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MG MGB GT V8 Factory Originals Technical - Don't know my amps from my elbow!

I suppose that my parents should have told me when I was a kid but they were too embarrassed. How many amps can you have drawing of a car electrical system at one go. I'm planning better electric fans and electric water pump etc and I'm worried that the whole thing might fall over. I have a suspicion that it is possible to approach this all scientifically and to calculate how much I can run. I already use a 72 amp alternator which I had left over from a Landrover and also a high power Bosch battery - 68 amp, I think. But is there a formula? - answer in very simple language, please.

You can have as many amps as you wish, so long as your alternator has enough power.

At some point, if you _really_ get carried away, the alternator will become difficult for the motor to turn (of course ... no such thing as a fee lunch). The hot rod shops would have you believe that my making everything electric, you'll have no power drain ... that may be true for dragsters running without alternators but for the rest of us the laws of physics/thermodynamics/whatever you call it govern.

So do I understand that the more electricity required, the harder the alternator is to turn? This, I don't understand. What is it that is producing the extra resistance/friction? I thought that it was merely some coils spinning inside a magnetic field and not touching the sides.
Could you also say that the more amps you are asking from the alternator, the quicker it is going to wear out?
What is the amperage of a pair of 90w headlight bulbs?

Divide the wattage by the working voltage to derive amps. Most people use 12 volts as the working voltage number on an automobile, but it can vary from about 11 to 14.7 volts depending on circumstances.
George B.

>So do I understand that the more electricity >required, the harder the alternator is to turn? This, >I don't understand. What is it that is producing the >extra resistance/friction?

Think of Hoover Dam. It takes a ton of effort to turn those generators, because they are being asked to spit out a ton of current. Of course it isn't touching, but there's magnetic resistance. Surely you've noticed that your car slows a little at idle when you turn the headlights on.

>>>>So do I understand that the more electricity required, the harder the alternator is to turn? This, I don't understand. What is it that is producing the extra resistance/friction? I thought that it was merely some coils spinning inside a magnetic field ....
Exactamundo, it's having to overcome the magnetic field - have you ever tried to pull two magnets apart ?
You will find an excellent primer on car electrics on Paul Hunt's site (fellow V8-er)...see and go 'electrics'

Fine, but no one is explaining why (presumably) the magnetic field is getting stronger the more the power draw and there fore requiring more work from the engine to turn the thing round.
I'm afraid that I had always thought that the magnetic field was a constant and that to get more power out of the alternator it had merely to spin faster which is simply a function of emgine speed. Tell me I'm wrong but tell me why.

I'll leave that to someone else - I failed A level physics (twice!)


I've prob missed point on this one but to me the Battery supplies power. The alternator is there to keep battery topped up.

Your fans and lights radio etc if on draw amps, your car electrics and electric water pump will be on all the time.

Being very simple at this stage, If you add all amps together and is more than 72 you will start draining your battery, assuming you are charging at full rate, which will not be so in a traffic jam.

My problem with uprated alternators is how far you can push the alternator wiring and this may need to be uprated to get full use out of alternator.

The very simple answer is if you do not drain the battery your system is Ok. If you wish to monitor situation best to get some add on guages.

I'm no electrician so above may be completly incorrect.

Marc - think of it this way: You don't get owt for nowt. Which means you can't get something for nothing. You can't make or destroy energy, only change its form. You want more amps to power lights and sound systems? Where is that energy going to come from (the majority of which is going to end up heating the atmosphere)? Electrically from the alt, mechanically from the engine, and ultimately from the petrol. If an alt could supply additional power *without* additional engine input it wouldn't need *any* engine input. So you could spin the alt with your little finger and it would go merrily on supplying whatever power you needed - so you wouldn't even need an engine.

An alt supplies a relatively constant voltage irrespective of rotational speed, and is capable of supplying any current within its design limits at a wide range of engine speeds - its output current at any time is simply controlled by the load at that time. An increase in load will tend to drop the output voltage due to the internal resistances (losses) within the system, the voltage regulator detects this and increases the voltage it applies to the field windings, which increases the magnetic field, which restores the output voltage to what it should be whilst now supplying more current. The crucial point is "increases the magnetic field" which requires more energy from somewhere i.e. the engine.

You can up the alt max output to a certain point but then you have to start considering the wiring. Some markets/years of MGB had doubled-up wiring from alt to solenoid and solenoid up towards fuse-box etc. for extra current carryng capacity. If you are adding many 'standard' loads you can use standard wiring for each load, but may have to put heavier gauge in from where they all come together back to the alt. If you are putting on a single big load it will need heavy gauge all the way, including any ground wire.

Within the specified max load for a given system and the specified (and working!) alt it is the alt that is supplying all the cars electrical load - the alt has an output of about 14v when running, the battery only about 12v. Thus the battery is continually being *charged* when running - it has to be or very soon it wouldn't start the engine.

Paul Hunt

Well of course I am very grateful for everyone's input but clearly I'm a poor student because I still don't understand why the alterntaor needs more grunt to turn it the more electricity demanded.
However, and by coincidence, I visited Beaulieu today (the National Motor Museum) and amazingly in the kiddies part there was a touch exhibit where you turned a crank handle to produce power and a set of headlights would come one. The harder you crank, the brighter the lights. The interesting part was that you could switch on more and more lights. I was amazed. It's really true! The more light swtiches I switched on, the harder that crank was to turn. I had someone switch them all off and you could really feel the crank handle needing less work. I am really impressed. I now truly believe and I am sworn to evangelise and spread the

...good news - but I still don't understand why and so far no one on this thread has explained.
Marc (contd)

Actually, PaulH did explain (sort of). You need a decent textbook on electro-magnetism to go much further. The important bit is "An increase in load will tend to drop the output voltage due to the internal resistances (losses) within the system, the voltage regulator detects this and increases the voltage it applies to the field windings, which increases the magnetic field, which restores the output voltage to what it should be whilst now supplying more current."
Now, back to playing with magnets when you were a kid: remember how some magnets are weak and some are much stronger when you try to pull them apart? This resistance is what the alternator belt and engine is overcoming every turn of the pulley. Greater use of electricity in the car causes the voltage regulator to increase the field winding current, which increases the magnetic field. Increasing the magnetic field is 'making the magnets stronger' which is what makes it more difficult to turn the pulley...
Now I do know that magnetism and electro-magnetism are 2 different subjects and there are some differences, but I think this is close enough to explain the fundamentals. Are you still with me ?

>...good news - but I still don't understand why and >so far no one on this thread has explained.


We love you dearly but stop being pathetic, lad. It's not that difficult a concept to master the basics of. People have given you half a dozen heads-ups -- it's not like we owe you a treatise on the subject. Run along, now ...


Nice, Harry.

Quite unnecessary. Friend of Sam from NY, maybe.

It's very simple. You must understand that it takes energy to make energy, so the alternator only converts the mechanical energy of the engine into electrical energy. An interesting characteristic of an alternator is that it produces energy on an "as demanded" basis, so the more amps you draw, the more energy is drawn from the engine to produce those amps. For example, if you're drawing 36 amps, it'll take only half as much mechanical energy from the engine to produce those 36 amps as when you're drawing 72 amps. Thus, if you were to measure the power output of the engine at the rear wheels with a very sensitive dyno you would discover that you'd get a slightly lower output with all of the electrical devices switched on. Also, remember that you can't take out any more than you can produce. If your alternator produces 72 amps, then you can only consume a maximum of 72 amps. Any more than that and you'll overload the system and things will start to burn up.

Having a bad day?
Steve S.

Piss on 'em, Harry; that was clever. AD
Art Dodge

Actually Harry I fundamentally disagree with you. Magnetism is an absolute bastard to understand. It's easy enough to show WHAT happens WHEN, as Marc found at the Science museum exhibit. But to explain HOW and WHY it happens is something else again. It annoyed me when a teenager that I couldn't grasp it well enough to pass A level physics and it's annoyed me on and off ever since. I sympathise absolutely and totally with Marc.

In my hazy recollection of physics, when you move a current carrying wire through a magnetic flux, the voltage generated is proportional to the rate of flux cutting. The opposing force is proportional to the rate of flux cutting and the current flowing.

In short, the voltage generated is proportional to the alternator speed (which is proprtional to engine speed). This should be more than 12V at any reasonable engine speed, but may at low speeds drop below 12V and is limited electronically.

The current flowing will be dependent on the electrical load (lights, music etc). In principle, the alternator will try to supply as much current as required. The current will heat up the wires carrying it (in proportion to the square of the current) and eventually melt them (either in the fuse, wiring or alternator) if too much is demanded.

Usually, there is no drain on the battery while the engine is running (they put a little light there to warn you if the battery is being discharged.

Harry, that WAS funny. Marc, your reply was clever, too! I think that the thing to remember is that you have to put energy in if you want to get energy out. It doesn't just come out of thin air (well, it can, but let's not go there right now). Marc, if that concept intrigues you, get the most basic book you can on motor and generator theory, and do some studying.

Regards, Joe
Joe Ullman


Iím glad your asked this because now I understand it better. These Q & A help more than the person asking, they help those that havenít asked and even help some of those that answer when they have to think it through enough to explain it.

Donít be discouraged by sarcastic replies, nobody understands it until they are taught, be it at age fourteen or forty.

Nice experiment you got to participate in. Itís like the best way to get someone to understand why cars have transmissions is to have them ride a multiple speed bicycle.

Back to the purpose of your question, if running an electrical device increases the load on the alternator, do you save horsepower replacing a mechanical fan or water pump with an electric fan or water pump?

The manufactures say yes and the law of diminishing returns would say you would use more horsepower when you lose a little converting from mechanical to electrical and again back to mechanical to run a fan that moves the same amount of air.

The fans donít always move the same amount of air though. A mechanical fan used more energy at high speeds when it isnít needed (fan clutches reduce this, but add weight that requires additional energy). Thermostatically controlled electric fans should shut down at speed if the coolant temperature drops. I would also expect the electrical load to be spread more evenly over time rather than during the time when horsepower is most needed, during acceleration.
George Champion

Having expended my ration of smartass comments for the day, here's the serious side:

There are two questions here:
1. How come the alternator's increased load makes the engine run slower if it's just a bunch of magnets spinning in bearings?
2. Physics of generating electricity is an enigma wrapped in a mystery. This is actually a statement, but we won't quibble.

Tackling last first: the fundamental physical activity at work when generating electricity is the induction of current. Basically put, it says that when a conductor is passed through a magnetic field, current is induced to flow. Practically, this means that as a wire, or chunk of iron, or 'x' conductor is moved through a magnetic field, cutting the invisible lines of flux, current will flow. In an alternator, or more correctly, an AC generator, laminations of iron (conductor) are spun near a series of magnetic fields, inducing current to flow or 'making' electricity.

Second, what about the varying load/varying ease of spinning?
By design, alternators limit their current output but not voltage. Thus, the need for voltage regulation, which is accomplished by varying the amount of field current flowing through the rotor. The higher the field current, the higher the output voltage.

The regulator senses system voltage and if sensing voltage is below the regulator setting, as increase in charging results by increasing field current. Conversely, higher sensing voltage will result in a decrease in field current and sysytem output. A vehicle being driven with fully charged battery and no accessories on will have a high sensing voltage. The regulator will reduce the charging voltage and current until it is at a level to run the ignition system and trickle charge the battery (2 to 4 amps). If a heavy load is turned on (headlights) the additional draw will cause a drop in the battery voltage. The regulator will sense this low system voltage and reduce the field circuit resistance. This allows more current to flow to the field windings, increasing the magnetic field, increasing the alternator output.

Thus, there is a varying magnetic field through which the conductors must be passed. If the magnetic field is weak, passage is eased. If the magnetic field is strong as a result of many accessories having been turned on, passage of the conductors through the field is made more difficult. This increase is felt by the sensitive operator as the engine pulls harder.

Art Dodge

Think of a magnetic field as almost concentric lines of force. (a pretty basic assumption, but one that works) The stronger the field, the more lines you have and the denser they are packed together. The alternator rotor is an electromagnet. The more current is applied, the greater the magnetism. This is a rotating magnetic field which cuts across the coils of the field winding, creating a current in them. The more lines cut across the conductor, the greater the current. The force to cause the magnetic lines to cross the conductors in the field windings is supplied by rotating the shaft, thereby converting mechanical energy to electrical energy.

NB: increasing the current in the rotor increases field density and increases the load on the shaft. The voltage regulator simply increases the current in the rotor in response to a drop in system voltage. Presumably you don't need that concept explained. ;)
Jim Blackwood

BTW, to explain the resistance of the shaft to turning: moving the magnetic lines of force across the windings of the field coil induces a current in those windings. This current in turn creates it's own magnetic field around the conductor. As luck would have it (or Physics I suppose) this field is opposite to the one inducing the current in the first place. This causes the shaft to resist turning. The more current, the stronger the field and therefore the more resistance.

So now you know the rest of the story.
Jim Blackwood

It doesn't take any more effort to move a given wire through a strong magnetic field than it does a weak magnetic field, and this is possibly what is confusing the issue.

The crucial thing is what Jim describes above. Luck? I suppose it is lucky, just think what would happen if the two magnetic fields *aided* one another ...

Paul Hunt

I think you'll find that the first and second laws of thermodynamics put paid to that idea Paul :-)

Paul, explain your comment please, Then explain electo-magnetic induction.

"I'll ask my chauffeur to explain it to you" ...
Paul Hunt


Problem with the light is that unless you drive in a darkened room you can't see it because its so dim (like me compared to boffins above) I end up taking bulb out to check and by that time battery is flat.

Anyone for astrophysics!


The in all the explanation of increased torque require to turn alternator as electrical load is increased no-one has mentioned the key concept that is:

A guy called "Lenz" (spelling unknown, I'm an engineer not a english teacher) created a law and Lenz's Law says a conductor passing through a magnetic feild has a current induced through the conductor and a force opposing the motion of the conductor. The amount of opposing force is proportional to the current being induced on the conductor.

In a alternator is alot of conductors passing through a magnetic feild. So as more current is drawn from the alternator the more force opposing the turning of the alternator.

If anyone knows how to put a picture on this page it would be so much easier to explain.

Clem Spriggs

I always found it easier to convert electrical problems into plumbing problems. Picture if you will:
Your alternator is now a water wheel. You are turning the crank and suppling water to the locals. Takes a bit of work to supply the water to your friends, does it not. Even though you do not even "touch" them? Now picture an increase in population. In order to supply each peerson with the same amount of water, you must either turn the wheel faster, or add more water to your buckets. The guy behind you with the whip is the regulator. When the volume of water starts to drop (the amps) he gives you a crack to speed you-up.

If too many people drop by, you will soon tire. The whip will no longer be able to motivate you, you will overheat and you will drop. No more water.

The reason physics is so hard to learn is that it is taught by people to whom it is all so obvious. These so called teachers are never quite bright enough to figure out why others do not "get" the concepts. Then, after we are bored and demoralized we go do something else with our lives.


Yeah, but electricity and water shouldn't be mixed.
Paul Hunt

Excellent explanation, Pete. You should have taught the Physics class that I was in.
Steve S.

Love this thread. Pete - brilliant word picture. Clem - Yes, I remember Lenz's Law now. The induced current - and therefore force - acting in the opposite direction is the linchpin of the explanation. Thanks.

This thread was discussed between 09/07/2001 and 21/07/2001

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