Super new guy, how’s this look?

[BDubya907]

I would seriously look at going 24V for your system. It’s a big system for 12V. Start looking at the amps you will be pulling, and the wiring and fusing it will require. This will help you decide whether 12V is even feasible.

I keep going back and forth between 12V & 24V. The few reasons I go back to 12V:

1. I really like that the Victron MultiPlus II will do all the switching for me, since I have split phase 50A shore / generator power. However, If I go with 24V, that's not available, and I'd need to get a smart phase selector.
2. I really like the idea of 24V allowing for smaller wire sizes, not so much for cost, but because of the reduced current and therefore reduced heat.
3. I'd like to be able to charge while I'm driving, and that seems to be much simpler at 12V with the stock alternator.
4. I believe availability of 12V solar panels / sizes is better?
5. Won't need a 24v to 12v converter for DC loads. Again, not a cost thing, but more of a "one more thing that could fail" thing.

I have revised my "rough" Energy Audit, specifically to remove the coffee pot, and to correct amount usage of a few things. I've also drawn out what I believe my system would look like, if it was a 12V system. I still have some wire sizes to figure out, and I need to determine what my solar array should look like, and the MPPT Charge Controller size, but those things will all be determined by whether I go with 12V or 24V.

Do you have any other insights that would help steer me in the best direction?

 

[Bluedog225]

48v. If starting from scratch. I think 24 will be a bit of a dead end going forward.

 

[MisterSandals]

I believe availability of 12V solar panels / sizes is better?

This is a no issue with the advent of MPPT SCCs.

Do you have any other insights that would help steer me in the best direction?

You have a good grasp of the issues. It’s a shame there isn’t a perfect solution. Alternator 12V to 24V is doable. 24V to 12V for the coach is doable too though I don’t like the idea (probably not as bad as it sounds).

Have you considered other brands to give split phase for 24V?

Magnum? Schneider? Samlex EVO? Outback?
I admit I am just throwing out names…I do not know this product area very well (maybe not at all).

Sorry for not steering…

Have you looked in the “Show and Tell” forum for inspiration? Lots of success stories there.

 

[BDubya907]

Alright, finally made it home from work! Got up on the roof of our motorhome to get measurements and lay it out. Looks like I should probably go with (2) separate arrays…one set of 200W panels, and one set of 100W panels. I need to fine tune my sketches and snap some chalk lines since the edge is curved and there’s nothing really suitable to measure from. So 2 arrays, (2) SCCs, and I’m just about positive I’ll build the racks to allow for tilting, on both the long edge and the short edge. I watched a video where they experimented with flat vs. tilted, and on flat they were getting 37-ish amps, and tilted they got in the mid 50s, so it looks like the effort was worth it for them.

So…
•clean up my roof schematic and get accurate measurements of available real estate.

•determine the layout of the two arrays
•do the math to determine wire / OCPD sizes
•design the tilting racks
•”install” it all on paper to make sure it works
•create a conductor schedule, so that I can have a comprehensive document with every single conductor listed, where it runs from and to (device & terminal) size, lug and hole size at each end
•redo my energy audit after measuring loads on stuff. This won’t give me “real world” durations, but at least real world watts.
•redo my system diagram
•start ordering more stuff
•clean my roof!

 

[Bluedog225]

I haven’t gone back and read the thread, but…those seem like small panels. Maybe don’t buy anything yet. folks are always getting ahead of themselves and end up regretting purchases. Planning first.

 

[BDubya907]

I haven’t gone back and read the thread, but…those seem like small panels. Maybe don’t buy anything yet. folks are always getting ahead of themselves and end up regretting purchases. Planning first.

I’ve kind of “tunnel-visioned” myself on Renogy panels, or HQST (same exact panel for about $10 less each) for no real reason other than they’re very easy to get and seem to be pretty well reviewed. I think the smaller size of them as compared to a larger wattage panel will allow me to make the most efficient use of roof space, but I still have a lot of research to do, math, and paper layouts.

Thanks for the tip though, I’m pretty good at focusing on what I think I want / need and fail to consider other options.

 

[Bobert]

I’ve kind of “tunnel-visioned” myself on Renogy panels, or HQST (same exact panel for about $10 less each) for no real reason other than they’re very easy to get and seem to be pretty well reviewed. I think the smaller size of them as compared to a larger wattage panel will allow me to make the most efficient use of roof space, but I still have a lot of research to do, math, and paper layouts.

Thanks for the tip though, I’m pretty good at focusing on what I think I want / need and fail to consider other options.

Think outside the box to be sure you chose what is best for you. I ended up using “used” 240 watt panels at 60 some odd dollars each when the shipping was figured in. I raised the panels above my vents using brackets I made from extruded aluminum bleachers that the local school threw away and my dad saved “Incase somebody could use it some day” this picture was taken before I covered the passenger side gapbwith aluminum and built an airfoil at the front to decrease wind resistance, prevent lift under the panels and make it look better.

 

[curiouscarbon]

i am late and didn't read everything, just wanted to share

for my project, for AC inverter i went with the multiplus 24/3000 instead of the 12/3000 because of the high amps needed to serve the max rating of the 12V one.

serving a 6000W surge with 12V is like 500A+ which was way too much for me to feel confident to engineer properly.

good luck and cool project

 

[BDubya907]

i am late and didn't read everything, just wanted to share

for my project, for AC inverter i went with the multiplus 24/3000 instead of the 12/3000 because of the high amps needed to serve the max rating of the 12V one.

serving a 6000W surge with 12V is like 500A+ which was way too much for me to feel confident to engineer properly.

good luck and cool project

I definitely hear you there. Certainly would be nice to drop down in wire, lug, and fuse sizes. The only reason I’m trying so hard to stay at 12V is because the MPii in 24/3000 isn’t available yet, and I’ve got a 50a RV. With my luck, I’ll just be turning on the main switch to test the new system out and the 24V will become available?

 

[curiouscarbon]

haha, victron might be watching this thread and launch the mp-ii 24v as soon as you upload a pic of an mp-i in operation ?

so many people here are looking to them for the mp-ii series.

thanks for sharing your project ?

 

[BDubya907]

Alright, so I've cleaned up my roof layout, and determined that the maximum use of available space would result 2 separate arrays. I believe this means 2 charge controllers, which is fine. I'd like to use Victron for everything that I can, as I'd like to have (1) display that covers the entire system. Looking at the Datasheet for the MPPT 150/70, I believe this is what I'll need to go with for each array, considering volts, watts, and Isc. Am I looking at this correctly? Also, I've convinced myself that I need as many panels as possible to recharge as quickly as possible, which would allow us to use as much as possible. The size of the 100W Renogy allows me to get the closest to 2000W when compared to the 175W or 200W panel sizes, with my available space. But damn...$1100 for 2 SCCs? Just so I can stay with 12V and take advantage of the MPii's built-in switching for 50A systems?

2 separate arrays, two charge controllers

Array "A" Would be (9) 100W Panels, arranged in 3S3P

• Voc is 24.3v per panel
• Isc is 5.39A per panel
• 72.9V / 900W / 12.35A for all (9) Panels
• Lowest Temperature would be -5f, so correction factor of 1.2
• Temp corrected to 87.48V / 900W / 10.2A

Array "B" Would be (10) 100W Panels, arranged in 5S2P

• Voc is 24.3v per panel
• Isc is 5.39A per panel
• 121.5V / 1000W / 8.23A
• Temperature correction factor of 1.2
• Temp corrected to 145.8 / 1000W / 6.8A

 

[littleharbor2]

Having panels tucked up against your A/C units will create shadowing that will basically kill the whole string's output. Depending on your trailers orientation to the sun your shadows will be a morning, afternoon or all day problem. I see the hash mark shaded areas on your plan drawing. Is this showing raised roof sections? If so the A/C shadow thing applies unless you get elevated enough to eliminate any shadows.

The Isc. numbers in your calculations look a little odd. Temperature corrections don't apply to current, only voltage, if that's why you're changing them.

 

[Bobert]

Alright, so I've cleaned up my roof layout, and determined that the maximum use of available space would result 2 separate arrays. I believe this means 2 charge controllers, which is fine. I'd like to use Victron for everything that I can, as I'd like to have (1) display that covers the entire system. Looking at the Datasheet for the MPPT 150/70, I believe this is what I'll need to go with for each array, considering volts, watts, and Isc. Am I looking at this correctly? Also, I've convinced myself that I need as many panels as possible to recharge as quickly as possible, which would allow us to use as much as possible. The size of the 100W Renogy allows me to get the closest to 2000W when compared to the 175W or 200W panel sizes, with my available space. But damn...$1100 for 2 SCCs? Just so I can stay with 12V and take advantage of the MPii's built-in switching for 50A systems?

2 separate arrays, two charge controllers

Array "A" Would be (9) 100W Panels, arranged in 3S3P

• Voc is 24.3v per panel
• Isc is 5.39A per panel
• 72.9V / 900W / 12.35A for all (9) Panels
• Lowest Temperature would be -5f, so correction factor of 1.2
• Temp corrected to 87.48V / 900W / 10.2A

Array "B" Would be (10) 100W Panels, arranged in 5S2P

• Voc is 24.3v per panel
• Isc is 5.39A per panel
• 121.5V / 1000W / 8.23A
• Temperature correction factor of 1.2
• Temp corrected to 145.8 / 1000W / 6.8A

View attachment 94572View attachment 94573

That layout looks really good. I will send one word of caution. When you have panels installed very close to the front of a vehicle they are susceptible to a tremendous amount of lifting from the wind while driving. I raised my array 8” off the roof making this factor even more pronounced and so I fabricated an airfoil to prevent this and it also give more strength to the front of the array and less wind resistance as a whole. If you google “solar panel flew off” you will notice that the loss of solar panels on the highway is almost always the front panels.

Having panels tucked up against your A/C units will create shadowing that will basically kill the whole string's output.

This is true however I have found that you seldom get anything like ideal conditions for solar when camping. The reason you want as many solar panels as you can get is not for the sunny days to get max output but rather the cloudy days when shade really isn’t a factor. It’s less than ideal placement of panels but it’s not like you have any other options.

 

[BDubya907]

Having panels tucked up against your A/C units will create shadowing that will basically kill the whole string's output. Depending on your trailers orientation to the sun your shadows will be a morning, afternoon or all day problem. I see the hash mark shaded areas on your plan drawing. Is this showing raised roof sections? If so the A/C shadow thing applies unless you get elevated enough to eliminate any shadows.

The Isc. numbers in your calculations look a little odd. Temperature corrections don't apply to current, only voltage, if that's why you're changing it

The hashed areas are just a visual reference of the space allotted to each array. The diagonal hashed are for “A” as the horizontal hashed are for “B”.

As far as the Isc and temperature, my understanding is that Isc is the current produced when the panel is not attached to anything. If the voltage changes, the current has to change, doesn’t it? So on a cold day, the Voc increases, so doesn’t the current have to decrease?

Voc at 77f is, let’s say, 24V, and watts are 100. Isc would be 4.16A
Temperature drops and now I have 28.8v, but still a 100w panel, that’s 3.46A. Where am I going wrong with this?

 

[MisterSandals]

As far as the Isc and temperature, my understanding is that Isc is the current produced when the panel is not attached to anything.

The Isc is the current when there is a short between the panel positive and negative. When not attached to anything, there is no current flowing.

 

[BDubya907]

The Isc is the current when there is a short between the panel positive and negative. When not attached to anything, there is no current flowing.

I’m an idiot. No more responding for me until I’ve actually gotten out of bed and am awake.?‍

Wouldn’t Isc, at -10f, be lower than Isc at 77f though?

 

[MisterSandals]

Voc at 77f is, let’s say, 24V, and watts are 100. Isc would be 4.16A
Temperature drops and now I have 28.8v, but still a 100w panel, that’s 3.46A. Where am I going wrong with this?

Wouldn’t Isc, at -10f, be lower than Isc at 77f though?

You are assuming that watts remain constant when in fact the watts can go over rated watt spec as voltage rises.

I "think" the current negligibly affected by temperature in the differences you are talking about (yah, i know about super-cooled super conductors but those are hundreds of degrees different).

For practical solar calculations, i assume current is unaffected by temperature in my calculations. (willing to be taught otherwise though!)

So:
24V x 4.16A = 99.84W
28.8V x 4.16A = 119.8W

 

[littleharbor2]

As far as the Isc and temperature, my understanding is that Isc is the current produced when the panel is not attached to anything. If the voltage changes, the current has to change, doesn’t it? So on a cold day, the Voc increases, so doesn’t the current have to decrease?

Voc. is open circuit voltage. No current can flow through an "open" circuit.

As to the shading issue, I believe you will be very disappointed with the results if not addressed.

 

[BDubya907]

As to the shading issue, I believe you will be very disappointed with the results if not addressed.

Voc. is open circuit voltage. No current can flow through an "open" circuit.

As to the shading issue, I believe you will be very disappointed with the results if not addressed.

I think the only way I can address shading is by putting as many panels up as possible. I’ll have them all tilt-capable, and with the super long summer days up here in Alaska, I’m hoping to be able to compensate for shading. Unfortunately I can’t remove the A/C units, and I can’t raise the panels up to be above them, so what else can I do besides put up as many panels as space allows? Maybe down the road I can remove the rooftop units and put in a mini split?

 

[MisterSandals]

I’m hoping to be able to compensate for shading.

Having 3P in the area around the A/C will help if shading affects as few strings as possible.