Environmental Engineering Reference
In-Depth Information
of 36 m
W
,or1m
W
per cell including interconnects. Because of the serious lim-
itations of NiMH at cold temperatures, some form of climate control system is
necessary (heating element that discharges the battery) or some additional energy
buffering is needed, such as an ultra-capacitor.
Two examples are presented here of hybrid electric vehicle performance in
different conditions [3,4]. In the first example Nissan performed test drives of
identical Tino hybrids, one with NiMH and one with lithiuim ion battery packs in a
cross-country route to examine ESS efficiency. The second example illustrates cold
weather influence on a Toyota Camry hybrid and Ford Escape hybrid during winter
testing at the Argonne National Laboratory.
Performance was improved by replacing the NiMH pack in the Tino hybrid
with Nissan's in-house spinel based lithium ion cells processed with thin electrodes
and laminate structure that provided twice the power (2.5 kW/kg) of NiMH, twice
the energy (140 Wh/kg) in half the pack size [5]. Thermal management was
facilitated by the laminate cell design. Two of the Tino hybrids were driven the
same route from Los Angeles to Las Vegas to Phoenix and back to Los Angeles and
their battery performance compared. The results are listed in Table 10.2 for
reference.
Table 10.2 Status of electric energy storage systems in Tino hybrid vehicles
d
SOC (%)
P
eff
(kW)
T
batt
(
C)
Ah
eff
(%)
Wh
eff
(%)
C/D power restraint
frequency (%)
NiMH
40-80
4.25
52
91
83
44
Li ion
30-85
4.28
49
98.8
95.1
2.2
Table 10.2 shows that for this field trial both the NiMH and lithium ion packs
operated over a similar state-of-charge window (
d
SOC), delivered roughly the
same average power (
P
eff
), and all the while the lithium ion pack ran slightly cooler
(
T
batt
) and had substantially higher Coulombic (Ah
eff
) and energy efficiency
(Wh
eff
). What is most revealing is what Nissan engineers refer to as the battery
pack charge/discharge (C/D) power restraint frequency (last column in Table 10.2).
That is, the number of times (as a percentage) that the battery management system
(BMS) was required to limit the battery discharge power or to limit the rate of
braking energy recuperation due to various factors, including the pack temperature.
It can be seen in Table 10.2 that lithium ion delivers far superior performance than
NiMH in real world driving conditions.
Argonne National Laboratories also performed road evaluations on commer-
cially available hybrid vehicles to determine the impact of battery temperature on
vehicle fuel economy and braking energy recuperation limits [6]. Figure 10.12
illustrates the rate at which the vehicle NiMH packs curtail fuel economy as the pack
temperature decreases in cold weather to
10
C and below. This is essentially the
