What's the point in alternator amperages?

[brianl703]

You could use a shunt located at the battery, then run small-gauge wires to the analog voltmeter.

There is nothing that requires the shunt to be near the gauge.

Take one of these:

http://www.extendcomputer.com/pages/EVPages/BatCurShuntPages/Shunt150Detail.html

or

http://www.extendcomputer.com/pages/EVPages/BatCurShuntPages/Shunt300Detail.html

connct it with say 18-gauge wires to a suitable voltmeter and there's your 150 or 300-amp ammeter.

(although there are probably better shunts out there with the more typical 1mV per amp scale).

 

[jsharp]

I assumed it was done only because of cost. As you say, all the old mechanical meters are current meters. In the old days you did need to run the current all the way to the meter, through it, and back. That was followed by using a low resistance shunt and using a more precise meter movement that was scaled and calibrated to correspond to the current through the shunt.

These days though we can measure current pretty easily inductively. Just not as cheaply as we can voltage...

 

[ikeepmychevytoo]

Thanks all of you very much for the wealth of information! For analogies, recalling of the Colomb from high school -that I forgot completely, confirmations, measuring tips... everything. This was more than I expected...

I know very basics like P=IV but nothing further

What really puzzling for me was that to see cars which have similar accessories with alternators that "alternating" on very large scale on amp specs. 55 and 94 sounded too diverse to work out in the same car.

quote:

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Originally posted by 1 FMF:
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?

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I confess, I used the terms "alternator" and "generator" interchangibly on purpose

The true tribology behind these is another black field for me. GM -still in '84- uses the term generator on its "shop manual". While not that old, yep I guess it's old. By the way, this is a Canadian '84 Chevrolet Celebrity w/ 173 v6 on it (hey, don't go... I love its ride

). Has HEI (no points) with distributor. I think regular alt. was 78 amp. for v6 and AC option, 94 amp. was for police, taxi or just an option but -with the light of above discussion- it might reasonably be regular for colder climates. Yes, original was supposed to be 94A Delcotron, then sometime it was replaced with 55A Bosch. My region is not so cold, a couple days -5~10ºC a year that's all the cold, so this may be why 55 amp. did well.

I will use MoolaKule's total consumed power idea to roughly calculate the power need then (depending on the result) I will probably go with Patman and haley10's ideas.

I'm a good boy for the batteries, but always one night I forget one accessory on and it drains down to 1 volt. So I don't know it is me or the alt. for that I can only go 2½ years per batt. I just wanted some perspective on the alt. issue, I won't buy "the cheap batt." just didn't want to ruin it too if its because the alternator.

Thanks again for every bit of info.

 

[labman]

I always thought the switch to voltmeters was to save money. A cheaper gage, and just tie it to any voltage source in the dash. For an ammeter, you need the shunt in the gage plus 2 heavy wires running into the dash and back. I guess you could put the shunt out at the battery and just run a small wire in, but that is still more expensive than a voltmeter. There may be concerns about having such a high current circuit inside the cabin too. It should be no big deal to design a gage to read higher amps. I am sure bigger trucks have them.

You really need both gages. An ammeter just tells you what is happening. You know discharge is bad, but how do you know whether it still should be charging or settled down just off 0?

 

[jsharp]

quote:

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Originally posted by brianl703:
You could use a shunt located at the battery, then run small-gauge wires to the analog voltmeter.

There is nothing that requires the shunt to be near the gauge.

Take one of these:

http://www.extendcomputer.com/pages/EVPages/BatCurShuntPages/Shunt150Detail.html

or

http://www.extendcomputer.com/pages/EVPages/BatCurShuntPages/Shunt300Detail.html

connct it with say 18-gauge wires to a suitable voltmeter and there's your 150 or 300-amp ammeter.

(although there are probably better shunts out there with the more typical 1mV per amp scale).

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The problem with shunts is that they create voltage drop in the circuit and have to dissipate quite a bit of power at these current levels.

A common meter shunt might be 200mv drop at a given current to make it work well with the typical digital voltmeter that has a 200mv full scale setting.

150 amps through this shut and you're dissipating 30 watts. Not a small resistor.

 

[brianl703]

Use a larger current shunt and an amplifier (which can be built for less than $5 in parts, even if you buy 'em at Radio Shack. Offhand I think the 741 op-amp would do the job nicely) between the shunt and the meter if a meter with a low enough voltage range is not available. The gain can be set on the amplifier to correct the reading so that 1mV equals 1 amp, too.

The 300-amp 100mV unit will only be putting out 50mV at 150-amps which is a much more tolerable 7.5 watt dissipation. (They recommend that maximum continuous be 60% of 300 amps, so you'd want to use the 300-amp unit anyway).

 

[XS650]

quote:

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A common meter shunt might be 200mv drop at a given current to make it work well with the typical digital voltmeter that has a 200mv full scale setting.

150 amps through this shut and you're dissipating 30 watts. Not a small resistor. [/QB]

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That's why it's smart to use the appropriate sized wire instead of a conventional shunt.

Any wire with current running through it is a shunt.

100mv is more than enough signal to make valid measurements.

I think you're only talking a foot or two of wire at it's rated ampacity to give a 0.1V drop.

A couple of feet of the appropriate ampacity wire should give you the desired 100mv drop at rated capacity and only run moderately warm when run at capaicty. Nice thing is that battery cables used as shunts rarely run near the alternator capacity for more than a few seconds at a time.

 

[99]

quote:

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Originally posted by ikeepmychevytoo:
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?

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A higher amp alternator is better able to handle higher electrical loading. Cars and trucks that came "fully loaded" with all the electrical toys always have a larger alternator than a "base" model. You should always replace your alternator with the same size, never go smaller. Alot of people that have the need go larger than stock.

 

[jsharp]

quote:

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Originally posted by XS650:

quote:

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A common meter shunt might be 200mv drop at a given current to make it work well with the typical digital voltmeter that has a 200mv full scale setting.

150 amps through this shut and you're dissipating 30 watts. Not a small resistor.

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That's why it's smart to use the appropriate sized wire instead of a conventional shunt.

Any wire with current running through it is a shunt.

100mv is more than enough signal to make valid measurements.

I think you're only talking a foot or two of wire at it's rated ampacity to give a 0.1V drop.

A couple of feet of the appropriate ampacity wire should give you the desired 100mv drop at rated capacity and only run moderately warm when run at capaicty. Nice thing is that battery cables used as shunts rarely run near the alternator capacity for more than a few seconds at a time. [/QB]

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Yes, a length of wire would make sense in this application. The problem you get into is if you get the voltage drop too low you're almost forced into some kind of electronic instumentation. Otherwise your trying to find a mechanical meter with a +- 100mv scale and having it be accurate and rugged enough to hold up in a vehicle. You can't really scale it either so your stuck with a +- range that matches the alternator output plus some margin and it makes a reading like 5 or 10 amps sort of iffy.

I'm with labman on one other thing too. It would be very nice to have a voltmeter and an ammeter. Given the amount of electroincs in new vehicles I can't see this costing much compared to some other things. I'd take that instead of interior lights that dim gradually when you close doors. I find things like that to be of no value at all...

 

[XS650]

quote:

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Originally posted by jsharp:
Otherwise your trying to find a mechanical meter with a +- 100mv scale and having it be accurate and rugged enough to hold up in a vehicle. You can't really scale it either so your stuck with a +- range that matches the alternator output plus some margin and it makes a reading like 5 or 10 amps sort of iffy.

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If you don't want to do it or don't think it can be done, then you can't do it.

A 100mv voltage drop at 150 amps when the normal current through the wire is more on the order of 10 amps most of the time is not a big deal.

Or as Brian said, use a simple op amp for grand total of about $2 worth of parts in addition to a cheap $4 meter. Then you don't need 100 mv. There are several relatively easy ways to do it.

You could use an LM324 that costs about 50 cents and which has 4 op amps on one chip and runs off a single ended power supply of 3 to 32 volts. Then you would have three op amps left over to provide signal conditioning for other interesting things.

If someone wants to do it, it's not very difficult.

 

[haley10]

I have a degree and worked in this field for many years. My profile say electrician, but that's my retirement job.

I won't try to clarify anything, but suffice to say if the Bosch 55A is working for you, I'd leave it alone. It's adequate for people that don't add a lot of aftermarket accessories and do normal driving.

A larger alternator won't appreciable affect your engine horsepower. That is a function of the work the alternator has to do i.e. how many electrical accessories you have on such as fog light, stereo, heater, etc..

 

[jsharp]

quote:

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Originally posted by XS650:

quote:

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Originally posted by jsharp:
Otherwise your trying to find a mechanical meter with a +- 100mv scale and having it be accurate and rugged enough to hold up in a vehicle. You can't really scale it either so your stuck with a +- range that matches the alternator output plus some margin and it makes a reading like 5 or 10 amps sort of iffy.

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If you don't want to do it or don't think it can be done, then you can't do it.

A 100mv voltage drop at 150 amps when the normal current through the wire is more on the order of 10 amps most of the time is not a big deal.

Or as Brian said, use a simple op amp for grand total of about $2 worth of parts in addition to a cheap $4 meter. Then you don't need 100 mv. There are several relatively easy ways to do it.

You could use an LM324 that costs about 50 cents and which has 4 op amps on one chip and runs off a single ended power supply of 3 to 32 volts. Then you would have three op amps left over to provide signal conditioning for other interesting things.

If someone wants to do it, it's not very difficult.

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Oh no, I know it can be done, this is the kind of thing that I do all day pretty much...

I was thinking out loud before about why cars don't give us that now and the answer is always that it might cost the manufacturer a bit more money than a simple voltmeter, not that it's that tough to do.

 

[XS650]

quote:

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Originally posted by jsharp:
Oh no, I know it can be done, this is the kind of thing that I do all day pretty much...

I was thinking out loud before about why cars don't give us that now and the answer is always that it might cost the manufacturer a bit more money than a simple voltmeter, not that it's that tough to do.

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The ammeter vs voltmeter debate for alternator equipped cars has been going on since just after dirt was invented.

There are good agruments for both sides but the volmeter side wins almost every time. Even cost no object vehicles where any small cost increase for an ammeter would be insignificant.

My preference is both

But not enough of a preference to add an ammeter.

 

[jsharp]

quote:

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Originally posted by XS650:

quote:

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Originally posted by jsharp:
Oh no, I know it can be done, this is the kind of thing that I do all day pretty much...

I was thinking out loud before about why cars don't give us that now and the answer is always that it might cost the manufacturer a bit more money than a simple voltmeter, not that it's that tough to do.

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The ammeter vs voltmeter debate for alternator equipped cars has been going on since just after dirt was invented.

There are good agruments for both sides but the volmeter side wins almost every time. Even cost no object vehicles where any small cost increase for an ammeter would be insignificant.

My preference is both

But not enough of a preference to add an ammeter.

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Agreed

If you ever want to build simple instruments where 10 increments are adequate resolution look at the LM3914, 3915, 3916 parts. They read voltage and will drive LED's or other types of displays directly. The ranging is set with a simple resistor divider and the inputs are well protected. You can even gang multiple parts if you need greater resolution.

Sorry to the OP. I guess we went

a bit...