Automotive/Car Power

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Expert: - 5/6/2007


Question
My question is really basic and relates to basically all cars, not only a specific type of car.
When a car is turned on, does it get it's power source (meaning headlights, CD
player, other electrical stuff) from the gasoline being converted to
energy(electricity) or its taking power straight from the battery? Also, can you explain
how the car gets its power when the power is only on, not the engine (meaning
when the car key is turned half-way).
I know my question might sound silly, but I am very curious about this topic and
couldn't find a real answer on the web. Thanks for the help.


Answer
Happy to answer a basic question, especially when it is well articulated!

Automotive electrical systems are, by and large, direct current, or DC.  In that regard, cars along with most other land, sea and air vehicle share a "common ground" (sorry about the pun!) in that they rely on a battery system as an initial - or startup - source of power.  

Cars, trucks and buses need a starting battery to provide the energy to run a DC starting motor required to crank the prime mover (engine) which moves the vehicle.  For design convenience, that battery-based system will also power all other systems and auxiliaries needed on the vehicle, including lights, engine ignition systems, wipers, ventilation motors and your radio and CD player.  Some items, the headlights for example, can be turned on at any time; unfortunately, they can also inadvertently be left on, discharging the battery!  Other items, the blower or your CD player for example, can be operated if the keyswitch is turned to the accessory position.

The problem with batteries is that they only STORE energy, and will discharge as loads draw that energy from them.  Therefore, they must be charged.  Further, most automotive electrical loads are really too large for a battery to supply for any length of time.  Therefore, it is imperative that a charging source of supply be available that will not only charge the battery, but will also carry the bulk of the loads that will be used when the car is on the road.

The charging and ongoing supply task is taken on a an electrical generator, run by the engine once it is started.  That generator can range up to a kilowatt (kW) or more on some cars.  The generator on modern (post 1964) cars is a rotating-field machine, specifically an AC three-phase alternator from which the output is rectified to DC for use by the battery and the rest of the automotive loads.  That an AC generator is used may raise a question, easily answered by the simple fact that the rotating-field generator is cheaper to make, requires less servicing, is more efficient and had higher power density than the old stationary-field DC generators once used.  Too, it is easy to rectify three-phase AC to smooth DC with low ripple.

I'll stop here as I need to get to bed - 0600 arrives much too soon!  I hope this helps.

Cheers ... EGK  

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Ernest (Ernie) Kenward

Expertise

The challenges I most enjoy are thoughtful technical questions of a trouble-shooting nature in both electrical, power electronic and mechanical systems, mainly automotive but also machine control and small-machine PLC applications. Please note, however, that I am NOT a walking shop manual! I DO, however, make it a point to have those manuals and other service literature for those vehicles I DO own, and highly recommend that anyone serious about maintenance or modification of their vehicles do the same; MOST of your answers WILL be found there. For that matter, I do NOT go out of my way to acquire shop manuals for any vehicle I do NOT own! That being the case, any general query to me along the lines of "What is the meaning of this code read from the ECU of my 2015 XYZ?" or "Where is the fuse for the windshield washer pump found?" (try your car's electrical distribution panel for a start!) will not go far. What I do offer is a pretty good collection of literature, insights and hands-on experience with 1950s to 1980's Ford products (plus a developing database of information and practice with the Mercedes diesel cars), along with an engineering perspective and the ability to design and implement custom control, electrical and mechanical subsystems for vehicles. For that reason, I am happy to make my thoughts and efforts available to those who are of like mind and/or are seriously making a point of learning about their vehicles. Use the Opportunity to Learn!

Experience

A key skill in my work and hobby pursuits both is STRATEGIC TROUBLESHOOTING. I am a senior instructor in Electrical Engineering Technology at a leading Canadian polytechnic, my areas being Electrical Power and Industrial Control, electrical and electronics design and manufacturing, and AutoCAD and related CAD/CAE software - plus equipment problem-solving and new equipment design and prototyping. Hobby-wise, I have 30-plus years of experience in auto restoration, mostly in electrical and mechanical systems. Ongoing projects include a 1959 Edsel Corsair, my 1978 Ford E250 class-B motorhome conversion, and the care and upkeep of my Mercedes 300CD. My vehicles become engineering test beds for electrical and mechanical upgrades as ideas present themselves. This includes the design and production of circuit boards to restore or enhance features for which no OEM replacement parts are obtainable, or where better specifications or reliability can be had via newer concepts. Regarding the E250 RV conversion, I designed and continue to revise a custom power distribution system, managed by a Programmable Controller (PLC); this has made most revisions as easy as uploading new firmware as I develop it. The "mini" PLC is a powerful device for custom automotive control systems. One good example (there are many) would be the Moeller "Easy Relay"; these offer a wealth of control, monitoring and variable-and-status display options for such projects. A good example project which has worked well is that one for my RV noted above, which has been on the job - revised in firmware only - for a decade now. It is a load management and charging control system to avoid the sulfation-induced early failure that often befalls deep-cycle batteries used in RV power applications. The battery installed in 2003 lasted long enough to more tnan pay for the PLC that contributed to its longer life ... and the PLC will be there for the next battery as well!

Organizations
IEEE - senior member ... past WCC Student Activities; SME - senior member ... past chair, greater Vancouver chapter chair 318; Edsel Owners' Club - have served in various capacities on chapter executive during seventies; have been Power and Driveline resource on the Edsel Owners' Club "E-team" for more than a decade.

Education/Credentials
Graduate of UBC

Awards and Honors
Certificates of appreciation from IEEE and SME for work in student and chapter activities

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