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Do I need a 30A or 40A SCC? Calculators disagree....

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zipsonic

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System is 12v, 4 Panel 100W, 18v-21.6v, 5.5A-6.11A. 2 Series, 2 Parallel.

Apparently rule of thumb is to size it to be 2/3 max output. 400W / 14.4 Volts = 27.8A. If it's sized at 2/3, that would require a 41A output. I think 40 would be close enough.

However, Victron's calculator says to use a 30A controller.

I would think that Victron knows what they're doing. Are their controllers that more efficient?

So, do I go with a 100/30 or do I go higher? There doesn't seem to be a 40A, so 100/50 is the next step. Any thoughts would be appreciated...
 
Victron prefers you over-panel their controllers, but 30A is enough. At 30A*14.4V = 432W.

If the 2/3 thing is a rule, I've never read that in a SCC's manual, and Victron's propensity for recommending over-paneling means you're going to push the SCC to its max regularly. Thus a manufacturer is recommending 100%, not 2/3.

Additionally, for the majority of the day, the panels will be at less than maximum output, so it will be below the 30A rating.

The 50A should be bought if you think you might want to expand the number of panels.
 
zipsonic said:
Apparently rule of thumb is to size it to be 2/3 max output. 400W / 14.4 Volts = 27.8A. If it's sized at 2/3, that would require a 41A output. I think 40 would be close enough.
Click to expand...
snoobler said:
If the 2/3 thing is a rule, I've never read that in a SCC's manual
Click to expand...

I have also never heard of this 2/3rds rule. Could you give a link to where you read that?

Most controllers aren't very concerned with exceeding the power output by modest amounts, they will just limit current, what matters most is not exceeding the current or voltage inputs.

Regarding the proper size SCC:

Panel Specs:
Voc = 21.6V
Isc = 6.11A

In 2S2P: Voc = 43.2V ( x 1.25 to account for extreme temperatures = 54V), Isc = 12.22A, Watts = 400W (400W / 12V = 33A)

So a charge controller that can handle an input of at least 55V and 13A and an output of about 30-40A would be ideal.

The Victron Smartsolar 100 | 30 would fit the bill.
 
There will be loss as heat but unless you are swimming in the shallow end of the price pool it's not going to be that great a percentage.

Although they don't make the efficiency graph public Victron does advise that their peak efficiency is 98%. Let's say we are providing 1200W to Victron 100/50 from the array and can sink the full amount into the load. We would be dropping about 25 watts as heat, assuming Victron has targetted peak at full load. I don't know if they have or haven't.

Now here's the catch.

Most controllers, Victron inlcuded, do not run at peak efficiency across the full power range. Victron privately has stated 95%, but I've not tested that myself. Let's assume it's true. Let's use a 250/100 Victron but still supply the same wattage to it. Now that we are running off the peak, we are dumping 60 watts as heat. The same temperature rise is not going to be seen as the 250/100 has more generous heat sinking which can dissipate the heat more easily but 60 is more than 25.

Without understanding the efficiency curve of a device simply buying a bigger one could well result in more power loss.
 
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zipsonic said:
I've read it in more than one place, and I've definitely seen the add 25% for environmental spikes. Here's the last place I read that 2/3 rule:

How Big of a Charge Controller Do I Need? | Solar Biz

How Big of a Charge Controller Do I Need?
www.thesolarbiz.com www.thesolarbiz.com
Click to expand...

You are likely thinking about panel VOLTAGE, not current. The guidance in the Victron manual is 22%. If a Victron system is rated for 100V, divide by 1.22 for cold weather voltage spike allowance, 100/1.22 = 82V, i.e., panel Voc input to the SCC should be 82V or less for a Victron 100/30.

While I generally regard The Solar Biz as a good resource, that's the only place I've seen those current recommendations, and that goes against Victron's own practices/recommendations.
 
You have to match the voltage you feed the controller to it's limitations. Do not feed higher voltage than the controller is rated.
 
Funny how you post this as I plan to run 3 panels and sometimes 4. Just ordered the Victron 100/30 as it seemed to fit for what I will need plus its smaller footprint. From what I have been seeing coming out of the panels I feel with 4 panels I'm not ever really going to spike near that 400 watt mark or these figures on the panels. So this controller is fine for my needs.
 
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RV Solar Power Blue Prints

Building a vehicle mounted solar power system? Let me help.
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This also recommends 40A for a 400W 2S2P setup. Doesn't cite the 2/3 sizing, but I figured that's why 40A was selected.

Thanks for all the great information and reasoning. I really appreciated the over-paneling posts. Seems to be a fair amount of conflicting opinions and recommendations.

*Edited for clarification*
 
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I've always been a big believer in NOT designing something close to the needed spec. There is no harm is going bigger. If it were me, I wouldn't think twice about getting a more capable SCC and the worry of over-paneling or exceeding limits becomes moot.
 
I am in agreement with those suggesting you oversize the SCC rather than try to hit it on the nose, or plan to panel to its limit. This, in addition to a safety margin, provides expansion capability if you find your panel production doesn’t meet your needs. Given the many considerations, I felt going above had more advantage than disadvantage. Further, though there is a price difference between the two options you’re considering, I would surely go the higher capacity route and put concerns of overloading the SCC in the rear-view mirror. Buy once, cry once.
 
I get the concept of overpaneling, but I really never want to leave power on the table, so I will almost always oversize my SCC to allow for future expansion. In this example, I would probably go for the 100/50 if you want to stick with Victron - to allow for panel expansion.

That said, my 5940W array is slightly over the 5800W my SCC can handle, but I really never want to add more than that, and even if I did, I'm not aware of a SCC that can handle more than 100A... :)
 
gnubie said:
...Most controllers, Victron inlcuded, do not run at peak efficiency across the full power range. Victron privately has stated 95%, but I've not tested that myself. ....

...Without understanding the efficiency curve of a device simply buying a bigger one could well result in more power loss...
Click to expand...
I have tested Victron & a few other popular brands by measuring the voltage & current on both sides of the controller. The real world efficiency is closer to 92% at peak voltage & current end of the scale - 4 panels in series pushing 25 Amps or more to the battery.
 
In my mind, if you've sized the battery pac properly for your needs, and have sized the PV array to be able to replenish that battery in an acceptable time, the SCC is nothing more than a vehicle for getting the PV power to the battery. Makes no sense to size it so it has to work at near or over max capacity. Size the SCC with ample "head space". The harder piece of electronics works, the more likely the chance it will fail. Mine is sized between 40 and 50%. If I ever want to expand, I can with no worry.
 
I just read my TriStar manual. Funny but this paragraph is not in the download manual from their web site. So I can't copy and paste, I will paraphrase. "To comply with -- NEC -- The current rating of the controller must be equal to or greater than 125% of the array's short circuit current output (Isc)."
My 45 amp controller "TS-45: 36 amps Isc maximum from panel array."
 
I haven't heard the 2/3 rule either. However, I do size differently based on PWM or MPPT. For PWM, I take ISC x 1.25 for over irradiance, since its output is not current limiting. For MPPT, I simply do the Solar Watts / Battery Voltage, as that's what most manufacturers have told me to do.
 
I'm glad this forum exists. This can be kind of frustrating for a newbie.

It seems there is no clear answer here. I would imagine that if its an NEC code, then manufacturers would push to comply. Especially if it pushes someone to a larger more expensive product. Also places that cite a "rule" while others who have been doing this for a long time have never heard of the rule.

I'm going to go the 100/50 route. It's a bit more expensive and I don't think I'll ever add more panels (not much more roof space), but it seems to be the "safer" route. But it's hard to learn and know what's right with conflicting information.
 
Zil said:
I just read my TriStar manual. Funny but this paragraph is not in the download manual from their web site. So I can't copy and paste, I will paraphrase. "To comply with -- NEC -- The current rating of the controller must be equal to or greater than 125% of the array's short circuit current output (Isc)."
My 45 amp controller "TS-45: 36 amps Isc maximum from panel array."
Click to expand...

This is a different issue than this discussion. This is about the ARRAY'S CURRENT INPUT INTO THE CONTROLLER, NOT THE BATTERY CHARGING CURRENT.

from:


Max PV short circuit current 35A and 60A for the input for 30 and 50, respectively.

OP is at at 12.2A INPUT to the controller - well under both.
 
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