Simple car alternator question.

i’m pretty sure that the alternator charges a battery quicker if the belt is turning it quicker (higer RPM’s). is this true?

Well, yes, up to a point. The maximum output point is certainly above the idle RPM, but it may only be at, say, 2000 RPM. Running the engine at 4000 RPM doesn’t help. You’d need to look up data for the specific alternator to really work it out.

Just for the fun of it, i’ll volunteer an uneducated answer.

Charging rate surely increases with rpms up to a point. After that , charging rate is steady.

Is the increse linear? I don’t know.
What is the rpm “saturation” point? I don’t know.

The battery only needs to regain the energy it expended in getting the starter to turn the crankshaft and initiate the motor’s internal combustion thing. I’d venture a guess that the battery can release energy (in starting) than it can accept a recharge (from the alternator). Even so, a minute or two of the car running ought to be plenty. At that point, you can pretty much disconnect your battery with no noticeable effect. Your alternator ought to be able to provide enough charge to run all components ‘at full blast’.

Indeed, many after-market pully kits allow you to use larger than stock pulleys to “underdrive” things like the AC, water pump and alternator to put less load on the motor (and thus more HP to the wheels!). I’d recommend true reading gauges.

just to add, this question is talking about a battery that was just charged after being burned out, if that changes anything.

Almost all modern alternators have the voltage regulator built in, it’s part of the field exciter, as I recall.

For older GM alterantors, they start charging at about 500 rpm, but don’t put out a “full” voltage (13 to 14V) until 900 to 1,000 rpm. I’m sure on newer cars, if the alternator is turning at any viable engibne speed, it’s putting out some power.

However, because it’s voltage-regulated, once it reaches whatever RPM provides full output, anything over that is essentially wasted. (Well, not “wasted”, but it’s not a direct increase: IE, you get 12V at 2,000 rpm, 24V at 4,000 rpm, etc.)

Basically (it depends on the model and year of the car) if you’re at 1,000 to 1,200 rpm, you’re getting the max voltage you’re gonna get out of the alternator.

At that point, you can pretty much disconnect your battery with no noticeable effect.

It may not have much effect on how the car runs, but it can have a profound negative effect on the alternator. Alternators rely on battery resistance to help regulate their charging. Taking the battery out of the circuit can cause the alternator to fry itself in short order. DO NOT RUN AN ALTERNATOR WITHOUT A BATTERY IN THE CIRCUIT.

…this question is talking about a battery that was just charged after being burned out…

I’ve never heard of a battery being “burned out.” What do you mean?

Well Gary, maybe you haven’t heard the term burned out. But I am sure you have been stranded with your car not starting before. :slight_smile:

I assume we’re talking about a battery that was “dead” or discharged, without enough power to crank the engine.

The proper thing to do is to fully charge the battery off the car. Alternators are designed to “top up” the battery charge, i.e. restore and maintain full charge after cranking and during electrical consumption of ingition, lights, accessories, etc.

Alternators are not designed to recharge significantly discharged batteries. Sometimes they will restore full charge, and sometimes they won’t, but either way they are overstressed in the attempt. Using the alternator to recharge a very low battery will shorten the alternator’s life.

<Johnny carson>I did not know that.</carson>

It, of course makes sense.

Thanks, GaryT.

Happy to be of service, Corbomite. It was possible to do that with the old DC generators, as they had current regulators independent of the battery. However, it’s a different scenario with alternators.

Note: the recent trend among car manufacturers is to call alternators “generators.” That doesn’t mean they are like what were called generators decades ago. They still produce AC which is rectified to DC. It’s just semantics.

[old guy mode] I remember back in the day when they were changing from generators to alternators seeing on the (B&W) TV a demo where they started the car, took out the battery, and drove the car a couple hundred miles. This was a Very Impressive thing (to me as a kid, anyway). Maybe they had some kind of load or regulator hidden away somewhere? [/ogm]


You should also know that most alternators require power to make power. If you don’t have a field current to start with, you probably won’t get anything out of the alternator. You are spinning a coil inside of an electromagnet. No electromagnet, no power.

[Training instructor mode]
Disconnecting the battery on a running alternator (AC generator) equipped modern car is one of the dumbest things you can do. Don’t do it!
The reason is that the battery acts as a stabilizer on the fluctuations in voltage from the alternator. In other words the battery acts like a shock absorber in the electrical system. If you remove the battery cable from a running car, besides burning out the alternator, under certain conditions the alternator output voltage could spike upward to the point that the electronic control units in the car are toasted from over voltage. In the industry this is considered a bad thing. :smack:

An alternator on a modern car is designed to do two things. First carry the electrical load from the car and secondly to trickle charge the battery. As Gary T pointed out the alternator is designed to keep a fully charged battery that way. An Alternator is not designed to fast charge a battery. Electrical loads in cars have grown to the point that if the alternator were to go to full field strength on a dead battery, it would destroy the battery. So the requlators don’t allow this. From a practical stand point what this means is that if you leave your lights on, getting a jump and driving to work probably won’t recharge your battery.

In the old days of DC generators, the total output of the system was maybe 15-20 amps. Subtracting what it took to run the car, you were left with between 12-18 amps, not too bad a charge rate. On the '03 model I am teaching on the output of the alternator is 170 amps. Back off, oh say, 10 amps to run the car and remaining 160 amps could destroy the battery in short order if the system allowed the alternator to go to full output.

Desert Dog, back in the days of DC generators, I had a battery stolen from my truck. I jumped started it (booster cables) and drove it 4 miles to my shop to put in another. No special thing was required. A DC generator won’t put out too much voltatge and since there was no battery who cares how much amperage the system put out?

JoeyG Alternators do require excitation current to start the charging process, but after a while there is enough residual magnetitism in the alternator to cause the unit to self excite (masterbate?) and start generating.

Gary T It’s not just a trend, it’s the law. The government has required since 1994 that all car makers conform to standardized language when naming many of the parts used on cars. This is why pre-94 Volvo called it a Lambda sensor, many people called it an Oxygen sensor, and Ford (gotta love Ford) called it an EGO sensor (exhaust gas Oxygen) Now everybody calls it a HO2S1. These standards are in an SAE standard (J1939?). Anyway, since an alternator generates electricy, it became know as a generator. Even though generator had previously been known as a unit that produced DC voltage way back when, and was replaced by alternators, which as you pointed out produce AC.

Getting back to the OP, on the cars in my school I can, with test equipment, get the alternator to produce about 50A at idle (800 RPM) and get full output by the time the engine has hit 2,000 RPM. The only time I have seen a need for a different sized pulley was on a RX-7 race car, we changed the pulley size to slow down the alternator. This was due to the fact that in a race the engine spent most of its time above 8,000 RPM. We didn’t want to spin the alternator to death.

thanks Rick. So, how long at 2k RPMs do you think it would take the alternator in a '94 formula firebird to charge the battery after being semi-dead (dead enough to not be able to start the car, but not completely dead)?

Rick, the battery in the last question was jump started. the question was how long after it was jump started.

Fuel, we’re not going to have a definitive answer to your question. It would vary depending upon precisely how discharged the battery was and upon the particular alternator’s charging rate.

In my experience, often about 30 minutes of driving will charge the battery enough so that it will start next time, and if it starts next time it will generally keep starting OK (assuming no problems in the system). As to whether the alternator will ever get the battery fully charged, some will and some won’t.

The longevity of both the battery and the alternator will be enhanced by getting the battery fully charged with an external charger.