PARALLEL or SERIES-PARALLEL?

[satanas]

Hey y'all ~ I'm losing my mind trying to figure out whether a purely parallel or series-parallel config is the best way to go for my setup. It's a 21ft Travel Trailer; planning to use only solar + off-shore option with the possibility of alternator charging via tow vehicle later on. I've been doing tons of research over the past few weeks and piecing everything together for my needs. I could really use some advice on this specific question...
I have a 24V/300Ah battery bank, a victron 100V/50A mppt charge controller, and [4] 200W/"24V"/8.33A rich solar panels. the benefits of using less equipment in series-parallel are enticing, but would series-parallel result in the voltage being too close to the 100V allowance of the charge controller? any guidance is much appreciated! Thanks again!

 

[MichaelK]

You could not wire 4S1P. Assuming your panels are ~30V, that would be 8.33amps at 30 + 30 +30 +30 = 120VDC

You may wire them in a 2S2P configuration. That would be 8.33amps at 30V + 30V = 60V + 8.33amps at 30V + 30V = 60V = 16.66A at 60V. Even below freezing, your Voc is likely to be only ~80Voc, comfortably below the 100V limit.

I don't think 1S4P will work if the panels get very hot. As the temperature goes up, the voltage goes down. At 28-29Vmp, your batteries might not fully charge, and the voltage is definately not high enough for battery equalization.

 

[satanas]

You could not wire 4S1P. Assuming your panels are ~30V, that would be 8.33amps at 30 + 30 +30 +30 = 120VDC

You may wire them in a 2S2P configuration. That would be 8.33amps at 30V + 30V = 60V + 8.33amps at 30V + 30V = 60V = 16.66A at 60V. Even below freezing, your Voc is likely to be only ~80Voc, comfortably below the 100V limit.

I don't think 1S4P will work if the panels get very hot. As the temperature goes up, the voltage goes down. At 28-29Vmp, your batteries might not fully charge, and the voltage is definately not high enough for battery equalization.

This is super helpful, thank you. The reason I was worried about exceeding the charge controller's limit in 2S2P config is because the solar panels have a Voc of 45.4V. Is this a degree that is rarely, if ever, actually met?

 

[MichaelK]

This is super helpful, thank you. The reason I was worried about exceeding the charge controller's limit in 2S2P config is because the solar panels have a Voc of 45.4V. Is this a degree that is rarely, if ever, actually met?

Nope! Voc will happen every day. As soon as the first ray of light hits the panel in the morning, the Voc will be exposed to the controller till power starts flowing. The controller will also see Voc when the batteries reach full charge, and the charging circuit opens.

Here is one critical point! How cold does your area get. The voltage of a panel goes up as the temperature goes down. At freezing, the voltage is typically 1.10-1.12X what it is rated for at 25C (room temp). So, on a frosty morning, 2 panels in series will have a Voc of (45.5V + 45.5V) X 1.12X = 101.9Voc.

I think it would be wise to find a new controller with a higher voltage limit. I think you can use the controller you have for the summer, but I think it would be a good idea to upgrade it before the fall cooldown starts.

 

[GSXR1000]

what is the amp of each panel?

 

[satanas]

Nope! Voc will happen every day. As soon as the first ray of light hits the panel in the morning, the Voc will be exposed to the controller till power starts flowing. The controller will also see Voc when the batteries reach full charge, and the charging circuit opens.

Here is one critical point! How cold does your area get. The voltage of a panel goes up as the temperature goes down. At freezing, the voltage is typically 1.10-1.12X what it is rated for at 25C (room temp). So, on a frosty morning, 2 panels in series will have a Voc of (45.5V + 45.5V) X 1.12X = 101.9Voc.

I think it would be wise to find a new controller with a higher voltage limit. I think you can use the controller you have for the summer, but I think it would be a good idea to upgrade it before the fall cooldown starts.

I'm in southern california and it *never* gets that cold. I was planning on taking the rig east through the Fall, so I may upgrade the charge controller just to be sure. Is there a detriment to getting a charge controller that has a voltage/amperage far beyond what the system generates? (besides cost, of course) ~ do you think a MPPT 150/45 would be better suited to this setup?

 

[satanas]

what is the amp of each panel?

I incorrectly stated it was 8.33A above, but it's actually 5.32A per panel.

 

[MichaelK]

I'm in southern california and it *never* gets that cold. I was planning on taking the rig east through the Fall, so I may upgrade the charge controller just to be sure. Is there a detriment to getting a charge controller that has a voltage/amperage far beyond what the system generates? (besides cost, of course) ~ do you think a MPPT 150/45 would be better suited to this setup?

I am also in southern California, and it most certainly DOES get to freezing. I'm located about 30 minutes away from Renogy. I scrape frost off my windshield several times over the course of a winter when going to work in the morning. Not every morning, but enough that I don't get bananas in the back yard.

Take a look at Epever's Tracer series. They make an affordable 50A in 150 and 200V versions.

Oversizing inverters is more of an issue with too big. A bigger charge controller might consume slightly more power, but you are not likely to ever notice.

 

[satanas]

I am also in southern California, and it most certainly DOES get to freezing. I'm located about 30 minutes away from Renogy. I scrape frost off my windshield several times over the course of a winter when going to work in the morning. Not every morning, but enough that I don't get bananas in the back yard.

Take a look at Epever's Tracer series. They make an affordable 50A in 150 and 200V versions.

Oversizing inverters is more of an issue with too big. A bigger charge controller might consume slightly more power, but you are not likely to ever notice.

Ah, yeah, some zones in the inland empire, the valley and coast definitely get super chilly. Where I'm at though it hasn't gotten to freezing in decades! Still, I think you're right and it would be good to size up, especially in the face of increasingly unpredictable weather patterns.
I got the charge controller from battleborn and it seems like they haven't shipped it yet, so I'm going to try to swap for the 150/45 victron charge controller since I got in on sale and don't believe cancelling is an option.

Curious about your take on oversizing inverters? I've looked into this and haven't found a lot of helpful info. I know we've sort of departed the initial question but you're super knowledgeable and I'm appreciating picking your brain on this topic!

 

[GSXR1000]

1s4P would be fine... 24 amps max
this would work

 

[svetz]

You might find this thread of interest too: Figuring out how many panels in series and parallel based on your MPPT

 

[MichaelK]

Curious about your take on oversizing inverters? I've looked into this and haven't found a lot of helpful info. I know we've sort of departed the initial question but you're super knowledgeable and I'm appreciating picking your brain on this topic!

I myself apply the 2X rule for most electrical items. Let's say I determine that the max load I am likely to run is 2000W. With the 2X rule I'd be looking at 4000W inverters. Same value applies to generators and even your pickup truck. I would be very uncomfortable trying to carry a 1000lb load of cement mix in a pickup bed rated for 1000lb.

The reality is that many manufacturer's are very liberal with the capacities of their products. Good for marketing. As long as you base your decision in the numbers they print, they get your money. If the unit fails under load; well they'll find a way to say it was user error. Or, just ship it back to China (pre-paid both ways) at retail shipping rates.

Using the 2X rule helps protect yourself when trying to run equipment at the ragged upper edge of what they are designed for.

The bigger inverter WILL consume more power. My solution is to just add another panel or two. Solar panels are now dirt cheap, so there is no good reason any more not to have the maximal number of panels to meet your needs.