits empirically