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Keys to Vehicle Accessory Overload Prevention

Wed, 02/21/2007 - 7:45am
By John M. Miller

As technological innovations for automotive improve, driving around in your father's Oldsmobile, complete with AM/FM radio, tape deck and air conditioning, no longer marks the height of automotive luxury. Today's standards of technological luxury include 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. When over electrification problems occur, there are actions that industry designers can take to avoid a potentially hazardous problem.

Increasing Accessory Power
As new technologies come out, the modern automobile's electrical and electronic content continues to increase at over 110W per year on average. The majority of the electrical burden stems from electrification of historically mechanical and hydraulic powered systems. The introduction of anti-lock braking systems in 1985 to 1989 has evolved from 8 kB of memory to today's standard of more than 128 kB of memory.

Table 1 summarizes the major electrical/electronic content of the modern automobile in five major subsystems: (1) Engine management, (2) Multimedia & Heating Ventilation & Air Conditioning (HVAC), (3) Body Electrical, (4) Chassis Electrification, (5) Lighting (Exterior & Interior), and (6) Future Systems.

At a minimum, the vehicle electrical charging system consisting of alternator, storage battery, and the electrical distribution wiring (i.e., PowerNet) must support the full complement 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 usage. Taking only engine management and multimedia and HVAC from Table 1, places a demand on the vehicle electrical system for 102A. 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 is a large alternator and at the nominal system voltage on the PowerNet of 14.2 V, it must deliver 2840 W.

When electrical loads from the remaining four subsystems are included in the vehicle load survey at various duty cycles, it becomes possible to overtax the alternator capability. In the present automobile, the PowerNet voltage drops until the system voltage matches the battery internal voltage of 12.8 V at which point the battery begins to

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Table 1
support a portion of the total electrical load in a process referred to as the battery contribution. This becomes a cycling event, as it occurs randomly for non-scheduled electrical loads such as automatic temperature control of the passenger cabin. The end result is a contribution to battery wear out as well as the eventual need for battery replacement.

When intermittent loads from subsystems for Body Electrical, Lighting, and Chassis Electrification are accounted for, the demands on the vehicle charging system are accessory overload. Automakers are therefore taking steps to reduce the electrical system loading caused by the pervasiveness of electronics including, improvements in the efficiency of existing electrical accessories, functional integration to eliminate duplication of control electronics so that housekeeping logic power demand is lower, and through down sizing actuators.
PowerNet Stabilization
Today automotive manufacturers are looking at the use of an ultracapacitor distributed module, or local energy cache, that provides PowerNet smoothing and stiffening in the locality of the ECU concerned.

Switching of high power loads imposes significant disturbances to the vehicles electrical distribution network, the 14V PowerNet. Electric power steering for example has power demands of 130A to as high as 160A in some recently proposed EPS designs. In the past it was assumed that EPS demands in the 85 A (1.2 kW) to 130 A (1.8 kW) range represented worst case PowerNet loading to the point that the EPS function would be compromised. Placing a transient burden of 1.2 to 1.8 kW on the

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Implementation of Ultracapacitor or Distributed Modules
PowerNet when the engine is near idle and continuous loads are already at 27 A plus 67 A, or 1.3 kW, means that the electrical distribution system voltage dips from 14.2 V to 12.8 V, the battery potential. This 10% drop in distribution voltage will be noticeable first as headlamp dimming and secondly as EPS performance degradation, not to mention the PowerNet transient being broadcast to all other connected ECU's.

The benefits of distributed modules, or localized energy caches, provided by carbon ultracapacitor technology are very effective in PowerNet stabilization and help smoothen and stiffen the power line at even remote branch circuits in the automobile.
What the Future Holds
The trend towards accessory overload will continue to expand along with the rise of automotive function and feature content. Ultracapacitor distributed modules are shown to be one way to lessen the PowerNet corruption caused by load switching and load shedding of electric actuators, pumps, fans, and compressors.

Distributed modules are the answer to avoiding loading the passenger vehicle with two or more lead acid batteries that would be necessary to provide sufficient filtering of the electrical system. At some point, it will become necessary for automakers to do a PowerNet reset and increment the distribution voltage up by a factor of three to the proposed 42V standard. This action will cut the distribution currents discussed in this article by a third. Moreover, interim systems are already moving in this direction by locally boosting the 12V battery supply to 30 V and higher for specific functions. EPS is one such function. Micro-hybrid and related idle-stop systems are further examples of the need to provide higher voltages in the automobile.

About the Author
John Miller is vice president of Maxwell Technologies, 9244 Balboa Ave., San Diego, CA; (858) 503-3300.

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