# Thread: how will the amperage of my alternator affect...

1. ## how will the amperage of my alternator affect...

i have a 55amp alternator and i was wondering if i use an opus 90W psu to power my computer - would everything be okay?

[i don't know much about electrical matters, i just read another thread where this guy said the alternator on his vehicle was 65amp and that it sucked...]

these are the things i plan to power - xenarc screen, epia mII mainboard, 1 full size hDD, maybe a laptop hDD for OS, slimline cd/dvd drive...and that's about it, i suppose...for now...

will it be okay? and could someone explain to me how this amperage deal works and what effect it has on performance?

thanks

2. While a 55 amp alternator is a bit on the small side for todays vehicles I think it should be fine. What it depends on is how much total power you are trying to use.(high powered stereo system, running heeater and defroster and headlights and so on all at the same time) Car are designed so th at you can run every thing all at once and not kill the battery. That being said they did not plan it out for us to add all the the other things we do. (carputers and such) A 55amp alternator is going to put out well over 700 watts. As long as you don't run every electrical device in your car at once for extended periods of time you will be fine.(I realize its unrealistic that you would anyway but just trying to make my point clear)

i don't have ac, so you can factor that out...i'm going to remove the double-din radio/cassette player and replace it with the screen, so you can rule out the radio as well, and i only have the stock stereo installed, so it's not like i'm pulling a lot of power...

so i'm on the bright side, huh? is it possible to install an alternator with a higher amperage? will it work with my vehicle? what is a good amperage?

4. Volts x Amps = watts

We have a constant 12 volts since this is in a car, which makes the rest easy.

12 x ? = 90

? = 7.5 amps

It is important to understand though that the 90 watts is not a measurement of draw that the PS puts on the electrical system. That number represents the high end that the PS itself can deliver before you risk burning it out. It is more likely you will draw less, depending upon the number of powered devices hooked to it (CD-rom, hard drives, fans, video card, etc.).

Back to the question though. When cars are spec'ed out, and they pick an alternator to put in the production model, they usually just pick two alternators for the whole line of cars. IE - they will have a high end one for the cars with all the power gadgets and A/C, and a lower end one for those without all the gadgets or A/C.

Just like most factory euipment, neither of these alternators are what would be considered high-end. They usually provide just enough juice or maybe a little less than would be needed to run all factory options at once (like the rear defogger, A/C, brake lights, head lights, stereo, power windows, etc.) The car manufacturers trust the battery to pick up any slack that the alternator may be short. This works since options like power windows, heated seats, rear defogger, etc. are only run for a short time.

But - the 55 amps you quoted may be a little short, depending. If your not running a strong stereo and amps, and no A/C, it is probably sufficient. Other questions are what kind of Motherboard you are using - Via? Athlon? these have large differences in the amount of power consumed.

If you did need to raise the available amperage up, there are many ways (too many to cover here). Some things you could do include replacing the alternator with a higher amperage alternator, add another battery such as a Yellow Top Optima (probably best choice, I'll deal more with that later), or change the pulley on your alternator (this can cause it to wear out prematurely though).

Back to the science for a second. Wattage is simply a measurement of power. It is power taken from your engine (horsepower) and converted to electrical energy (watts) by the alternator. So simplified:

horsepower = wattage

This means that as you go up in wattage, you must also go up in HP. So anything you do to draw more amps will take more HP away from the engine. This has little to do with what you asked about, but is intended to point out that using a bigger pulley or larger alternator will result in less power at the wheels (hey nothing in life, especially wattage, comes for free).

And back to the question: Your best choice is probably adding another battery such as the Yellow Top. Not just because a battery will provide a much larger 'pool' of power to draw from than an alternator could ever do, but because you will probably need one of anyway, due to the fact that cars cut power when you crank the engine to start it. What this means is that every time you start the car, the PC will begin booting, power will be cut while cranking, then power will be re-applied once the engine is started. Like yanking out the power cord during boot and plugging it back in a few seconds later, this will eventually cause a hard drive crash.

Using another battery and the proper power supply such as the Opus fixes that startup problem as well as helping with the amperage. A battery slowly pulls energy from the electrical system to re-charge, rather than all at once. This makes for a 'smoothing' effect in the power system, which in turn means a reduced number of Amps drawn at any one moment in time than might be neccesary otherwise.

I hope this helps and is at least a little clear... I am out of coffee today so forgive me :P

If you do still want to replace your alternator for some reason, be aware that all that is important is the it physically fits your bracketing in the engine. Get as much power as you think you might need and just be sure it fits.  'Fits' here means that it will work in your physical area (IE pulley in same position, same brackets, etc.)

Also I forgot to mention and falconey reminded me; if it were me I think I would put A/C as a priority over the carPC

5. not to be funny, but if you got a car without a/c and you live in alabama(which I'm sure gets just as hot as Georgia) then you should wait to put a carputer in until you get a new car. I'm not as technically sound as the previous writers in this thread but you'll be pushing the hell out of that alty by adding a carputer. 55amps is really low, which means the builders of your car had no intentions of it having any electronic intensive equipment(i.e. a/c)

6. Well I think I may have gone too far in detail on my message but to clarify a few things since I dug up some Mountain Dew...

A different pulley will not raise the amperage per se (an alternator only puts out what it can) but it will increase the available amperage at slower engine RPM's. This has the net effect of charging the battery faster in stop and go traffic, etc. So that will help you get by with less.

My main points were:

Dont confuse 90 watts on the PS with what you need to supply it with. These are two different things.

A computer all by itself uses little power. The add-ons like video displays, sound cards, hard drives, GPS, and your (hopefully) new head unit will. You will probably need to upgrade to cover the new stuff you will add, rather than for the computer itself.

Lastly, if you do need more power, you could probably tell AutoZone the make and model of your car, and when they get to the questions about whether or not you have A/C, 4wd, whatever.. just say you have that. When they finally pull it up just ask what the amperage is. If it is higher than yours, it is the better alternator, and it will most likely fit fine (they do change the pulleys for cars with A/C often times though, so you may have to swap those).

My last point was if you are feeling up to it, you can make most any alternator work if it will fit, and you pay attention to the regulator. Regulators are either in the alternator, or seperated. If it is in the alternator, and you can bracket the alternator in and swap pulleys; congrats, you have a better alternator. If it is seperate, just make sure you get the regulator that normally is for that alternator.

In the end, after the regulator, all alternators put out 12vdc with the rated amperage. So basically if you can fit it, get it spinning, and provide the right regulator, it will work.

7. Originally Posted by slushieken
Using another battery and the proper power supply such as the Opus fixes that startup problem as well as helping with the amperage. A battery slowly pulls energy from the electrical system to re-charge, rather than all at once. This makes for a 'smoothing' effect in the power system, which in turn means a reduced number of Amps drawn at any one moment in time than might be neccesary otherwise.

If you would like proof of this, get your hands on a clamp based ammeter, put it on your alternator's positive output wire, and measure. Then connect your battery to another via jumper cables. Watch the readings. current flow will always go up (from the alternator) with the second battery connected. Now slushieken's Vette would be OK, as a lot of new vehicles have very high output, in the 130-150 Amp range (my Thunderbird's is 140 Amp from the factory)

As for a 90 watt PSU killing your alternator, I seriously doubt it. As long as you are not loading the processor at 100% all the time, you should be ok, as the PSU's max output is 90 watts, and the load from a PC is hardly static. It pulls the majority of its juice at the PC's startup, when it has to spin the disks up from a dead stop, and I/O is through the roof as it loads the O/S. If you are running a Mini-ITX joby (like the Via Epia) then I would not even worry about it. The Via epia Nehemiah 1000 MHz only uses about 32 watts at 100% CPU usage... Also, since the Opus 90 watt PSU is a switching power supply, it uses buck/boost switching regulation in lieu of linear regulation (old power wasting, inefficient stuff like in cordless phone cradles). Switching power supplies don't attempt to dissipate power, they handle it by turning off their inductors for longer periods of time (shorter pulse width). Linear regulators cannot do the same thing, so the voltage difference in the input and output voltages has to go somewhere, so they use it up and dissipate it as heat. In general switching power supplies are in the range of 75-85+% efficient vs 40-60% (or less) for linear. What that means is the Opus will not be drawing a static load on the vehicle's electrical system at all times, and will be working less during low power times (mp3 playback, idle, etc) and drawing less current. It'll only juice up hard when there is a need for the power on its output.

-Phil

And I agree with philstuf about not wanting to run two batteries in parallel. Most people who do this are into the competition scene and often run an isolator so that one is for the car and the other is for the sound system. They often times run expensive alternators.

9. Originally Posted by ACHIEVER
And I agree with philstuf about not wanting to run two batteries in parallel. Most people who do this are into the competition scene and often run an isolator so that one is for the car and the other is for the sound system. They often times run expensive alternators.

Those are so shiny and sparkly though

Eye candy is good.

10. Another thing to keep in mind is that with today's cars (from about 89 on up) the alternators, as a general rule, only provide enough power to maintain the battery and all running accessories. Maintain, NOT charge. That means that if you add something to the electrical system, it can cause the battery to put out more power than it is getting from the alt.

I discovered this issue when I mounted some small fog lights on my 92 Eclipse, and with the computer running, would find my accessories (radio, defog, running lights) dying one by one at idle after about 10 minutes. This was with a new battery and alt, both matching the stock components.

Quite a few certified mechanics verified that the stock alt is not enough to power 3rd party accessories. They all recommended a better alt. I trusted their advice, since the people I spoke to had nothing to gain by trying to sell me a better alt.

Not to say this is your potential issue, but you might want to look into your car's power output vs its requirements for all stock accessories turned on. That will tell you if the alternator will be able to handle the extra load of your carPC.

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