Sizing the drive system - Propulsion Systems for Hybrid Vehicles - page 189

Unlike more advanced batteries, the NiCd unit has more restricted discharge
and charge acceptance rates as noted by (4.16) and (4.17). Furthermore, the very
restricted working voltage swing of only 28% is more of an issue for the combi-
nation ultra-capacitor bank. This is due to the ratio of minimum to maximum
working, or the voltage swing factor, s u , being only 71% (4.20). This means that
variation about nominal voltage U c nom = 1.25 V is constrained to +16% and 12%
according to (4.18) and (4.19).
Part of the system sizing study is the selection of system voltage. Work done
on this topic generally focuses on losses within the electrical distribution system
(EDS) of the vehicle. An overarching requirement is that voltage at the loads
remains within 97% of the source (battery) terminal voltage. This distribution
system efficiency requirement both drives the selection of cable sizes and places a
lower limit on distribution losses noted in Figure 4.29. In low voltage systems the
distribution system losses have a marked dependence on system voltage [32]. This
effect is illustrated in Figure 4.31, where load power is a parameter.

100%

95%

90%

85%

2 kW

80%

75%

70%

65%

6 kW

60%

55%

17 kW

50%

10 20 30 40 50 60 80 100

Bus voltage (V)

Figure 4.31 Electrical system efficiency dependency on voltage (courtesy Ford
Motor Co.)

To realize a net distribution and inverter component efficiency of 95%, as
listed in Figure 4.29 and noting that a power electronic inverter at high voltage will
have > 97% efficiency nominal, we see that our distribution system must have
> 97% efficiency. In Figure 4.31 it is easy to see that this means system voltage
levels of several hundred volts (off the chart in Figure 4.31 having a logarithmic
abscissa). For this example, a nominal system voltage, U b nom = 500 V, is assumed.
From this we calculate the required number of cells in a series string in the battery
module. Equation (4.21) quantifies the procedure:

U b nom
U c nom

N bc ¼ ð 4 : 21 Þ