question

RV solar charger options

I am running into confusing information about solar chargers. I have room for 10(15 if I get creative) 450-watt solar panels on my roof. each with an open circuit voltage of 47v and an expectation of about 10 amps real-world output. looking at chargers for a 12v system nothing makes financial sense. Victron's biggest is 250/100, which maxes out of about 1100w charging on a 12v system. I would need 5 \$1000 controllers..... I am hoping I am doing some math wrong. On the small end however the 100/50 can do 700 watts and I could do one per panel and only come out at around \$1500 total (10 chargers).

My goal would be 2 banks of 5 panels as that's how they will be arranged on the roof and while they will be adjustable when parked if one side is shaded I don't want the whole system to suffer.

currently looking at the Canadian Solar BiHiKu CS3W-450MB-AG as I can get them cheap \$300/each shipped. but am open to suggestions to get the most out of the system.

the roof is 9.5ft by 36ft of usable space. no restrictions. I have low-profile ACs and will be building a rack above everything with linear actuators for tilt control.

The inverter is the MultiPlus-II 2x 120V and currently, I have 1000AH of usable storage.

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That's the bugger about 12V. You need massive current to charge it.

4500W/14.4V = 312.5A

There's just no way around it at 12V. Have you considered 24V? 48V?

You're also going to need a lot of battery to be able to handle that much current.

4500W is a MASSIVE amount of power for a 12V system. Is your choice of the MP-II 12/3000 2x 120V driving it? You could get 2X MP 24/3000 120V in parallel split phase with a workaround for 30A shore power.

You'd ultimately spend about the same on hardware, have 2X the power and probably save a bit on thinner copper.

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It was already a 12v system when I got it. I changed the 1000-watt inverter to the MultiPlus-II 2x 120V. I am fine when I have shore power but I need to get some solar going for boondocking. I have a 7500-watt diesel generator that is quiet but I don't want to run it during the day. the goal is to make enough power throughout the day to run basic loads ( we sit around 600 watts on average in the day and 200-300 at night) this keeps the fridge going and random things like TVs.

AC is not my concern. we usually don't need it at night and if I need it during the day I can fire up the Genset.

I am just trying to figure out my options for adding solar. the panels are cheap enough but it seems crazy to have 10 solar chargers for one rig. I don't even know where I would put them all.

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Okay, so we're locked in with some hardware.

Have you actually calculated what your boondocking needs are, or is this just a scorched earth/overkill approach?

Consider that 4500W of panels is just a massive amount. Yes, you have space, but do you need to/should you use it all?

Something you can do is over-panel, i.e., have way more solar than the MPPT can handle, but you produce your peak MPPT output for a longer period. As long as you stay within the MPPT voltage and input limits.

5X 100/50 would take care of MOST of your needs.

How much battery do you have or do you plan to have? Chemistry (Gell, sealed, flooded, AGM, lithium)? To support that array and overnight storage requirements, you're going to need a lot.

Those are bifacial panels, so you'll never get any benefit from the bifacial aspect even mounted on the rack. With extreme tilt (45°), you might see a little.

Look at the NMOT rating:

https://cdn.shopify.com/s/files/1/0079/6003/5417/files/2132_110-1281_Spec_Sheet.pdf?v=1650047373

450W STC (conditions which rarely exist in real life and even less frequently on an RV roof) panel produces only 337W in more realistic conditions.

Here's what I would do if I wanted to cover the whole roof, and I was stuck at 12V:

Assume the 10X panels produce only 337W, so plan for 3370W total.

3370/14.4 = 234A

Put 2P panels on each 100/50 MPPT (5 MPPT). Your entire array would be in parallel, so shading would be of no concern whatsoever. Shaded panels drop out while the others hammer away.

This might eliminate the need for the complexity of tilt/adjustment. Even if you're in a substantially northern latitude, you probably wouldn't care that your flat array is only getting 40% of STC... that's still a LOT of solar.

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do a mix of serial and parallel

that will also help with solar partial shading

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but what mix? That's my problem. To do a mix I need to run almost the top-of-the-line solar chargers and need 5 of them. \$5000 makes no sense when I could do \$1500 for 10 small units and put one per panel. I was hoping to do 2 inverters but I dint see any option that would work. 10 units are gonna be a mess of chargers and wires but it will give the best shade resistance. each panel will be on its own. negates the need for solar optimizers.

I was just hoping I was reading something wrong and someone could give me a better combo of solar chargers.

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scott-guenther ·

Per my above comment, there is no realistic reason to get 10 units... Your panels will rarely produce 450W and will more regularly meet their NMOT rating of 337W.

STC ratings mean 1000W/m^2 of solar intensity on the panel and the CELLS at 25°C. The reality is that only 20% of the sunshine is converted to electricity and the 80% is converted to heat. This means that even in relatively mild 25°C ambient conditions, most panels heat to 40-50°C in these conditions, and that drops their output notably. Other factors like imperfect tilt, atmospheric clarity, elevation, etc., all work to influence output - mostly to the downside.

Two parallel panels per 100/50 would be more than sufficient to cover the vast majority of circumstances you will ever see. Having all panels in parallel will also maximize your shade tolerance meaning shaded panels won't influence the output of unshaded panels.

Spend \$1500on what you need to take care of 85-90% of your panel capability or spend \$3000 to ensure you get that last 10-15%.

Your choice to stick with 12V is driving this requirement.

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I use 5x 330Wp in parallel for 1650Wp in total with one SmartSolar 150/100, that rarerly sees 100A output, so you may consider the 150/100 as a cheaper option to 250/100, you don't need the high string voltage.

That is enough to power our catamaran on the hook, 2 fridges, a residential 6 drawer deep freezer on 120V, 4 burner induction hob, residential baking oven, watermaker, washing machine, all the electronics... (Victron Quattro 5000). 1000Ah LiFeYPO4 4 x 1000Ah cells, usable energy 15kWh 1200Ah at 0.3C discharge rate. we rarerly use A/C.

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sorry to revive an old thread. 5 x 100/50 might work. I am currently looking at 3 x 250/100 and doing 9 x 600-watt panels.

I had an odd thought. an MPPT controller takes only what it can so overkill is not an issue. but can I link all the panels together and run 150/100amp and just hook all 3 to the same wringing? I can't find a good answer on Google if multiple controllers can be on the same array.

to simplify the question can I take 1 panel and hook 2 charge controllers to it or does that cause issues?

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It's been long enough that I don't recall the conversation, and I'm too tired to go back through it. If I understand your question...

One array per MPPT. You may not share panels between MPPT. It's a shit show.

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snoobler ·
any explanation as to why it's a bad idea to use two mppt controllers on one array? i don't doubt that its a bad idea as I cant find any examples but I also cant find any reasoning. my electrical knowledge says volts matter and amps are used as needed so it seems legit that extra-waisted amps could be used by another mppt but I don't understand exactly what they are doing and why 2 or more controllers would "fight " each other.

I have scoured the internet for answers and get nothing as to the science or physics involved with how mppt controllers work and why 2 or more would not work together.

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scott-guenther ·

An MPPT is a variable DC-DC converter. It uses a tracking algorithm to determine the optimal PV voltage and current to deliver maximum power. It periodically checks itself by doing a "sweep":

The spikes and dips every 10 minutes are the MPPT testing conditions.

Two or more MPPT on the same array will constantly interfere with each other varying their load trying to find the maximum power point. Your array will output far less than it can if connected to two MPPT.

Example (2 MPPT, 1 array):

#1 pulls a load to find the maximum power point.

#2 pulls a load to find the maximum power point.

#1 sees the power drop due to the load from #2, so #1 pulls less load.

#2 pulls more until it finds the maximum power point.

#1 tries again and pulls a load.

#2 sees the power drop off due to the load from #1, so #2 pulls less load.

#1 pulls more until it finds the maximum power point.

#2 tries again and pulls a load.

#1 sees the power drop off due to the load from #2, so #1 pulls less load.

#2 pulls more until it finds the maximum power point.

#1 tries again and pulls a load.

#2 sees the power drop off due to the load from #1, so #2 pulls less load.

#1 pulls more until it finds the maximum power point.

----The following repeats as a group, but alternating #2 and #1----------

#2 tries again and pulls a load.

#1 sees the power drop off due to the load from #2, so #1 pulls less load.

#2 pulls more until it finds the maximum power point.

https://community.victronenergy.com/questions/211461/mppt-synchronized-charging.html

1 Like 1 ·
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Sorry once again to revive this thread but I feel like starting a new one will hurt the info listed before and the struggles with large Arrays and RVs

12V is obviously not going to work and Victtron doesn't seem to want to make a 48V 120x2 so I am working out how to fix this. Here is the current thought and I want some input.

this is the current setup. yellow is high voltage 120/240. it goes from an automatic generator switch on one side of the chassis to the 120x2 then back to the main panel. all power runs through the 120x2. If outside power and generator power is present the Genset wins.

Blue is the 120x2

Green is the 60amp 240 Generator

Gray is the 12v batteries. about 17,000 watt hours. industrial AGM. I can get them dirt cheap.

Here is the thought.

I add in an eg4 6000xp 240 unit and add its output to a 50 amp 240 plug. when plugged into shore power it won't be used at all and power will go directly to the Victron. when not on shore power the main power cord will be plugged into the EG4, providing 50 amps of power. the batteries will all be charged through solar only. I would love to use the passthrough but I can't find an inverter other than Victron that will let me limit input amperage and I rarely actually have 50 amps input. usually 30 and sometimes 15. I might just get a 48v charger if I need one but I never should.

the EG4 can take 8000 watts of solar so we are good there. It's 48 volt (forgot to add the extra batteries but I am thinking of another 45,000 watt hours of 48v batteries. the will also be an Industrial AGM and I have space in the center of the chassis so we are good for weight and weight distribution.

Am I missing something? the only downside is I have to remember to plug in the EG4 when not on shore power but with the plug being right there it will just be part of the pack-up procedure and I will store it with plug-in. I could put in another auto transfer switch but it's not a huge hassle.

Edit: Just want to clarify, I am not married to the EG4 6000XP. It's just cheap and does everything I need (I think). I also want to clarify I don't need 240. for anyone unfamiliar with 50Amp RVs. they take 240 but nothing is 240 in the rig. It has 2 separate phases. when plugging into a single phase such as 15amp or 30amp the dogbone connector combines the 2 sides and feeds both the same 120v. This only works because nothing will ever use 2 hots and the panels are well balanced. you will never see all the air conditioners on one leg, they are divided up evenly so the load stays even.

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