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Science Vs Fiction as it Applies to Charging System Design


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I would like to know something similar to this. But with alternators

 

Basically getting a bigger alternator and what size cable it needs for certain length runs.  Because i cant fit a 1/0 lug in the stock junction box

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On 5/7/2019 at 5:49 PM, SnowDrifter said:

Let me take a stab at this. I like these fun little challenges

I write this with a couple assumptions:
1. The amplifiers used are likely class D with an efficiency of 70%
2. The wire used is standard OFC 0 gauge. No over sizing.
3. Wires used adhere to AWG standards, which as per the chart, will be spec's to have an equivalent cross sectional area to the solid copper
4. With above, the wire used will have a resistance of 0.00009827 ohms/foot
5. Wiring for this scenario requires use of a ground path. As frame/chassis of vehicles have a large variance, I will assume that a ground will also need to be wired, giving a total circuit length of 30 feet.
6. Due to variability in termination techniques and quality, all connections will be treated as zero resistance. This includes fusing.
7. Wiring used will be assumed in a chassis configuration - i.e. wired individually rather than an insulated bundle.
8. The able above lists current capacities for wires. Those will not be used. The code lists for an AC system, and there are some difference in how voltage drop works in AC vs DC (read up on power factor if you want to know more). Instead, I'll be referencing an ampacity chart provided by lincoln electric which establishes that 0ga <50ft at a 60% duty cycle(pretty close to music eh?) to be 350a.
9. While to my knowledge, there are no published tolerances for a 12v electrical system, I am opting to use electrical code NEC 210.19, which states a maximum allowable voltage drop for feeders+ branch circuits to be no more than 5%. Interpreted as: Total drop for the circuit can be no more than 5%. So I'll use that.
10. I am interpreting playing music at 30kw as a 50% load cycle, with an average draw that needs to be supplied as 15kw. As for all intents and purposes, we're treating the 30kw/15kw number as regulated power, I'm not going to get into differences in current draw in response to voltage droop across lines. It would make this long and complicated.



30kw power at 70% efficiency requires 42,857w of input power
15kw of power at the same requires 21,429w of input power

13.8v at 5% drop is 13.11v, and 0.69v drop

So for 30kw as our maximum, 42,857w/13.11v= 3,269a
And for 15kw, 21,429w/13.11v=1,635a

Now for the wiring:
0 gauge at 30ft gives 0.00009827ohm/ft * 30ft = 0.0029481ohms/wire

Target resistance:
30kw: 0.69v/3,269a = 0.000211ohms
15kw: 0.69v/1,635a = 0.000422 ohms

Coupling that with our wire, which is simplified by X=wire/target, where X is runs of wire, target is our allowable resistance, and wire is the resistance/wire
30kw: 0.0029481/0.000211 = 13.97, or rounded to 14 runs
15kw: 0.0029481/0.000422 = 6.98, or rounded to 7 runs.

14 runs yields 3269a/14 = 234a/wire. Below the 350a limit set above
7 runs yields the same, below the 350a limit set above.

Soooooooooo
Bearing in mind the (numerous) assumptions above, such a system could be handled with 7 runs on music(50%), or 14 runs at burp(full power)

SD - I'd like this answer a LOT more if you edited it to incorporate the duty cycle of music.  [Yes . . . I see what you did with the 15kw calculations - but let's get the enthusiast there more directly.]

Let's all stay tuned and give SD the chance to edit accordingly.

Tony Candela - SMD Sales & Marketing
Email me at [email protected] to learn about becoming an SMD Partner!

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On 5/7/2019 at 9:15 PM, ToNasty said:

I would like to know something similar to this. But with alternators

 

Basically getting a bigger alternator and what size cable it needs for certain length runs.  Because i cant fit a 1/0 lug in the stock junction box

Really, it's all about current over distance and how that affects voltage drop.  Maybe I should build a calculator for such on our web site . . . hmmm . . . 

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Tony Candela - SMD Sales & Marketing
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13 minutes ago, snafu said:

Really, it's all about current over distance and how that affects voltage drop.  Maybe I should build a calculator for such on our web site . . . hmmm . . . 

Please do. It would be much appreciated.  Im just a dumb concrete worker and framer and floor installer and plumber.  Electrical is not my forte

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1 hour ago, snafu said:

SD - I'd like this answer a LOT more if you edited it to incorporate the duty cycle of music.  [Yes . . . I see what you did with the 15kw calculations - but let's get the enthusiast there more directly.]

Let's all stay tuned and give SD the chance to edit accordingly.

I was always under the impression that music is a 50% duty cycle. If that's inadequate or there's more to it, then let me get a pen and paper to take notes!

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1 hour ago, SnowDrifter said:

I was always under the impression that music is a 50% duty cycle. If that's inadequate or there's more to it, then let me get a pen and paper to take notes!

Correct.  Hence my above request for clarity in your explanation.

Tony Candela - SMD Sales & Marketing
Email me at [email protected] to learn about becoming an SMD Partner!

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1 hour ago, snafu said:

Correct.  Hence my above request for clarity in your explanation.

I've edited premise 10 with my understanding on the subject

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Tuned in

2007 Chevy Tahoe (SOLD)

12 ~ FI Audio X series 10" w/BP option

2 ~ DC Audio 5.0K @0.67

3 ~ DC Audio 5.0K @1.0 

2 ~ PPI 3 way sets (not installed yet)

1 ~ RF T400-4, 1 ~ RF T600-2, 1 ~ RF T600-4

4 ~ CT Sounds 5.25" Strato comps  (rear fill only)

1 ~ XS Power D4800

1 ~ XS Power D3400

8 ~ XS Power XP3000

160 stock alt, Mechman 370 Elite, 185 DC Power

320+ Sq. Ft. Sound Deadener

Pioneer AVH-X5500BHS

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12 hours ago, SnowDrifter said:

I've edited premise 10 with my understanding on the subject

I like it.  While SD's post is lengthy, it's also accurate.

When it comes to charging system design, here are the factors you must consider:

1 - Intended use of the amplifiers - music / SPL / both

2 - Duty Cycle of the media (a function of #1) - heavily compressed pop music (or rap / bass discs) has a duty cycle of 50%, sine waves have a duty cycle of 100%

3 - Voltage available

4 - Current required

5 - Voltage Drop you're willing to live with (SD uses 5%, I tend to like 3%)

6 - Law of Diminishing Returns

Once you have these answers, then it's a simple process of using Ohm's Law to arrive at the answer to a question like this.

#6 is an ass kicker for sure.  For example, you add three additional runs of cable and pick up .2dB on the meter.  That's not insignificant if you compete - could make the difference between winning and losing - but the human ear can't detect the delta.  Then, you add three more runs and pick up another .15dB on the meter.  That's a total of 6 runs to pick up a third of a dB.  Again, maybe that's what it takes to win.  But, where do you stop?  Personally, I don't think 90% of the members here at SMD are die-hard SPL competitors.

Keep in mind when we're talking about thousands of Amperes of current and minimizing voltage drop, every single thing matters.  Are you using the best materials available to you?  Are your terminations done in such a way that voltage drop has been minimized.  Etc.  This process is referred to as minimizing your losses and it's what all of the world's best designers and engineers utilize.

SD - well done.  PM me your stuff so I can mail you the TORK2.

 

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Tony Candela - SMD Sales & Marketing
Email me at [email protected] to learn about becoming an SMD Partner!

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