Environmental Engineering Reference
In-Depth Information
Other conditions are not so benign. For example, in hybrid vehicles the traction
battery is classified as an emissions related component. This means that its ability
to continue functioning must be continuously monitored and checked. When the
battery, or other ESS, component loses 20% of its capacity it will be considered
worn out. For an advanced battery, wear out usually means upwards of 200,000
charge-discharge cycles. In a hybrid propulsion system, this number of events
should be sufficient for 10 years and 150,000 mi warranty interval.
There are currently efforts under way to extend the warranty interval to 15
years and 150,000 mi for hybrid batteries. This means that the battery, as an
emissions system regulated component, must sustain its capability over the given
warranty period or be replaced by the manufacturer if it wears out sooner. Detec-
tion of wear out requires accurate and reliable monitoring. This has led to various
suppliers working on battery life models and other means to detect battery wear
out. The most notable of these activities has been the effort led by Johnson Controls
Inc., in cooperation with the MIT-Industry Consortium on Advanced Automotive
Electrical and Electronic Systems and Components and its member companies, to
develop a battery available energy monitor. This model has been shown to give
accurate and repeatable results for lead-acid battery systems, but
it remains
unproven for long term monitoring.
4.7 Supporting subsystems
It should be understood that hybrid vehicles require electrically augmented steer-
ing, braking and climate control systems. The vehicle steering system must be full
electric assist, or electric over hydraulic, as a minimum to ensure that steering boost
is available even with the engine off, regardless of whether the vehicle is at rest or
in motion. Similarly for the brakes since engine vacuum is not available during
idle-OFF mode. In fact, some mild hybrid implementations use separate elec-
trically driven vacuum pumps for the brakes during engine-off periods. Cabin cli-
mate control
is the most energy intensive engine-off
load. The following
subsections elaborate on each of these topics.
4.7.1 Steering systems
As a general rule of thumb, when a vehicle steering mechanisms rack load exceeds
about 8 kN, a low voltage, dc brush motor, electric assist may be inadequate for
acceptable steering boost performance. The range of rack loads from 8 kN to
roughly 12 kN defines a transition during which 14 V electric assist must give way
to 42 V PowerNet systems. The low voltage 14 V power supply is not adequate to
source the instantaneous power demanded by steering systems having high rack
loading. Above 12 kN of rack load, regardless of vehicle type, the electric assist
steering is best served from a 42 V PowerNet vehicle power supply.
BEVs will generally operate their electric assist steering from the traction
battery. However, this requires attention to high voltage cabling and proper circuit
protection. For distribution voltages greater than 60 V, it is accepted practice to
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