What's the point in alternator amperages?

ikeepmychevytoo · May 30, 2005

I have a question for those who could bear with my electrical illeteracy.

There are cars with AC and many other amperage draining accesories from the similar eras. One come with -say- 47 amp generator while the other may come with a 94 amp generator.

To be more direct, mine original was replaced with a 55 amp Bosch. When digged through the RPO codes I found it's original generator was an optional 94 amp Delcotron. However car operates perfect for years with the 55 amp... so what's the point with the 94 amp? What do I miss since Bosch? Am I expected to replace batteries sooner, or do I comprimise from spark strenght at the plugs? This alternator can keep the batt. charged and everything operates. With my previous Bosch batt. it would even crank without the slightest dimming when the headlights on!

Novodays battery is dieing slowly (2½ years and I was harsh on it). This time should I consider replacing the generator too? What matters if I pick a 78 amp or 108 amp Delco or else leave the 55 amp Bosch alone?

ikeepmychevytoo · May 30, 2005

brianl703 · May 30, 2005

Well, I can tell you that the stock 75-amp alternator in my 1988 Mustang is widely considered to be barely adequate to keep the battery charged under severe-load conditions:

Driving at night (headlights on) in bad weather (defroster on high, wipers on too) in stop-and-go traffic (engine mostly running at idle). (This car does not have an electric engine cooling fan).

That's one example where an alternator of a given size might be perfectly fine 99% of the time, but it's that 1% of the time that really pushes the alternator to it's limit.

Also, a larger alternator will potentially last longer than a smaller alternator in the same application, because it'll be built better (better cooling, bigger rectifier).

mountravlr · May 30, 2005

A heavier than recommended alternator will load the engine more than one rated at a lower amperage.

motorguy222 · May 30, 2005

An amp is the speed at which electricy moves.

The higher the amp,the faster it moves.

The higher the amp,the more power it has in respect to the voltage.

A 12 volt Alternator will only put out 12-14 volts when it is working properly.

The amps that the system puts out can vary depending on the electrical need of the car in question.

The amperages that are needed varies according to what the car has,such as A/C,electric windows,rear defroster etc.,etc.

The more options that a car has,the higher the amperage will have to be.

The electrical current will have to move faster to keep up wiht the demand in a system that has several options(electrical needs),this means that a higher amperage alt. is needed.

[ May 30, 2005, 05:23 PM: Message edited by: motorguy222 ]

Gary Allan · May 30, 2005

quote:

A heavier than recommended alternator will load the engine more than one rated at a lower amperage.

Maybe for the peak hp drag ..but the smaller alt will just make up for it in the duration of the event, assuming that the demand end of the equation is equal. Both will charge until they run out of V differential. The smaller one just makes you dip into the battery deeper.

Bret Chase · May 30, 2005

quote:

Originally posted by motorguy222:
An amp is the speed at which electricy moves.

The higher the amp,the faster it moves.

The higher the amp,the more power it has in respect to the voltage.

A 12 volt Alternator will only put out 12-14 volts when it is working properly.

The amps that the system puts out can vary depending on the electrical need of the car in question.

The amperages that are needed varies according to what the car has,such as A/C,electric windows,rear defroster etc.,etc.

The more options that a car has,the higher the amperage will have to be.

The electrical current will have to move faster to keep up wiht the demand in a system that has several options(electrical needs),this means that a higher amperage alt. is needed.

this may be a point of view thing, but electricity always moves at the speed of light. an amp is a measure of current or "electron flow" (coulombs per second), a volt is the measure of potential (1 volt = 1 joule per coulomb). if you multiply the two you get the amount of power that can be deliver in watts.

anywho,

a larger alternator allows you to have larger and longer continuous draws before the battery is depleted. a bigger alternator allows you to run your winch longer, run a bigger amplifier, run more aux lights, etc.

-Bret

labman · May 30, 2005

If it ain't broke, don't fix it. If your battery is staying charged up and the system never, or even seldom, shows low voltage problems, leave well enough alone. Batteries may even last better with a slower rate of recharge from a smaller alternator, as long as it is never discharged too far.

Gary gave a good explanation of the engine load for a large and small alternator. The amperage is the rate of charge. Compare it to volume in a time period to water flow. The engine still has to produce the power for so many amp hours. Once the battery is recharged, the voltage regulator reduces the power produced to equal the power being used.

Eddie · May 30, 2005

Bret Chase has it correct. Ed

kenw · May 30, 2005

let's get some basics here, we'll use the garden hose analogy:

Amps pull, volts push.

Amps=the amount of water that can be drawn out of the hose in a given period of time, usually defined as limited by the diameter of the hose. Amps needed are determined by the load.

When you pull more amps than the source can provide, the voltage will then start drooping. A dead short pulls near infinite amps. A larger alternator will be able to provide more amps before the voltage will start drooping. As will a larger battery.

Volts= the pressure of the water coming out of the hose, determined by the pressure at the pump (or water tower, or whatever)

Watts: the combination of volts x amps. 100 watts can be 1 amp x 100 volts or 10 amps x 10 volts. Power factors and such complicate things but we won't go there today.

A larger alternator will only make more drag when the load pulls more amps. At the same output, both alternators are probably presenting the nearly same drag on the engine since it's just bearing loss at that point.

The voltage regulator regulates voltage, not amperage. An alternator without a regulator outputs voltage based on the RPM, the higher the RPM, the more voltage. The regulator simply keeps the voltage output at a fairly constant 13.5-14.5vdc, regardless of the amps that are being drawn. The regulator will pass whatever amps are drawn by the load, up to the limit of the alternator's capacity, and simply make sure that the output is 14.5v (or whatever).

brianl703 · May 30, 2005

The voltage regulator is temperature-compensated and will reduce the voltage output as the alternator gets hotter (due to batteries needing less charge voltage at higher temperatures).

Higher current draws cause the alternator to heat up more.

1 FMF · May 30, 2005

quote:

Originally posted by ikeepmychevytoo:
To be more direct, mine original was replaced with a 55 amp Bosch. When digged through the RPO codes I found it's original generator was an optional 94 amp Delcotron. However car operates perfect for years with the 55 amp... so what's the point with the 94 amp? What do I miss since Bosch? Am I expected to replace batteries sooner, or do I comprimise from spark strenght at the plugs? This alternator can keep the batt. charged and everything operates. With my previous Bosch batt. it would even crank without the slightest dimming when the headlights on!

Novodays battery is dieing slowly (2½ years and I was harsh on it). This time should I consider replacing the generator too? What matters if I pick a 78 amp or 108 amp Delco or else leave the 55 amp Bosch alone?

what year/type of car? You say generator, so either the car is old, or you are

I will refer to it as alternator,

just to verify cause i'm confused, the original "generator" that was in there was a 94-amp delcotron? But you had replaced it with a 55-amp bosch alternator? And the car either came with a 55-amp, or an optional 94-amp, alternator from the manufacturer and your car originally had a 94-amp?

Reason I ask, and this might depend largely on the year and technology available but if the car came with the optional 94-amp and not the 55-amp then there's probably a good reason for it. One might be if that model car came with a carburetor and breaker points type ignition vs electronic fuel injection and hi-energy ignition system. Or maybe different headlights, fuel pump, heated seats.

Another big thing to remember is the alternator will not output it's rated number of amps at idle. That is dependent on how fast the belt is spinning it, which means you have to have engine rpms usually 2000 or higher. A higher amp-rated alternator may be able to output more amps at idle rpm than a lower amp one, 94 amp vs 55 amp for example. So depending on what accessories are on the car (truck or van?) you may need to take that into consideration.

The alt. amp rating is fairly simple- the amp rating is an indication of it's power output. Voltage is fixed, and everything runs on the same voltage. So the more amps the alt. can output, the more accessories you can safely have on the car before exceding the amount the alternator can put out and not be able to recharge the battery. Many short trips with an underrated alternator will result in the battery not getting a full recharge, and batteries that are allowed to sit not fully charged go bad. For technical info on maintaing car batteries, see howstuffworks.com. Car batteries should last close to 5 years with proper maintenance. And, you don't want to have an alternator/generator that just meets the power requirements where it's running 90-100% all the time. Always get the highest rated unit that is practical and affordable.

Depending on the year and technology, you may want a new alternator. They will do a better job of recharging the battery properly, and not overcharge it. Older alternators just put out 14v constantly and long runtimes would overcharge a battery and shorten their life, especially in hot weather.

MolaKule · May 30, 2005

OK, some clarifications.

From water analogy, Voltage is the pressure, flow is the current; a flow of charge, where the flowing of electrons constitute a current. The total volume of water due to pressure and flow is equal to the power.

The total power requirements or loads is what the alternator is rated for.

On a hot summer day, you might need for example 1.3 Kw or 1,300 Watts. This means that at 14.5 volts regulated, the alternator is pushing 90 amps of current, since P = IV or I = P/V = 90 Amps. The engine is supplying at least 2.5 Hp to the alternator. Actually, 1,300 Watts is equal to 1.7 Hp, but due to alternator inefficiencies, the power requirement is higher.

The only time the battery needs charging is right after starting. Afterwards, the alternator is supplying all the power to the accessories and ignition.

The current limiting in most alternators is done by the "saturation" of the magnetic core (iron) and the heat sensing thermistor at the regulator inside the alternator. The alternator actually produces three-phase AC and is rectified or transformed to DC.

As one person has already stated, the voltage regulator is heat sensitive and as the temperature goes up, the current is regulated back in order to keep the copper windings, core, and regulator from overheating.

A larger alternator has an advantage in that it is able to produce more current as the temps increase, up to a point. So the largest alternator that can physically fit will always be the better one.

One of the electrical problems I have seen with rated alternators is the fuse wire from the alternator output to the firewall increases in resistance over time and one actually gets lower voltages at the fuse block as the temps increase. If possible, take a volatge meter and check the voltage at the alternator and at the fuse block at idle with the just the lights on. You shouldn't see more than few tenths of a volt difference (voltage drop) between the two points.

brianl703 · May 30, 2005

If you connect the voltmeter between the alternator and fuse block, you'll directly measure the voltage drop along that wire. No need to remember two numbers and subtract one from the other that way. Also you can get a more precise reading of the voltage drop because most meters are 3 1/2 digits (1999 -- the 1 is considered 1/2 digit) which will only give you two digits past the decimal place (hundreths) if measuring between 2 and 19.99 volts.

If measuring below 2 volts, they will give 3 digits beyond the decimal place, or thousandths. That way you can know your voltage drop to the millivolt level

[ May 31, 2005, 12:10 AM: Message edited by: brianl703 ]

VaderSS · May 30, 2005

Funny this comes up. When I replaced the alternator in my grandmother's '82 bonneville, years ago, I had two choices. One was the stock size and was well over $100. The exact same quality level alternator, but in a smaller size, was $35. I went with the $35 alternator and never had a bit of trouble. I figure, I could always rebuild the original if it was an issue, but it never was.

On the voltage drop thing, I recently replaced the stock battery cables in the Impala with 1/0 wiring. I was shocked at how small the stock wiring was. It was the same size as the Festiva's wiring. The starter cranks with MUCH greater urgency now, and the voltage, measures at the PCM, stays much more consistant.

motorguy222 · May 31, 2005

Amperage

n : the strength of a electrical current measured in amperes

Source:www.Dictionary.com

jsharp · May 31, 2005

It's too bad newer vehicles come with a voltmeter or with no gauge at all. I liked the old ammeters. You could always tell if you had enough alternator output to keep up with what you were running.

As is it, a voltmeter only tells you current battery state and you have to watch it over time to decide if you have enough alternator output.

Gary Allan · May 31, 2005

Now someone correct me if I'm wrong ..but the transition to volt meters was probably due to the higher amp output of the alts. You have to run the amps through the gauge and run a high current shunt in parallel with the analog meter windings. These seemed to fall out of vogue when altenators went abouve 60 amps in max current output (the reason you can figure this is that there are virtually no automotive amp gauges that read over 60 amps)

Voltage is much easier to manipulate since you merely choke the input with a resistor (called a mulitplier for analog mulitmeters - yes, old news hardware). All meters are current meters (analog anyway)..and all have ratings for FSD (full scale deflection).

brianl703 · May 31, 2005

A voltmeter will only tell you current battery state without the engine running. Otherwise, it's measuring the voltage output of the alternator which is almost always (unless something is wrong) greater than the battery voltage. Given that the open-terminal voltage of a fully-charged car battery is ~12.6 volts, any reading over that would indicate that the battery is not being discharged.

XS650 · May 31, 2005

quote:

Originally posted by Gary Allan:
Now someone correct me if I'm wrong ..but the transition to volt meters was probably due to the higher amp output of the alts. You have to run the amps through the gauge and run a high current shunt in parallel with the analog meter windings.

All you need to do is measure voltage drop in the lead to the battery. Current = Voltage/Resistance.

Some cars have used a fusable link near the battery and use it as the shunt. No need to run hefty wiring into the cockpit.

What's the point in alternator amperages?

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