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# Vehicle power question

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#### Njguy

##### Member
Is there a way to view how much power/voltage your alternator is producing, vs how much your car is consuming? In volts and or amps. Don't know how to explain it any better than that.

Alternator producing volts/amps...okay. I follow you there. You can easily measure how much voltage and amps it is producing by hooking up a meter.

But what on earth are you talking about when you say your car is consuming volts or amps?

Do not tell me you are connecting an alternator to your electric car's motor so that it produces electricity to recharge the battery as it runs so that it runs forever.

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Alternator producing volts/amps...okay. I follow you there.

What on earth are you talking about when you say your CAR is consuming volts or amps?

What I mean is, vehicle consumes power through electronics, headlights and such. Alternator produces power. How can you rig up two ammeters to judge the vehicle's consumption vs the alternator's production.

Do not tell me you are connecting an alternator to your electric car's motor so that it produces electricity to recharge the battery as it runs so that it runs forever.

Have no idea why you would come to that conclusion from a simple question. And I do not own an electric vehicle.

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What I mean is, vehicle consumes power through electronics, headlights and such. Alternator produces power. How can you rig up two ammeters to judge the vehicle's consumption vs the alternator's production.

Are there separate wires running from alternator to battery, and battery to everythign else? If not...it could be a problem.

If it's just alternator to everything else (battery included. it could be a problem.

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Are there separate wires running from alternator to battery, and battery to everythign else? If not...it could be a problem.

If it's just alternator to everything else (battery included. it could be a problem.

Ok never mind that then. Instead, would a voltmeter holding at a steady voltage indicate that the alternator is supplying enough power? My main question is, how does the alternator only produce enough amps for what is needed. Is it internally regulated?

Your car alternator has a voltage regulator, probably internal to alternator case, that regulates the voltage to charge the battery and run any loads. This is typically about 13.4 to 13.7 vdc at run steady state. The regulation is done by how much field winding current is put into the rotor field winding. Alternators in cars are three phase A.C. generators. There is rectifier diodes (six) making a full wave, three phase bridge circuit. These diodes are press-fit into alternator case for heat sinking.

The alternator will produce whatever current is needed by the system to try and maintain the 13.5v regulated level.

The size of the alternator determines its maximum current capability. Once the alternate reaches its maximum capability it 'slips poles' meaning it will not produce anymore output. This is not detrimental to the alternator.

Most cars have at least a 50 amp alternator. Larger alternators of 70 amp or 100 amps are also used. Police cars are usually upgraded to higher current alternator to run all the lights and radio equipment.

Easiest way to measure current is with a clip-on ampmeter with D.C. current capability. Just clip it around a single wire, being alternator output, headlight, radio, etc.

Pre-1962, there-bouts, cars with D.C. generators do not inherently limit current like modern alternators so their regulators also have a current limit regulator in addition to voltage regulator. If full voltage is applied to field windings the brushes will glow like an arc welder. (not good for generator)

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If you want to know the power from the alternator and the total power consumed you would need two ammeters and a voltmeter. You would connect one ammeter in series with the output of the alternator, and one ammeter (would need to be a bidirectional type ammeter made for automobiles) in series with the small wire from the battery that runs the accessories (not the large cable that goes to the starter).

The voltmeter can be connected anywhere.

The power out of the alternator is then alternator amps times voltage.

The battery power is battery amps times voltage. Since the power can be into or out of the battery depending on whether the battery is charging or discharging, then the battery power can be plus or minus.

The total power being consumed is then the alternator power plus or minus the battery power.

Perhaps I can help. Older cars had an ampmeter or ammeter which was in the line between the main line(say starter terminal) and the battery. When you first switched on it showed a discharge of power. When you started and the alternator produced current it showed a charge going into the battery.

This is still the case but you don't get the meter any more just the battery light which goes out when the alternator starts charging .

So if you just want to see whats happening , put an aftermarket ammeter in . While there. put a voltmeter in too which will show you the voltage accross the battery all the time.

If you want to show the amounts consumed and produced separately as I think you do then you would have to find a way of separating these .I don't know how you would do that because the alternators inbuilt regulator watches and only produces enough current to keep the battery topped up as you drive .

To test and measure the alternators output in isolation you put a full voltage wire from the positive to the field winding-small or no resistor .The alternator will then produce according to revs. Very risky with the new vehicles as you can blow it all up ,computer and all!

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ok i get it. Thanks for the info guys. Anyone know on averafe what the lowest rpm is to keep the alternator at 13 volts?

ther should be a 'cutout' unit that regulates the voltage...

ok i get it. Thanks for the info guys. Anyone know on averafe what the lowest rpm is to keep the alternator at 13 volts?
Typically an alternator will put out 13V at near 600 RPM engine idle (but little power) with the alternator speed at 2-3 times the engine RPM (as determined by the pulley size).

The answer to how much electrical power the alternator produces is ALL OF IT! If you "borrow" some charge from the battery during cranking or slow idle, the alternator replaces that charge when the engine is running faster. In the meantime, it is running the ECU, the fuel pump, the cooling fan, the radio, the running lights, the AC clutch&fan, etc, etc.

An alternator(at speed) is a current amplifier Iout=k*Ifield, where k ≈ 25, i.e. 2A of field current makes 50A of output current.

The Voltage Regulator watches battery voltage, and controls the field current to make the battery voltage a nominal 14.4V. If you turn on your 20A headlights, the battery voltage would sag slightly, the VR sees the battery sag, it cranks up the field current, the alternator puts out more output current to just balance the added headlight load.

Due to the field inductance, it takes the alternator about a 1/4sec to respond, so when a big load is addec suddenly, during that 1/4sec, a little charge comes out of the battery. As soon as the alternator output increases, it carries the total load again, with a small extra amount to replace the charge that was just borrowed from the battery. At the end of the day, the alternator produced ALL the energy that was consumed, because the state of charge of the battery is maintained.

My Cessna has the traditional zero-center ammeter in the positive lead to the battery. At slow idle, if all the lights, radios, avionics, pitot heat, etc is turned on (total load ~50A) , the alternator cannot keep up with that much load, and the ammeter will show a net discharge of 20 to 30A.

If you rev up the engine, the alternator speeds up, and can deliver up to 60A. At that point, the ammeter shows a net charge of 10A, because the alternator is supplying the 50A of load, and there is 10A left over to recharge the battery. The ammeter stays on the charge side while the battery regains the charge it gave up while the engine was turning too slow. It gradually tapers to zero, indicating that everything is back in balance, with the battery nominally fully charged...

If the alternator fails (or you turn it off in flight by pulling the field breaker), you immediately would see the total current consumption of the entire aircraft as a discharge on the ammeter. In the event of a failure, it is up to the pilot to react by shedding unnecessary loads, and by landing as soon as practical.

An accurate digital voltmeter monitoring the battery/main bus voltage will also tell you if the alternator is carrying ALL of the load, or if some load current is coming out of the battery. If the battery voltage remains at the nominal VR set-point voltage (say 14.4V), then you can be sure that no current is coming out of the battery (some could be going in).

If the engine is at idle, and the load exceeds what the alternator can produce (at that rpm), then you will see the battery voltage sag below the VR set point. As the engine/alternator speeds up, you will see the battery voltage come back up to the VR set point, at which point you can be sure that the alternator is carrying the total load, with some left over for recharging the battery.

ther should be a 'cutout' unit that regulates the voltage...

'Cutouts' are used with generators. Alternators use a "Voltage Regulator" (VR) or "Alternator Control Unit" (ACU)

Thanks guys your a wealth of information!

Perhaps I can help. Older cars had an ampmeter or ammeter which was in the line between the main line(say starter terminal) and the battery. When you first switched on it showed a discharge of power. When you started and the alternator produced current it showed a charge going into the battery...

Minor nit. The cranking current from the battery to the starter motor is never routed through the ammeter. The charge/discharge zero-center ammeter is typically -50A - 0 - +50 full-scale. The 500 to 1000A of cranking current would blow it up. Instead it is wired so that the alternator output feeds the main distribution bus (loads) and the ammeter is inserted in the wire that goes to the positive terminal of the battery (the starter is fed from a separate wire). That way, it reads the NET current in-to/out-of the battery.

Minor nit. The cranking current from the battery to the starter motor is never routed through the ammeter. The charge/discharge zero-center ammeter is typically -50A - 0 - +50 full-scale. The 500 to 1000A of cranking current would blow it up. Instead it is wired so that the alternator output feeds the main distribution bus (loads) and the ammeter is inserted in the wire that goes to the positive terminal of the battery (the starter is fed from a separate wire). That way, it reads the NET current in-to/out-of the battery.

Err Thats correct and I did not say it does , or I hope thats what I said.

The line is attached to the positve point (screw and nut) to which the main 60 amp cable attaches to the starter on most cars I have seen , if you have a look. The starter current does not flow through the ammeter

In the circuit:

|-------./.------------------Starter-----------------------(-)
|
|
|---------------------------+Battery-----------------------(-)
|
A
|
|------------B-------------- Alternator--------------------(-)
|
C
|
|----------------------------Fuse box supply--------------(+) to lights, radio, wipers, etc.

An amperimeter inserted at A shows the current the battery is delivering or accepting, which can be charging minus consumption.

An amperimeter inserted at B shows the current the alternator delivers to the battery plus to the car circuits consumption

An amperimeter inserted at C shows the current the car circuitry consumes.

Choose where/what you want, can install more than one amperimeter.

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