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Old 02-02-2015, 15:04   #46
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Re: Mixed AWG Solar Install

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Originally Posted by colemj View Post
I think the voltage drop may not be as important here - particularly if you have a PWM controller and not an MPPT. The panels will put out 17V, and you need 15V at the controller. So a 10% drop is not going to hurt you as much as you may expect.

This is even more so when you consider that in your cruising grounds, the panel output will be less than rated max do to heating. We get ~15% less when hot.

FWIW, we have 8AWG, running about the same length as you, handling 15A with no problem or noticeable decrease in terminal output. Your last calculator above shows this as 1.87% voltage drop at 15V.

Mark
Careful with this statement. A 10% voltage drop is a 10% drop in power and will hurt you exactly as much as you expect, if you expect 10%, and that is before any loss due to the efficiency of the charge controller and battery lines.
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Old 02-02-2015, 15:15   #47
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Re: Mixed AWG Solar Install

Looking first at the 10' run of 8AWG, that alone will cause a 0.3+ voltage drop from 12V to about 11.6 at 25A. Using 4AWG there would bring the voltage back up over 11.8V, and the loss to only .128V.


So that first run of 8AWG cable is going to cost you 2/10ths of a volt, which is like having a 20% discharged battery.


The 25' of 4AWG will have a similar voltage loss, so you could say that regardless of how you set up the system, your first run of 8AWG to get inside the hull? Will represent fully HALF the voltage loss in your entire system.


Probably bringing your entire loss into the 0.4V range instead of a 0.2V range that you could aim for.


Tolerable? Probably. Overall about a 3.3% loss, when you COULD be having half of that loss. As long as snaking the new cable is more trouble than an avoidable 1.5% power loss in the new system...no problem.
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Old 02-02-2015, 15:28   #48
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Re: Mixed AWG Solar Install

Precisely why I suggested a parallel/serial configuration, with twice the voltage and half the current. Much less loss overall without messing with the 8 awg cable.
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Old 02-02-2015, 15:55   #49
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Re: Mixed AWG Solar Install

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Originally Posted by Ashevar View Post
Careful with this statement. A 10% voltage drop is a 10% drop in power and will hurt you exactly as much as you expect, if you expect 10%, and that is before any loss due to the efficiency of the charge controller and battery lines.
Actually, it is not in this case. The panels OCV is 17.8V. The batteries need 14.2V during the end of bulk charging, but will be pulling the panel voltage down to between 12.2-14.2V before that point.

Solar panels are (in simple terms) constant current supplies - not voltage supplies - and they provide that current over a range of voltages.

As long as the panels can provide sufficient charging voltage, there is no appreciable loss of the current available for charging. In this particular case, there is a potential loss because of the MPPT availability, but in his environment, even that isn't very significant.

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Old 02-02-2015, 16:06   #50
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Re: Mixed AWG Solar Install

Quote:
Originally Posted by hellosailor View Post
Looking first at the 10' run of 8AWG, that alone will cause a 0.3+ voltage drop from 12V to about 11.6 at 25A. Using 4AWG there would bring the voltage back up over 11.8V, and the loss to only .128V.

So that first run of 8AWG cable is going to cost you 2/10ths of a volt, which is like having a 20% discharged battery.

The 25' of 4AWG will have a similar voltage loss, so you could say that regardless of how you set up the system, your first run of 8AWG to get inside the hull? Will represent fully HALF the voltage loss in your entire system.

Probably bringing your entire loss into the 0.4V range instead of a 0.2V range that you could aim for.

Tolerable? Probably. Overall about a 3.3% loss, when you COULD be having half of that loss. As long as snaking the new cable is more trouble than an avoidable 1.5% power loss in the new system...no problem.
This is not the case for his application or solar in general.

First, the panel voltage is 17.8V, not 12V. That 0.3V in 10' of 8AWG will leave him with 17.5V, which is a whopping overhead to the 14.2V that he needs for charging.

Second, this is NOT like having a X% discharged battery. This line of reasoning is a complete non-sequitar and makes no sense.

Third, as long as the voltage is high enough for charging, the current is available for charging. Voltage drop is the wrong way to assess this situation, except for MPPT - but this is a minor issue here.

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Old 02-02-2015, 16:09   #51
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Re: Mixed AWG Solar Install

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Originally Posted by socaldmax View Post
Hmmm...

those voltages seem awful low. Not from cable losses, just low compared to the 17.6v that the panels were posted to produce at max. power.
17.6V is not max power voltage, but open circuit voltage. When connected to a load, and in the heat of the equatorial sun that these are in, his posted voltages are what would be expected.

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Old 02-02-2015, 16:17   #52
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Re: Mixed AWG Solar Install

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Originally Posted by colemj View Post
Actually, it is not in this case. The panels OCV is 17.8V. The batteries need 14.2V during the end of bulk charging, but will be pulling the panel voltage down to between 12.2-14.2V before that point.

Solar panels are (in simple terms) constant current supplies - not voltage supplies - and they provide that current over a range of voltages.

As long as the panels can provide sufficient charging voltage, there is no appreciable loss of the current available for charging. In this particular case, there is a potential loss because of the MPPT availability, but in his environment, even that isn't very significant.

Mark
There is no way around a measurable voltage drop across a line dissipating a proportionate amount of power. That voltage(and power) drop, while likely far smaller than 10%, isn't free. You can gain efficiency elsewhere but a line loss is a line loss and that usable power is gone as heat.

Pushing 25 amps through 20' of 8 awg is going to dissipate 8W no matter what you have on either end of the wire.
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Old 02-02-2015, 16:40   #53
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Re: Mixed AWG Solar Install

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Originally Posted by colemj View Post
17.6V is not max power voltage, but open circuit voltage. When connected to a load, and in the heat of the equatorial sun that these are in, his posted voltages are what would be expected.

Mark
I'm not sure where you got that from.

According to Belizesailor on page 1 of this thread,
Quote:
Max voltage (Vmpp) for these panles is 17.6, open circuit voltage (Voc) is 21.9.
That's why I expected to see close to 17.xxV coming out of the panels. Even with a 21.9Voc, he could run them in series pairs and still not exceed the rated 50V max. controller input.

I've measured panel output a number of times, and with MPPT controllers and the exception of the sun being very low in the sky, I usually see Vmax or very close to it, within 1 volt.

I have seen PWM controllers that dragged the solar panels down to very close to battery voltage while charging, precisely why I'm strongly against PWM controllers, especially since there are some true MPPT controllers that are cheaper than some of the PWM controllers.
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Old 02-02-2015, 16:54   #54
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Re: Mixed AWG Solar Install

BelizeSailor also posted that he is using Kyocera 130W panels. Those have a thermal voltage coefficient of -0.0821 V/C and a NOCT temperature rise of 25C. So, at 25C ambient temperature the panels would be operating at 50C, or 25C above rated temperature. That equates to a Vmp voltage drop of 0.0821 x 25 = 2.05V. So, at actual operating conditions the Vmp would be expected to be ~17.6 - 2.05 = 15.55V, very close to the actual observed conditions.

[Edit for clarity]This is why similar sized 32-cell panels (OP's are 36) disappeared from the market, because they start with a Vmp that is about 2V lower than a 36-cell panel, and after you factor in the operating condition voltage drops they frequently don't make sufficient voltage to charge a nominal 12V battery (which requires 14-14.5V to get fully charged).[/Edit]

Sorry to those who don't like theory and equations here, but they actually explain a great deal, and good to see that the empirical field measurements actually tie quite well with the theory.
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Old 02-02-2015, 17:03   #55
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Re: Mixed AWG Solar Install

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Originally Posted by socaldmax View Post
I'm not sure where you got that from.

According to Belizesailor on page 1 of this thread, .
That's why I expected to see close to 17.xxV coming out of the panels.
His panels are very similar to mine (I have the Kyocera 120's), and I know these panels very well. When under a load, and in the equatorial sun, 15.5V or so is what they produce. A maximum Vmpp is only a specification, but is meaningless in practice other than comparing different panel specs.

He is not seeing any appreciable voltage drop in practice regardless of what everyone thinks the theory should be. He is also not experiencing any loss of useable current regardless of what everyone thinks the theory should be.

He is as fine as he is every going to get with these in this configuration.

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Old 02-02-2015, 17:05   #56
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Re: Mixed AWG Solar Install

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Originally Posted by Dsanduril View Post
BelizeSailor also posted that he is using Kyocera 130W panels. Those have a thermal voltage coefficient of -0.0821 V/C and a NOCT temperature rise of 25C. So, at 25C ambient temperature the panels would be operating at 50C, or 25C above rated temperature. That equates to a Vmp voltage drop of 0.0821 x 25 = 2.05V. So, at actual operating conditions the Vmp would be expected to be ~17.6 - 2.05 = 15.55V, very close to the actual observed conditions. This is why 32-cell panels disappeared from the market, after you factor in the operating condition voltage drops they frequently don't make sufficient voltage to charge a nominal 12V battery (which requires 14-14.5V to get fully charged).

Sorry to those who don't like theory and equations here, but they actually explain a great deal, and good to see that the empirical field measurements actually tie quite well with the theory.
Yes - this.

Except that these are 36-cell panels, not 32, and will pretty much always produce enough voltage for typical battery charging (especially for his gel batteries).

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Old 02-02-2015, 17:08   #57
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Re: Mixed AWG Solar Install

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Originally Posted by colemj View Post
Yes - this.

Except that these are 36-cell panels, not 32, and will pretty much always produce enough voltage for typical battery charging (especially for his gel batteries).

Mark
Yes, I understand. That's why they work, and drop to ~15.5V. If they were 32-cell panels they would have a Vmp ~15.5-16V, which in specs looks like enough to charge a 12V battery. But when you knock 2V off of that for normal temperature conditions you get down to ~13.5V, which after controller losses isn't enough. Voc would allow a 32-cell to get there, but at significant power loss, as the knee is quite sharp.
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Old 02-02-2015, 17:09   #58
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Re: Mixed AWG Solar Install

Ugh. Glad I never bought any of those panels.


Something just occurred to me...

all the more reason to run those suckers in series.


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Old 02-02-2015, 17:29   #59
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Re: Mixed AWG Solar Install

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Originally Posted by socaldmax View Post
Ugh. Glad I never bought any of those panels.


all the more reason to run those suckers in series.
Well, up until a relatively short time ago, these 36-cell panels were all that was available. And the newer higher voltage panels have physical dimensions that don't always fit a boat.

Running in series is a good idea with a single MPPT provided shading isn't an issue. This is rarely the case on most boats, and doing so requires multiple MPPT's and $$.

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Old 02-02-2015, 17:47   #60
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Re: Mixed AWG Solar Install

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Originally Posted by svlamorocha View Post
Kirchhoff's voltage law (KVL)[edit]

Voltage drop is additive, that is embedded in the definition of voltage/potential.. "Kirchhoff's second law", "Kirchhoff's loop rule" or "Kirchhoff's second rule".
Thanks. Will read up on that.
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