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power supply distribution in car

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banh

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normally, in car system, how is the power supply distributed?

as far as i understand, there is battery, and there is alternator.

so, for example, power supply to the dashboard is from only battery or from both battery and alternator?
 

v_c

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Alternator is driven by a belt that is attached to the engine. The alternator shaft speed is roughly three times the engine speed due to the pulley ratio. So if you see your engine speed idling around 600 rpm, then your altenator is roughtly rotating at 1800rpm. The output voltage of the alternator is regulated at around 14V. This output is attached using a thick cable to the battery. Things that are more power hungry are connected closer to the alternator whereas the loads that are voltage sensitive are connected closer to the battery. Normally, the alternator is supposed to be able to deliver current to all of the loads and have enough current capability to charge the battery. However, there are cases (for example when the engine idling -- at at a stoplight and there is lots of power consumption), the alternator may not be able to provide all the current required. In that case, the balance of the current demand is taken out of the battery and the system voltage will go lower than the usual 14V.

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v_c
 

IanP

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Have a look at this simplified schematic of car's electical circuit ..

As you can see, when alternator generates power, supply to most of car's receivers comes from both: battery and alternator, and the battery works as big smoothing "capacitor" ..

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IanP
 

v_c

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I think today's lead-acid batteries are mostly designed to start the engine and have a load-balancing effect. But the alernators are sized so that you don't draw continuous current from the battery. In other words, you are supposed to put back the charge that you took out of the battery in normal driving conditions. If you don't put the charge back, or if the state-of-charge battery goes too low, the battery will begin to age and eventually it will go flat. Everytime you take too much charge out of the battery, the battery is cycled and each battery has so many of these "cycles". Once you use them up --- the battery is done!

But I agree with you ... the loads will draw currents from the battery (when they need peak power). And some vehicles have undersized alternators and/or way too many electronics that draw lots of current. These vehicles must replace their batteries every couple of years due to heavy cycling.

This is a good discussion ....

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v_c
 

banh

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However, there are cases (for example when the engine idling -- at at a stoplight and there is lots of power consumption), the alternator may not be able to provide all the current required. In that case, the balance of the current demand is taken out of the battery and the system voltage will go lower than the usual 14V.

So it means that every subsystem in the car should have a switch-over circuit, i.e. to switch between battery or alternator when whichever voltage is higher?



from IanP's diagram, it seems that battery & alternator are all "mixed" together?

what does it mean by the battery as a big "smoothing capacitor"?

From alternator (after being regulated): 14V, and from Battery : 12V --> does it mean that all systems
should work with 12-14V power supply? originally i think most systems work will 12V supply..
 

IanP

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Mind you, so called "12V lead-acid" fully charged battery has ≈13.8V voltage, so if you connect something to the car power system allow for that margin ..

In most cases alternators are three-phase generators equipped with three-phase rectifiers; the output voltage is not "pure" dc, but has ripples (see picture below).
A battery can be treated as almost ideal voltage sources, so its internal resistance (impedance) is close to zero; if a voltage with ripples is connected to a battery the ripples will be "smoothed" by the battery and the voltage at the battery pins will be, or should be, pure dc ..
Connecting an alternator (with rectifier) to battery means that current flows to battery at the peaks of ripples and is taken from battery in between .. Obviously, the former has to be higher than the latter, and therefore battery will remain its charge ..

Regards,
IanP
 

    banh

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v_c

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Actually, everything is connected to the same bus with everything seeing roughly the same voltage which should be about 14V when the alternator is operating properly. Note that the battery voltage of 12V refers to the open circuit (no load) voltage of the battery which is actually about 12.6V (2.1V/cell times 6 cells). So no "switch over" circuit is required.

Battery is can be viewed as a large capacitance (it does store charge after all). Since the battery behaves as a large charge reservoir (with a large effective capacitance value), it is behaves as a pretty constant value voltage source, so when large current demands and spikes are demanded from the system, the battery provides these currents without its voltage changing much, so in effect keeps a smooth voltage in the presence of these transient loads.

In fact, most of the electronics placed in the vehicle bus will work down at 6V and probably as high as 27V. When you crank the engine, the voltage does go down quite low and there are systems that need to be functioning at these voltage levels. Also very high transient voltages can exist in the system, most of these are short-duration (low energy) pulses and there is sufficient protection in most subsystems like engine electronics to supress and filter these. There are however other conditions such as reverse battery (-14V) or jump start (28V - from a tow truck) conditions that present sustained dc voltage levels --- and the subsystems are designed to survive these as well.

Best regards,
v_c
 

    banh

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