diy solar

diy solar

MPPT sizing

Followed most of what you’re saying :)

Unfortunately I don’t have the RV with me but can assume simply based on where we’re forced to park at times, that there will be some shading. I suppose there’s only so much a person can do. I think I can get most of the panels in front of the AC unit so shading from it shouldn’t be an issue. I am planning a tilting mount but more to allow me to get to the wiring and to clean underneath. Wouldn’t stop me from tiling to to avoid shade the odd time.

What is the reasoning for not needed fusing with only two in parallel? Less current I’m assuming but can’t quite see how that would be in the case of a shorted panel.
 
Here is another thing to think about - how difficult would it be to run that extra set of wires from the roof?

The two 100/50’s are cheaper than a 150/100. I think two SCC would out preform a single larger one when some shading issues occur.

Just a thought
I wish it were easy. The unit came pre-wired with a single 10AWG pair into the basement. Knowing RV manufactures it’s probably routed around 4 corners and attached with 67 staples.
 
What is the reasoning for not needed fusing with only two in parallel? Less current I’m assuming but can’t quite see how that would be in the case of a shorted panel.
This may help answer your questions.


 
On my October camping trip I had the two (non-tilted) roof panels that were never in the shade and the two (tilted) ground panels that did get some shade. In almost all cases when I checked, the ground panels outperformed the roof panels. All four panels are the same make/model. Each set goes to it's own Victron 100/50 solar charge controller.
 
On my October camping trip I had the two (non-tilted) roof panels that were never in the shade and the two (tilted) ground panels that did get some shade. In almost all cases when I checked, the ground panels outperformed the roof panels. All four panels are the same make/model. Each set goes to it's own Victron 100/50 solar charge controller.
It could have been that the ground was acting as a heatsinks, keeping the panels cooler than those mounted on the roof. Most types of panels have increased efficiency at cooler (not necessarily cold) temperatures.
 
It could have been that the ground was acting as a heatsinks, keeping the panels cooler than those mounted on the roof. Most types of panels have increased efficiency at cooler (not necessarily cold) temperatures.

I don't think heat was a problem. It got down to -8° F at night and there was at least 12" of snow on the ground.
 
So I'm new to solar and trying to read up and I'm finding something confusing me about mppt controllers. Mppt controllers are not only more efficient but also allow for higher input voltage with less amperage, therefore saving on cost for bigger gauge wire right? But im confused about the input.

For an example this one (https://store.santansolar.com/product/epever-3210-30a-mppt-solar-charge-controller/) says max pv input when using a 12v battery is 390w, and max pv open circut voltage is 92-100 depending on temp.

If i 2s3p 240w panels with voc 37.2 each and 8.37a isc each, that would be 74.4v at 25.11a correct? But each panel being 240w, that would give me 1440 watts of input? I don't underatand what I'm missing here. Can someone please explain?
 
I am assuming your battery bank is 12v. If it is higher smaller mppt’s can be used. I will reference Victron mppt’s.

An array that produces 1440watts divided by 14.4 volts gives 100 amps going into the 12v battery. So you need a mppt that can produce 100 amps - like a Smart Solar 150/100.

On the input (panel) side, you cannot ever exceed the max voltage. The time you do that is one cold clear morning. In your referenced array, your panels are 37v in a 2s- so 74v. Your mppt needs to handle 100v.

The mppt takes the power in and adjusts the voltage and amperage to match the batteries. So it can change 1440 watts of 74v and 25a into 1440watts of 14.4v and 100a.

Your array of 2s3p may be better off as a 3s2p with a mppt 150/100.
Or break it up into three 2s with each having a 100/40 controller.
Hopefully I helped...
 
If i 2s3p 240w panels with voc 37.2 each and 8.37a isc each, that would be 74.4v at 25.11a correct? But each panel being 240w, that would give me 1440 watts of input? I don't underatand what I'm missing here. Can someone please explain?

Not 74.4V "at" 25.11A. Nor would each 240W panel be 37.2V "at" 8.37A.

"Voc" means "Volts open circuit", i.e. at zero amps. "Isc" means "Amps short circuit", i.e. at zero volts.

The 240W figure comes from 30.4 Vmp x 7.89 A Imp = 239.9W, because it measures 30.4 V at 7.89 A (momentarily, under "STC" or "Standard Test Conditions" which means panel is at 25 degree C and exposed to 1 full sun (1000W/m^2) but hasn't warmed up yet.)

"YMMV" (Your Mileage May Vary) and "PTC" or "Performance Test Conditions" of realistic temperature and breeze will produce lower power.


"
Specifications:

  • Rated Power: 240W
  • Open circuit voltage (VOC): 37.2 V
  • Max power voltage (VMP): 30.4 V
  • Short circuit current (ISC): 8.37 A
  • Max power current: 7.89 A
  • Power Tolerance 0/+3%
"
 
Not 74.4V "at" 25.11A. Nor would each 240W panel be 37.2V "at" 8.37A.

"Voc" means "Volts open circuit", i.e. at zero amps. "Isc" means "Amps short circuit", i.e. at zero volts.

The 240W figure comes from 30.4 Vmp x 7.89 A Imp = 239.9W, because it measures 30.4 V at 7.89 A (momentarily, under "STC" or "Standard Test Conditions" which means panel is at 25 degree C and exposed to 1 full sun (1000W/m^2) but hasn't warmed up yet.)

"YMMV" (Your Mileage May Vary) and "PTC" or "Performance Test Conditions" of realistic temperature and breeze will produce lower power.


"
Specifications:

  • Rated Power: 240W
  • Open circuit voltage (VOC): 37.2 V
  • Max power voltage (VMP): 30.4 V
  • Short circuit current (ISC): 8.37 A
  • Max power current: 7.89 A
  • Power Tolerance 0/+3%
"
Thanks, but my question was more about the mppt. From what I read about mppts it said to make sure the voc and isc of the panels did not exceed the ratings of the mppt. So if I used those panels in a 2s3p config that would be the numbers I used, which would be within the ratings of the mppt and would be 1440w, but the mppt also says not to exceed a 390w array for charging a 12v battery, hence my confusion
 
Thanks, but my question was more about the mppt. From what I read about mppts it said to make sure the voc and isc of the panels did not exceed the ratings of the mppt. So if I used those panels in a 2s3p config that would be the numbers I used, which would be within the ratings of the mppt and would be 1440w, but the mppt also says not to exceed a 390w array for charging a 12v battery, hence my confusion
My first comment for you is, 'I hate the "AN" Controller Series'. If you search the board, you will see lots of complaints from people trying to use the "new" and biggest one. I personally have replaced two failing and smaller "AN" controllers in the RVs of other people, using Tracer "BN" and Victron models respectively (on which I have never had a call-back).

And the original poster was not discussing a Solar Controller as small as the one you just pointed at. However, with regard to yours:

EpEver specified the "maximum array size" by simply multiplying the product of 30A maximum current (which will occurs on the output side from the SCC) times "13.0 Volts". This was ridiculous. A small error was made by using "13.0 Volts", even though the MPPT will be expected to charge the batteries with higher Voltage (typically 14.2V for Lead-Acid, or maybe as much as 14.5V for LFP batteries) while running "Bulk/CV" charging. But their giant mistake is assuming that "390 Watts" of panels will always be performing at 390 watts - they will, in fact, hardly ever reach their "STC" rated output levels. And if they do, that will occur for only a few minutes around solar noon.

So it is perfectly OK to oversize the panels, the SCC just refuses to take in power which it doesn't want to use for charging the batteries. With better controllers, (such as EpEvers's own "Tracer BN" series, and with the Victron MPPTs) I am comfortable with oversizing the Array - relative to the maximum output power of the SCC - by a factor of 1.5x to even 2x. At least one manufacturer rates their SCCs to be able to handle an array sized at 3x the maximum output of the SCC, though I do not remember at this moment which company that was.

I feel that any time your Array is or "needs to be" to be more than 2x the size of your controller, you should just get a bigger Controller. If I was choosing an MPPT for a "1440 Watt array", I would choose either a pair of 40A controllers (splitting the array), or a big one. But it all depends on the needs of the battery bank, and the needs of the ultimate "consuming appliances".
 
Back
Top