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Old 11-23-2023, 02:19 PM   #1
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MPPT sizing

Happy Thanks Giving to all
trying to figure out what size MPPT controller for a 400 watt bi-facial solar panel array. The manufacturer states the bi-facial panels can produce an additional 23% of power. I am looking at 500 watts max output according to my limited math skills. So do I go to 30 amp or next size up?
I'll be using this array to keep my 400 amp hr lifepo topped off while at anchor. We might add more panels later but for now we'll with 400 watts.
Any suggestions or guidance would be greatly appreciated

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Old 11-23-2023, 03:06 PM   #2
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400/13 = 30.7 amps s0 the 30 was already at the limit IMO
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Old 11-23-2023, 04:07 PM   #3
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Go with 40 amps and you will have a decent margin to cover the backside power production. I read one post here that said the backside was smaller than expected, maybe 10-15%.

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Old 11-23-2023, 04:15 PM   #4
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I know there's a jump in price, but your next set of panels will be more efficient than these are. I'd go with the Victron 50/100 MPPT. For an extra hundred bucks you'll have flexibility and options.

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Old 11-23-2023, 04:23 PM   #5
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Mppt

Thanks for the imput guys. I'm thinking on going with a victron blue tooth 100/50 incase I decide to increase the array. So how many watts can I run through 50 amp MPPT safely without melting anything down?
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Old 11-23-2023, 04:57 PM   #6
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Mppt

Hi guys
Just snagged a 100/50 victron smart solar at 185. Reading on a description it stated it could handle up to 700 watts. So that would allow me an additional 200 watts in the future. Thanks again everyone.
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Old 11-23-2023, 06:36 PM   #7
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We've got 525 watts of solar installed going through a Victron 100/30 MPPT controller feeding 450Ah of Trojan T-105's. The MPPT controller seems to cut back output at about 29 amps at the battery side so you shouldn't have to worry about melting anything down

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Old 11-23-2023, 07:36 PM   #8
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Here's an overview of how to size solar chargers. First, start with the solar panels.



Max Solar Power Output: The nameplate rating of the panel(s) is the starting point, but you should size for 125% of the nameplate. NEC requires it for land system. This covers various situations where solar irradiation can exceed the standard test conditions, and temperature can be below standard test conditions. If you have bifacial panels, you need to further oversize the controller.



Max solar array voltage: This is the max voltage that the panels can reach. It's important that this not exceed the controller's input rating since in most cases it will damage the controller. Like with Max power, you need to start with the nameplate max current (Voc, or Voltage, Open Circuit). But then over size is by 125% to accommodate low temps and high irradiation. I don't think bi-facial panels will increase voltage, only current, but am not certain. Read the panels spec sheet carefully to be sure.


Now let's look at sizing the controller with the above in mind.



Max Controller Current: This is the max battery charge current, and it will occur at max solar power output, and minimum battery voltage. Note that some controllers will just limit output if you try to exceed the max rating, where others may be damaged, so be careful about undersizing. So let's say you have 400W of nameplate rating, and that can increase to 500W with bi-facial. Take 125% of that and you get 625W. Now assume a low voltage of 12V (you might want to go even lower), and you have a max current of 625W/12V = 52A. With a 24V battery system it would only be 26A.



Max Controller Voltage: This is the max solar array voltage that the controller can handle. Most controller will be fried if you exceed this, so don't be tempted. Let's say that the panel's Voc is 60V. Take 125% of that and you get 75V. To me, a 75V controller would be cutting it too close so I would go with 100V (or something close to that).



Everytime I go through this process, I discover that I need a higher voltage and/or higher current rating than I guessed.
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Old 11-24-2023, 12:52 PM   #9
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MPPT sizing

Mr. Twisted Tree.

Great break down for MPPT sizing. I ordered the Victron Smart-Solar 100/50 yesterday but according to your formula I'm 2.5 amps shy off max output of the panels. Is the MPPT controller supposed to limit the amperage going into the battery bank or the output of the panel? Victron rated their MPPT
at 700 watts. There must be a thread on installing solar somewhere in here but can't find it. Any info on installing would be greatly appreciated
Thanks All
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Old 11-24-2023, 02:23 PM   #10
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Originally Posted by J.T kearney View Post
Mr. Twisted Tree.

Great break down for MPPT sizing. I ordered the Victron Smart-Solar 100/50 yesterday but according to your formula I'm 2.5 amps shy off max output of the panels. Is the MPPT controller supposed to limit the amperage going into the battery bank or the output of the panel? Victron rated their MPPT
at 700 watts. There must be a thread on installing solar somewhere in here but can't find it. Any info on installing would be greatly appreciated
Thanks All
Cheers
J.T.

You are probably OK on the amp rating. I believe the Victron MPPTs will just limit themselves, so under some cases you might forfeit a little power for a short time - no big deal.


Did you check the Voc for the panels? They can vary quite a bit from mfg to mfg.
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Old 11-24-2023, 04:13 PM   #11
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MPPT sizing

Mr. Twisted Tree
According to the sight at Bouge rv they rate this 200 watt bi facial at 22.5 +/_ 3 % VOC Does that sound right?
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Old 11-24-2023, 04:43 PM   #12
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Mr. Twisted Tree
According to the sight at Bouge rv they rate this 200 watt bi facial at 22.5 +/_ 3 % VOC Does that sound right?
Cheers
J.T.

I presume. So you will use two in series for 400W?


If so, that's 45V * 1.25 = 56V, so you are well under the 100V max.
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Old 11-24-2023, 05:34 PM   #13
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Quote:
Originally Posted by twistedtree View Post
This covers various situations where solar irradiation can exceed the standard test conditions, and temperature can be below standard test conditions. If you have bifacial panels, you need to further oversize the controller.
And on the water is the perfect place to exceed standard test conditions. I went 20% over with an MPPT for a south facing solar panel on our beach patio. The controller stopped working after a few days and I called the vendor. He asked if there was snow on the ground. No, just smooth water. Smooth like a mirror. And it was a cold, crisp day with my panel tilted enough to pick up the reflection. The cold solar panel with reflected light put out more than 20% over it's rated amperage. Go big.

I just read a study from the Netherlands where they were surprised to find actual production of bifacial panels were producing as compared to rated capacity. The odd thing was the highest production was when the panels were south facing but placed vertical. Bifacial panels can get more light to the backside in that orientation, but the bigger factor was heat dissipation. They didn't heat up as much and the cold air circulation was better. Who would have thunk it.
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Old 11-24-2023, 07:24 PM   #14
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MPPT sizing

Mr. Twisted Tree and all
If I were to wire two 12 volt panels in series won't that generate a 24 volt current verses in parallel. Just thinking of my cc batteries I replaced with 12 volt lithium. Paired up a couple 6 volt in series to to get 12 volts but still the same amp hrs as a single battery. So in my small mind wouldn't that be putting 200 watts at 24 volts into my MPPT? Does the MPPT sort that out on the other side (output to battery)? Maybe I ate to much turkey yesterday!!!!
I'm going to hang the panels off the stern rail of the flybridge so I'm hoping for a lot of refractive light from the water.
Thanks again all
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Old 11-24-2023, 08:49 PM   #15
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Mr. Twisted Tree and all
If I were to wire two 12 volt panels in series won't that generate a 24 volt current verses in parallel. Just thinking of my cc batteries I replaced with 12 volt lithium. Paired up a couple 6 volt in series to to get 12 volts but still the same amp hrs as a single battery. So in my small mind wouldn't that be putting 200 watts at 24 volts into my MPPT? Does the MPPT sort that out on the other side (output to battery)? Maybe I ate to much turkey yesterday!!!!
I'm going to hang the panels off the stern rail of the flybridge so I'm hoping for a lot of refractive light from the water.
Thanks again all
J.T.

You could wire them either in series or parallel. Either way you will get 400W.


And MPPT controller takes whatever the panel voltage is and converts it as needed to appropriate battery voltage, and it does it in such a way that maximizes the panel output.
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Old 11-25-2023, 12:57 AM   #16
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thanks Mr. twisted tree
Is there a preference between series and parallel connection and where are the key fusing points in the line in and the load out?
great info again
Thank you
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Old 11-25-2023, 09:20 AM   #17
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JT,

Wiring the panels in series increases the voltage going to the MPPT controller allowing longer runs from panels to controller or smaller wire guage. As TT mentioned the controller outputs the correct voltage for the 12v battery. The advantage with parallel wiring for panels is less effect from shading on the total output of the panels, which on a boat can be a real concern.

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Old 11-25-2023, 10:16 AM   #18
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JT,

Wiring the panels in series increases the voltage going to the MPPT controller allowing longer runs from panels to controller or smaller wire guage. As TT mentioned the controller outputs the correct voltage for the 12v battery. The advantage with parallel wiring for panels is less effect from shading on the total output of the panels, which on a boat can be a real concern.

James

Agreed, however I don't think the shading impact on series vs parallel panels is so clear. It depends on a lot of things, including how shading covers the panel(s), blocking diode configurations, etc. I think the best defense against shading is to use multiple controllers, but that adds expense not only in controllers, but also in disconnect switches, fuses, breakers, etc. What I do know for certain is that shading is a problem on boats. There is just no escaping it on 90% of boats. The remaining 10% are just annoyingly fortunate.


As for fusing/breakers/switches, here are some guidelines. Also follow ABYC guidelines to avoid getting dinged by a surveyor. And besides, they are good guidelines.:


- Between the battery and the controller you MUST have a fuse or breaker, and it needs to be located as close to the battery as practical. This is to protect the controller and wiring from the high current capacity of the battery, not the other way around. I prefer a breaker because it makes working on the solar system much easier when you can easily disconnect the batteries.


- You do not need a breaker/fuse on the solar input side of the controller, but I think a breaker or switch is a really good thing to have. In land based system I'm pretty sure they are required. Once again it's about being able to work on the system safely by having an easy way to disconnect the solar panels from the controller. This is especially important in higher voltage systems. Pay careful attention to the voltage and current ratings of any breakers or switches, especially if you have higher voltage panels. It's harder and harder, and more and more expensive to find breakers and switches that have high DC voltage ratings.


- In some cases you need fusing between parallel panels/strings, depending on how many strings you have in parallel. This is to prevent all the surviving panels from dumping all their current into a failed panel. In the panel specs you will find a max fuse size spec. That's the most current that can run through the panels without creating a hazard. To figure out if fusing is required, you need go back to the max current that calculated earlier (125% of Isc). That's how much current each panel/string can produce, and you then need to figure out how many such strings/panels you can have before exceeding the max fuse rating. That's how many strings you can have if one fails before you need fusing. So let's say the fuse limit for your panels is 15A, and your panels can produce 7A for each string. Two strings will produce 14A and is under the 15A limit, so that's the max you can have dumping current into a failed string. In that case you can up to three strings before you need fuses, allowing for one failed string, and two surviving strings. If you have 4 strings, then three survivors can produce 21A which will overload the failed string and create a hazard. In that case you need to install 15A fuses in each string before they are combined together. And again, watch the voltage ratings. Breakers are available up to about 125-150VDC, and above that you have to use fuses, and particular fuses rated for higher voltages.


One other thing unrelated to fuses and switches.... Be certain to wire the battery negative on the controller before any other connections. More than one controller has been smoked by doing otherwise.
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Old 11-26-2023, 11:47 PM   #19
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MPPT sizing

This is really challenging, what's not to love !!!!
OK so I'm going from a 550 watt system potential at my solar array to my 100 voc/50 amp MPPT with a on/off switch between the two on a 10 gauge cable. If I were to tee my panels together in series would I need a larger cable to the MPPT. Also from the MPPT to the battery via fuse and on/off switch what size cable should be used? Don't want to undersize and loose efficiency from the panels to the battery
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Old 11-27-2023, 08:25 AM   #20
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Originally Posted by J.T kearney View Post
This is really challenging, what's not to love !!!!
OK so I'm going from a 550 watt system potential at my solar array to my 100 voc/50 amp MPPT with a on/off switch between the two on a 10 gauge cable. If I were to tee my panels together in series would I need a larger cable to the MPPT. Also from the MPPT to the battery via fuse and on/off switch what size cable should be used? Don't want to undersize and loose efficiency from the panels to the battery
Life Long Learning Cheers
J.T.
If you run the panels in series you double the voltage. The amps stay the same. No increase in cable size.
For the mppt to battery cable size, you need to factor in the distance to the batteries.
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