Back in the day (not all that long ago), driving around in your father's Oldsmobile, AM/FM radio, tape deck, and air conditioning marked the height of automotive luxury. Today, many could not imagine a family road trip without satellite GPS, personal DVD players for each passenger, climate control, heated seats, cruise control, wireless access, and voice simulators to alert drivers that lights are on, the engine needs servicing or the road is becoming slick.
As the proliferation of automotive electronics and electrical accessories rises, how are manufacturers keeping up by providing sufficient power supplies? What happens in the event of voltage surge or sag? Is passenger safety jeopardized at the expense of greater luxury? What solutions are available to ensure that highly electrified vehicles maintain adequate power supplies?
Accessory power on the rise
Automotive electronics and electrical content is increasing at over 110W per year per car on average with no end in sight. Much of the recent rise in electrical burden is coming from electrification of historically mechanical and hydraulic powered systems. The introduction of anti-lock braking systems (ABS) in the late 1980s with an electronic control unit hosting approximately 8 kB of memory has evolved into modern ABS with more than 128 kB of memory in fifth generation hardware that is only 40% the package of early systems. Power requirements of modern electrical/electronic systems are typically lower than when first introduced due to design refinements and innovations.
The table below summarizes the major subsystems (and loads) of the automotive electrical/electronic content of a modern automobile:
Multimedia and heating, ventilation, and air conditioning (HVAC)
Lighting (exterior and interior)
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The electrical loads listed in the table are a mixture of continuous and intermittent functions. However, at a minimum the vehicle electrical charging system consisting of an alternator, storage battery, and the electrical distribution wiring (i.e., PowerNet) must support the full compliment of engine management functions along with most of the multimedia and HVAC functions on a continuous basis as well as portions of the remaining categories depending on driving conditions and customer use.
The two base categories from the table (engine management and multimedia and HVAC) place a demand on the vehicle electrical system for 102A. This is very significant and to illustrate the current state of accessory overload one must note that to support a load of 102A without battery augmentation, the alternator must be rated approximately twice this, or 204A. The reason for this is that the vehicle alternator can only generate at half its capacity at low engine rpm and at idle. This requires a large alternator, and at the nominal system voltage on the PowerNet of 14.2V, it must deliver 2,840 W.
When electrical loads from the remaining four subsystems are included in the vehicle load survey at various duty cycles, it is possible to easily overtax the alternator capability. When this occurs in the present automobile, the PowerNet voltage drops until the system voltage matches the battery internal voltage of 12.8V at which point the battery begins to support a portion of the total electrical load. This effect is referred to as the battery contribution.
The battery contribution becomes a cycling event as it occurs randomly for non-scheduled electrical loads, such as automatic temperature control of the passenger cabin or steering events, or deterministically for customer selected loads such as audio or navigation aides. These cycling events are what contribute to battery wear out and eventually lead to the need for battery replacement.
The last point to note regarding the table is that when intermittent loads from subsystems for body electrical, lighting, and chassis electrification are accounted for, the demands on the vehicle charging system truly are accessory overload. Future systems will continue this trend.