Environmental Engineering Reference
In-Depth Information
environmental conditions can subject it to lows of 40 C and under-hood condi-
tions can raise it well above the 70 C limit. Generally, vehicle batteries are located
in more benign locations such as in the trunk, behind or beneath the rear seat or
beneath the vehicle floor pan. The trend in battery electric vehicles (BEV) is to
package beneath the vehicle floor pan or centre tunnel, over most of the vehicle
length and protected from water splash and gravel impacts by a shield plate. BEV
and plug-in hybrid electric vehicle (PHEV) battery packs are now being installed
with provision for cell or module heating for colder climate operation. In
Figure 10.5 the General Motors Chevy Volt, a range extended vehicle, has its
16 kWh lithium ion pack in T-shape along the centre tunnel. Some battery che-
mistries are very sensitive to overcharge and overdischarge. Lithium ion systems
are a good example. Lithium ion has far more chemical energy than electrical
energy storage, so its stability and charge/discharge must be carefully monitored
and controlled, especially in high cell count series strings where any imbalance
may take certain cells to overvoltage (OV) or undervoltage (UV) conditions.
Figure 10.5 Chevy Volt PHEV car and phantom view and its battery pack
10.1.1 Lead-acid
Lead-acid secondary cells are used pervasively in automotive systems as standard
SLI batteries in conventional vehicles, BEV, low speed neighbourhood EVs, NEV
and low end hybrid vehicles. Recent improvements to SLI batteries since the
development of maintenance free batteries in the 1970s have been the use of cal-
cium as a hydrogen getter, other additives such as antimony for sulphation control,
better current collectors and expanded grid assemblies. The typical Pb-acid system
has a cell potential of 2.1 V, gravimetric energy content of 35-50 Wh/kg and
volumetric energy of 100 Wh/L:
Pb þ PbO 2 þ H 2 SO 4 ¼ 2PbSO 4 þ 2H 2 O
ð 10 : 16 Þ
Lead-acid batteries are typically characterized at a C /20 discharge rate, where
C is the capacity of the battery (Ah) at 20 h rate. Higher discharge rates incur higher
internal losses and lower resultant useful power. Figure 10.6 illustrates the voltage-
current discharge behaviour of a Pb-acid battery with discharge rate as a variable
and temperature as a fixed parameter.
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