is there a general guide line of how big an alternator needs to support howmanywatts

[rookanga 10+ year member]

I know I need to upgrade my alternator, one of the big three... But my question is not can this alternator power my system, I was wondering if there is a guideline of how many amps an alternator produces to wattage an amp puts out before the alternator can't keep up anymore, say (for example purposes) 100amp equals to 1000 watts without the lights dimming, not saying that exactly, but is there a guide line that people can suggest before upgrading my alternator

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[fasfocus00 5,000+ posts]

math, figure out current draw full tilt and match up your alternators current ability at idle or a specific RPM. there is no guideline, just math.

 

[akillj 10+ year member]

You can do some math to ensure you have enough amperage. Let's say you have a 60 amp stock alt. Your car should use about 40-60% of this during running / normal conditions.

60 * .6 = 36a of power to run the car / maybe some accessories on. (We'll use the high estimate)

Let's say you're running a 5000w amp (RMS).

You can estimate how much current (amps) it's going to consume by dividing input voltage (14.4) by rated output power (5000w)

But first, since no amp is 100% efficient, we need to figure out how much power it's going to consume, to create that 5000w. Class D amps by design are very efficient. They're usually up to 90% efficient at high ohm loads. Let's assume you're running it at 1 ohm, which we can estimate would give you about 70% efficiency. This is not 100% accurate, just an estimate.

5000 / .7 = 7142w

It's going to consume 7142 watts to create that 5000 watts.

7142 / 14.4 = 496a.

496 + 36 = 532 (

You will need 532 amps of alt to sustain this amp constantly while running the car and a few accessories. This would apply if the amp was running a constant tone, like in competitions. But we all know music isn't just a sine wave. So with a few batteries to ensure the voltage is stable, you can get away with a lot less alternator.

Full equation: (AmpOutput / ExpectedAmpEfficiency) / AltOutputVoltage = Amount of Amps required to run that amp constantly. Add that answer to (StockAltAmperageRating * 0.4-0.6) = Amount the car draws when running + a few accessories.

Add the answers to both those and you get the maximum your entire vehicle could possibly draw at one time. (Not counting for other mids/highs amps, but it will usually be minimal)

 

[hispls 5,000+ posts]

Any of this **** above is pure guessing. There's far too many variables to say what you will need. Buy the largest alternator that can be properly turned by the belt setup you have in your vehicle, if you have trouble after that, add a second alternator or more batteries.

 

[anger 10+ year member]

Any of this **** above is pure guessing. There's far too many variables to say what you will need. Buy the largest alternator that can be properly turned by the belt setup you have in your vehicle, if you have trouble after that, add a second alternator or more batteries.

May I ask what you mean exactly by '..that can be properly turned by the belt setup'...? Is that regarding the size of the pulley?

Im shopping alts as well and still a little unsure as to what I should be looking for.

TIA

 

[Based]

yeah the best bet is to just get the biggest alternator you can get from either Mechman or Singer that is just plug and play. I have a 320a from Singer on the way and I'm gonna be running just 2400 watts to maybe upper 2K watts. I probably didn't need a 320a alt but I just got it because I'd rather not have to buy a bigger alternator after I already bought one that was just too small.

Oh and about the belt thing, just email either Singer or Mechman about it. Go to their sites and make inquiries. My alternator for my vehicle some times needs a slightly smaller serpentine belt to prevent slipping which causes the car's electronics to basically shut off (power steering, water pump, etc.). Whoever you decide to go with will be able to tell you whether you need a smaller belt or not.

 

[n2audio 5,000+ posts]

There's really not.

Like Hispls pointed out -- there are too many variables (that vary drastically) to predict what will be adequate and what won't.

Plus "working" for one person might be considered failing by another.

Headlight dimming, for example -- some people consider that to be unacceptable at any degree. Others consider it to be typical of a high output sound system.

Clearly - the bigger the stock alt, typically the better off you'll be. However, big stock alts are big for a reason (a lot of stock electrical needs).

In most cases I'd say the average stock electrical system will run a 1000w set up reasonably well. Although there are plenty of exceptions even to that basic assumption.

The one thing you can count on is that adding amplifiers WILL increase the work req'd by your alternator. Even if things appear to be running perfectly well, your alt's functional life will be reduced.

 

[rookanga 10+ year member]

I kinda figured there wasn't a clear cut answer for this question, but got a lot of good info, now adding a battery will of course help out, but how much would extra batteries help, I was thinking about 2-3 all together, basically I took my system out of my truck, since I got a wagon (a lot better on gas lol) right now I have a 18 fi sp4 running on a sundown saz2000, I have yet to put this in my new vechicle, but I also want to upgrade and add another sundown 2500 to it, so I'm trying to figure out, what would be the best route for everything... Yes adding the biggest alternator would work, but I'm also not trying to break the bank for over killing it when I didn't need to also

 

[akillj 10+ year member]

Any of this **** above is pure guessing. There's far too many variables to say what you will need. Buy the largest alternator that can be properly turned by the belt setup you have in your vehicle, if you have trouble after that, add a second alternator or more batteries.

How is anything I said "Pure guessing?" And how is "Buy the largest alternator that can be properly turned by the belt setup you have in your vehicle" not pure guessing? If I said anything incorrect, please tell me what it was so we may all learn here. I explained how to mathematically calculate the maximum possible load an electrical system would see, as to avoid voltage drop, and get the most power out of your amplifiers. I don't see how 'buy the biggest alt you can' isn't guessing, but calculating it is?



There are plenty of actual sources of information (as linked above) where you can get real information on getting what you need.

 

[hispls 5,000+ posts]

May I ask what you mean exactly by '..that can be properly turned by the belt setup'...? Is that regarding the size of the pulley?
Im shopping alts as well and still a little unsure as to what I should be looking for.

TIA

Depends on how the belt + pulley is set up. Some cars just will always give you trouble with belt slipping if you try to get too strong of an alternator on them. Simply asking someone who builds alternators for a living should be sufficient. The popular shops should give you an honest answer of what you can use without running into problems.

How is anything I said "Pure guessing?" And how is "Buy the largest alternator that can be properly turned by the belt setup you have in your vehicle" not pure guessing? If I said anything incorrect, please tell me what it was so we may all learn here. I explained how to mathematically calculate the maximum possible load an electrical system would see, as to avoid voltage drop, and get the most power out of your amplifiers. I don't see how 'buy the biggest alt you can' isn't guessing, but calculating it is?

Short of actually measuring current over a period of time you are completely guessing at current draw. Amp "efficiency" is a complete ******** stat when it comes to real world performance, and Normal "music", over time, is about 15% of the power of full-power sine wave.

So given that without just wild guess work we don't know what someone needs, would they be better off trying to buy the smallest alternator they think they might be able to get away with or the largest one they're comfortable buying?

I'll stand by my advice.

 

[akillj 10+ year member]

Depends on how the belt + pulley is set up. Some cars just will always give you trouble with belt slipping if you try to get too strong of an alternator on them. Simply asking someone who builds alternators for a living should be sufficient. The popular shops should give you an honest answer of what you can use without running into problems.



Short of actually measuring current over a period of time you are completely guessing at current draw. Amp "efficiency" is a complete ******** stat when it comes to real world performance, and Normal "music", over time, is about 15% of the power of full-power sine wave.

So given that without just wild guess work we don't know what someone needs, would they be better off trying to buy the smallest alternator they think they might be able to get away with or the largest one they're comfortable buying?

I'll stand by my advice.

What is your source on the "music over time is about 15% the power of a sine wave?"

What if i'm listening to Decaf music, where bass lines can be constant through the entire song? If Decaf stuff and a sine wave are recorded at the same level (say, -6 db), the bass amp would see, and produce similar results to the sine waves. I don't see how that would be anywhere near "15%." Could you explain further on why amp efficiency is irrelevant / "********?" It takes an amp X power to create Y power. There's nothing more really to it. I don't see how if one wanted to calculate max load, that they wouldn't consider efficiency a valuable factor.

I'm not trying to be a jerk here, i'm here trying to learn as much as the next guy. I would agree that buying more is always better. Even more of a reason to get a rough calculation (like I did above^). It's not about getting as small of an alternator as you can. In fact, quite the opposite. If you take my example into a real-world scenario, you would need no more than 532 amps. That would be a MAXIMUM possible load (For amps + car running, anything else would be too small to even bother calculating in. I simply calculated the two biggest draws on the electrical). You would never see more than that. The rest would just be wiggle room.

In most setups here people running big amps / subs, the amps themselves are obviously going to be the top draw on the electrical. Next up would either be the car itself, or the mid/highs amps if they were powerful enough. If you can satisfy the amps than really anything below that (like the small amps for tweets or the car itself) isn't much to worry about imo.

 

[akillj 10+ year member]

Another note to OP and anyone else reading:

I find that a good guideline is to look at your monoblock amp FUSE rating. (Combined fuse rating if more than one monoblock) It will tell you what kind of class of alt we're talking about.

My buddy has a 7.5k (Fuse rating 750) and dual 370 alts. (740 amps). He says it runs great. The fuse rating gives you an idea how much power it's going to consume (but not exceed, obviously. Otherwise the fuse(s) will blow).

 

[n2audio 5,000+ posts]

The fuse rating gives you an idea how much power it's going to consume (but not exceed, obviously. Otherwise the fuse(s) will blow).

It's a rough estimate at best. It takes roughly 2x the rated current to blow a fuse instantly. An amp fused at 50A COULD draw 60, 70 maybe even 80A dynamically w/o blowing the fuses.

However, if you average out actual current draw of an amp playing music at full volume it is likely 1/4 to 1/3 of the fuse rating.

Which all goes back to the original question and that there's not a clear answer.

 

[adulbrich 5,000+ posts]

It's a rough estimate at best. It takes roughly 2x the rated current to blow a fuse instantly. An amp fused at 50A COULD draw 60, 70 maybe even 80A dynamically w/o blowing the fuses.However, if you average out actual current draw of an amp playing music at full volume it is likely 1/4 to 1/3 of the fuse rating.

Which all goes back to the original question and that there's not a clear answer.

What about wire? I have SHCA 1/0 OFC and was planning to fuse it at 300 amps. Will the fuse still protect the wire in the event of a short? How many amps can the wire realistically handle? Its about 4 feet of wire.

 

[n2audio 5,000+ posts]

It's fine.

Current demand is getting serious at that point.

300A should protect the wire, but with that much current demand there are some bad things that can happen before the fuse opens.

Personally, I'd be concerned about battery damage.

It's the nature of the beast though.