Alternator and battery: I thought it was a simple question

Hi, all. I’m not much of a car guy or an engineer, and I have what I thought would be a couple of pretty simple questions, but apparently they are not so simple, because I can’t seem to find a straight forward, reliable answer. I am confused as to how an alternator works along with the battery. After doing a little research, here is what I’ve concluded: (correct me if I am wrong)

  1. The alternator essentially generates an A/C and then converts it into D/C and as a result, charges the battery.
  2. The alternator also powers the electrical components of the vehicle.

Here is where I am confused:

I have always heard that the battery basically helps start the car and once the car is on, the alternator takes over and creates the energy to power the electrical components. That is, the battery does not serve a true function once the car is running because the alternator takes over. I have also read that in older cars, this is true and once started, the batteries could even be taken out of the vehicle and it would still drive. But from what I understand, the newer cars need the battery to help the alternator function, but I am confused because I thought that the alternator was powered by mechanical energy resulting from the engine.

So in essence I am asking:

  1. Where does the alternator get its energy from? The battery or the engine?
  2. What is the function of the battery (if anything) when the car is running?
  1. Alternator gets its energy from the engine.

  2. When running, battery serves as a storage device and helps buffer minor voltage variations from the alternator output. Also, in cases where the alternator fails to supply sufficient current while driving, the battery will act as a temporary current source to keep the car running until it gets depleted. If the alternator dies, a fully charged battery will keep the car running for a while.

The reason that the battery should not be disconnected while the engine is running is that, in modern cars, the resulting voltage spike can damage the computer.

Here’s a mechanical analogy of how an alternator works. Take a 3 foot long plastic pipe and half fill it with golf balls, the pipe is just big enough a golf ball will fit. Cap both ends. If you shake the pipe lengthwise back and forth, the golf balls will roll to one end of the pipe and smack “bang” into the end cap, then the other, right? You’d hear “bang”, “bang”, “bang”, etc. If you put a one way door at the end of each side, a golf ball would emerge on each shake in that direction. Then on the shake in the opposite direction another golf ball will emerge. etc etc.

The golf balls emerging from the ends, that’s the analogy of the current coming from the alternator. The one way door is the diode. It’s an alternating current because one golf ball is moving one direction, the next is moving in the opposite direction. You can probably imagine some arrangement of add’l tubes where you could get all the golf balls moving the same direction, that’s what the diode configuration in an alternator does.

In an actual alternator, the engine rotation, connected to the alternator drive pulley, causes a rotating magnetic field to impinge on some fixed coils of wire. Current comes alternately out of one end of the coil of wires, then the other. The physics of how electrons behave in a rotating magnetic field doesn’t exactly correspond to the above pipe analogy, but you get the idea. Physical motion from the engine is transferred to the alternator which causes current to emerge.

The relationship between the alternator and battery is sort of like the relationship between an incoming stream of water and a lake that a city draws its water supply from. Sometimes the lake level goes down b/c the city needs more water than the stream supplies, and sometimes the lake level rises b/c the city needs less than the stream is supplying. Which is happening at any given time, it just depends.

Likewise, whether the electrical needs of the car are being met from the battery or the alternator depends on how much electrical power the car needs at the time, vs how much electrical power the alternator is putting out. Sometimes the battery needs to supply a little extra umphhh, other times the alternator has extra power, so that is used to charge the battery up.

The alternator and battery form a team in modern cars. They depend upon each other to function properly. The battery prevents large voltage spikes from the alternator damaging the car’s electrical system. On modern cars, the alternator should never be run without the battery connected.

Edit: I should add that the OP doesn’t ask a simple question. It took many scientists throughout the world close to hundred years to finally figure out.

Back in the late 50’s I think Chrysler came out with the alternator instead of a generator. To show how superior it was, they started their Chrysler or Plymouth, took the battery out, and drove it to or from New York or someplace cross country with no problems. These days though I think even the alternators are actually generators. That’s about all I know unless you want to talk golf. I actually thought the car ran off the battery, and the alternator just continued to recharge the battery. Either way I really don’t care as long as it works.

Cars still have alternators (AC devices) but they CALL them generators because the Society of Automotive Engineers said to. The Alternator supplies all the electrical power for the car from the engine. The battery helps voltage spikes and is the starter but nearly every car made at some point in its operation at low speeds with everything turned on (say a snowstorm) will drain the battery because the alternator is not big enough. So much stuff is electric now, manufacturers can’t fit a big enough alternator. Scary, Huh?

Huh? Modern alternators incapable of supplying sufficient power to run all the car’s systems? Mustangman, where’d you get that? And where do they now call alternators generators?

I suppose there is a certain validity to calling modern alternators generators because the assembly puts out DC, actually the regulator within the assembly rectifies the alternator’s AC, but I’ve never heard one called a generator. A generator is configured to put out DC without the need to rectify it.

In addition to being a chemical store of electric energy, a battery consists of oppositely-charged plates separated by a thin insulator.

Well, what else has oppositely-charged plates separated by a thin insulator? A capacitor! So, in addition to being backup power, a battery has the same “voltage-smoothing” effects that a capacitor has.

Electronic devices, which are ever more common on cars, are very sensitive to voltage fluctuation. That’s why it’s not a good idea to run a modern car without a battery.

Three phase generators have been around for a long time. Up until about 1960, cars had DC generators. Chrysler was the first to use a three phase generator with a full wave rectifier bridge to turn the three phase AC to DC. Alternator was a marketing term, technically it was still a generator.

I think one thing the OP is confused about is that the new generators electromagnets in them to create the magnetic field instead of permanent magnets. The electromagnets need excitation current in order for the generator to have the needed magnetic field. Mechanical energy is needed to turn the coils in order to generate current to charge the battery and operate the systems on the car.

It take two things for an alternator/generator to produce electricity, excitation current and mechanical input. As RPM increases (mechanical input increases) output current increases and voltage increases. To prevent the voltage from going too high, the exciting current is regulated. That is as the RPMs increase, current increases, voltage goes up. The regulator reduces exciting current so that current decreases and voltage goes back down. The regulator controls the exciting current in order to maintain the proper voltage.

The voltage is developed by the resistance of the load. The load is made up of all the things that use the electricity. A partially discharged battery, headlights, computer, ignition, fans etc all make the load. Load is resistance and the larger the load, the lower the resistance. As the load increases, resistance decreases and voltage goes down. The regulator feeds more exciting current to the alternator so the current increases to meet the demand and the voltage goes back up.

The generator/alternator has limits. At low engine speed, the maximum current that it can deliver is limited. Most of the time, the alternator/generator can produce all the current needed, but under some conditions such as heater fan on high, headlights on, low engine speed or engine at idle, it may not produce all of the vehicles power requirements and the battery has to make up the difference. It would not be uncommon for a stock alternator to only produce around 13 amps at idle while being able to deliver 46 amps at 3000 rpm. These numbers came from a generator on a Toyota that I once had.

High powered audio systems can draw more current than a stock alternator can deliver, same for some equipment used in emergency vehicles and police cars. There are larger generator/alternators available for this market, but they use more energy (gas) to do this and they rob a little more HP from the engine.

For those who are sticklers for details, I do know that the automotive generator/alternator uses fixed coils for the output and a rotating coil for the magnetic field instead of the other way around.

Keith, I admittedly forgot about the high powered audio systems. I was thinking only of stock vehicles.

Emergency vehicles are typically equipped with two high output alternators and two batteries, with power management systems to control it all properly. They need the capabilities to sit still for hours on end with the light bars, radios, etc. running. Ambulances and fire trucks have even greater power needs. However, generating all of this power takes a lot of energy. You’ll not find emergency vehicles will small engines.

The alternator is equipped with a voltage regulator to maintain the proper running voltage, about 13.6 volts…It needs the battery to provide a voltage reference, a starting point by which it adjusts its output voltage…If you remove the battery, the voltage regulator can get confused as the voltage in the system will become unstable as electrical loads and alternator output keep changing. This is true now and was was true years ago…

Caddyman you are mostly right, the battery acts like a buffer. The voltage regulator uses a reference diode to determine the correct voltage, but the loads in a car can change so fast that the regulator has trouble keeping up without a battery.

The difference between now and many years ago is that the electronics is a lot more sensitive today. Mechanical points, tube type radios and mechanical gauges could withstand voltage spikes, although they would shorten the life of some of the light bulbs in the vehicles.

@the same mountainbike I got the info about alternators lacking enough power at times to run all the accessories from General Motors electrical engineering department when I worked for them.

Several projects my division (and sister divisions) was developing were electrical devices formerly driven mechanically. Electric power steering, electric brakes, electric air conditioning, electric rear wheel steering. Add in heated seats front and rear, heated windshields and rear glass, super-duper stereos, and dash displays and you’ve got yourself a formidable electrical load.

Combine that with overdrive transmissions to reduce engine RPM for better mileage and you’ve got yourself an overload problem driving 30 mph in stop-and-go traffic while snowing at night. Cadillacs had it particularly bad because they had all the accessories. Another division made alternators that reached 180 amps, I think. They would have preferred to have it poking out of the hood because it ran so hot.

Toss in that ALL the ECU controlled accessories require a quiescent current draw when the engine is off for various reasons and now you’ve got a constant parking drain on the battery. Yeah, it is small but it all adds up. Park your Caddy or Lincoln for 4 weeks at the airport and return to sub-zero temps and it will struggle to start.

Interesting. I’ve never heard of that before. Other than a defective unit, I’ve never seen an alternator on a stock vehicle that gave me cause to suspect that it could not support the car’s equipment at idle. Then again, I never turned on the heated seats and neckwarmers while testsing! I learn something new every day.

I think the heated windshields and rear windows got the boot a few years ago, tho. They drew massive amounts of current 30-40 amps each and they made windshield antennas and radar detectors useless. I imagine they played heck with Garmins and cell phones, too (hmm, texting while driving might be blocked). Seat heaters are more useful, IMHO.

Heated rear windows are still the norm in my part of the country. They don’t seem to interfere with anything except the removal of stickers from the inside of the glass.

Wish there were a system to block texting while driving… and a warning light to warn of a loose nut behind the steering wheel.

A DC generator is actually an alternator that uses a mechanical rectifier. The brushes and commutator is nothing but a mechanical switch. that rectifies the AC generated in the armature into DC.
What made AC generators practical for cars was the development of the silicon diode rectifier.

Unlike rectifiers, brushes and commutators are reversible. It will just as efficiently invert DC into AC for the armature to run the generator as a motor.

Thank you all. Your responses were all very helpful. The reason I asked is because my girlfriend was having trouble starting her car yesterday. Her headlights would come on, but the engine wouldn’t crank. After a jump start, and a few minutes of driving, the car died. About 15 minutes after the car died, it started (reluctantly) after a second try. She decided not to risk it dying again in rush hour traffic, so I just picked her up. I was under the impression that the car would not just die while running if the battery was the issue. Thus I thought the alternator was the culprit. I also thought it was the alternator because the car started later on. But I’m really not sure. Her battery is pretty old, and she mentioned buying a new one anyway, but I don’t want her to buy a new battery and have a faulty alternator drain it quickly. I guess the best thing to do would be to borrow a voltmeter to test them both?

Continuing to drive on a battery that is going out can easily take the alternator out with it. A battery with a shorted cell will put a constant load on the alternator as the alternator futilely attempts to charge a battery that won’t charge.


If your girlfriend’s battery is pretty as old . . . 4 or 5 years, or more . . . just replace it. Costco has excellent prices on batteries, if you’re a member, and can install it yourself

Anyways, after replacing that battery, get the charging system checked out, and you might want to check for parasitic draw

Take a good look at the belt. If it’s glazed or cracked, replace it now

Some retail auto parts stores will load-test your battery and alternator gratis. Takes very little time as they have all the equipment. No harm asking. In case you wonder how they can do it for free, well they’ve discovered they find quite a few defective batteries this way, and can recoup their costs on replacement battery sales.

One interesting development in auto- alternators is the amount of computer involvement incorporated in the charging system these days. New cars have a host of accessory systems drawing power, some from the electrical system, and some the engine. Things can get out of whack if too much total power is being drawn. On many newer cars, the computer reads the various sensors, estimates the % of battery charge, etc, and decides based on the situation how much power the alternator should be putting out. For example if the AC compressor is on, and the engine rpm is low, even if the battery charge is low, the computer may decide an add’l alternator load on the engine is no good. So it will put the alternator in low power output mode (by reducing the current to the rotating electro-magnet), in order to prevent drivability problems, like engine stalling.

This level of sophistication seems like a good thing; that is until something goes wrong. Diagnosing charging system problems may soon become the job of a computer technician.