Newbie DIY Solar

Art92

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Aug 17, 2021
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Hello everyone. Needing some help with setting up my first off Grid solar system. Some background , recently bought land and planning on boondocking in my RV for the time being . I was looking to setup an independent 24V system from the RV were i would just plug in the rv and also use for other purposes outside of the RV . So far i had bought some components based on Will’s video but now getting cold feet to bring it all together . So far I have

-24V 5000w 10kw surge pure sinewave inverter
-12 235W 37v used panels
-2 x 12v Ampere time 300ah plus batteries.

I am now looking for a solar charge controller but the numbers are not adding up. My question is if I wanted to be able to make use of all my panels what kind of MPPT charge controller would i need and what would be a possible way of connecting the panels together to be able to get the most from the components I already have . I would really appreciate some help. TIA
 

MisterSandals

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I am now looking for a solar charge controller but the numbers are not adding up.
What numbers? Have you done an energy audit to see how much energy you need or want?

Your pieces that you bought, look like this:
12x 235W x 5 solar hours = 14,100Wh production each day (you can find real hours of production)

2x 300Ah x 12.8V = 7680Wh storage
You will recharge this battery from empty in 7860Wh / (12 x 235W) = 2.7 hours

7680W / 5000W inverter = 1.5 hour runtime at full power (unlikely i know)
But do you know the standby power consumption of this monster inverter?

What is the Voc of you panels?
Do you have a budget or an idea of whether you want reliability ($$) or economy SCC?
 

Art92

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Aug 17, 2021
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Thanks for your response! Am having a problem with the numbers for the Charge controller, wouldnt my panels bring in too much power and blow the cc? i am wanting more of reliability, willing to spend a little more.

the Voc on the panels is 37v . I was going to try and have 6 in series and then the in parallel My math was adding up to too much V and amps input for the Mppt CC i was seeing on Amazon.
 

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MisterSandals

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too much V and amps input for the Mppt CC i was seeing on Amazon.
This is a common scenario for quite a few panels in series. What solar controller do you have your eyes on?

A reasonable solution for 6 panels is to put them in a combination of series and parallel. So maybe a 3S2P (37Voc x 3) and (8.35A x 2) or 2S3P (37Voc x 2) and (8.35A x 3).

The Voc and the SCC's max input volts are the key number to match up. In the 8.35A x 3 case, dealing with 25A in fusing between panels (need fuses when 3 or more in parallel) increases wire size, fuse size and cost...especially for longer wire runs from array to SCC.
 

MisterSandals

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Is there something else you're looking for that you decide to start another identical thread?

If I am off base, it wouldn't be the first time but feel free to redirect me or rephrase your question.
 

rmaddy

Full-time Solar-powered Trailer Life
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12 panels is a lot. You might consider using 2 charge controllers, each with 6 panels. You could have one set facing more south-easterly and the other set more south-westerly. This would give you all-day charging (on sunny days). You could run a lot on the inverter during the day without draining anything from the battery.

A lot depends on how much power you really need each day, how much during the day and how much at night. A 5kW inverter is pretty big and your battery capacity could be too low if you regularly need to that much power.
 

rmaddy

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Is there something else you're looking for that you decide to start another identical thread?
I was wondering the same thing. Making a duplicate post after getting help on the first just causes confusion.
 

MisterSandals

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Some new math to see how things shake out:

6 x 37V = 222V
If you're not in a location where it gets really cold, this might work with a 250V SCC. If you know the historical low temp (or temp you want to handle without your SCC getting toasted by over voltage) we can do the math.

12 x 235W = 2820W solar
2820W / 25.6V battery = 110A max charging from that array. That is a LOT.

Is it possible to break up the array into 2 parts to spread the charging out over a longer period? You could face 6 panels east-ish and 6 panels west-ish and charge at 55A for nearly twice as long (earlier in the east and later in the west).

Otherwise, charging at 110A you might need a couple of big 50-60A SCC's and those will be expensive and probably challenge the charging limits of your batteries (which i have not calculated).

2 different SCC's give you redundancy and you may have to spread out the charging to morning and afternoon anyway.

Still hoping to hear what SCC you have in mind. If you want suggestions, there is quite a bit of experience and earned biases from the folks here.
 

Hedges

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2820 W PV / 7200 Wh battery = 0.39C charge rate.
That is probably OK as most LiFePO4 batteries accept 0.5C at room temperature (didn't see spec for yours when I searched.)
You should program low-temperature charge cutout in BMS for a temperature where 0.39C is acceptable. A zero degree C cutout would be too low.
If you can keep the battery at a moderate temperature, then that protection would never have to kick in.

If you put PV panels at multiple angles, peak power could be 0.7 times as much, so charge rate 0.27C which would allow colder temperature charging (still not zero degrees C, which should probably be limited to 0.1C or 0.05C). You would also only need about 80A of charge controller rather than about 117A. Battery would be kept fully charged later into the evening.

Some charge controllers (Victron) can work with a battery shunt and additional devices to regulate battery current. You could limit battery charging to 0.1C or 0.15C which ought to be sufficient for most conditions (except one or two hours of sun during an otherwise cloudy day). It would let more power come from PV when loads like inverter were drawing current.
 

Art92

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Aug 17, 2021
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My bad, wasnt sure if i had posted in the correct forum. To be honest I am not even sure how much power we need 🙈. I was able to get the inverter on auction for $100 so i was trying to make everything else work with the inveter. I bought the RV as a fixer upper and do not have appliances and am not using the RV’s Ac because it was taking to much power. I am looking to invest in a portable unit or window AC unit that is more efficient and more efficient appliance. At most we need lights ,a little cooling, run a fridge 24/7 , run three monitors and charge a laptop for abt 10hours while im working . These are the major appliances i will be running . Our land is in the Texas hill country and we get about a good 8hours of sun and if we dont get another winter storm(knock on wood) the temps dont drop too much. At this point I want to see what would be a good configuration to make the most of the components I already had and to be able to use the power through the day and at least make it overnight until the sun comes out the next day as we are not staying there full time but looking to spend a couple days there at a time and also have it as a short term rental. I hope this gives a little background on our power needs .
 

MisterSandals

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Unless you have the biggest RV in Texas, I am not sure you will get 12 big panels on the roof. If you are able to get them in use, either on your roof, or setup on a ground mount or even propped up on cinder blocks, you will have a LOT of power.
If you have any numbers for how many watts your appliances use or how many kWh per year or anything, we can run some numbers to make sure this works as well as possible to meet your needs.

It sounds like you will be counting on this to keep your food cold and power your work computer(s) as a necessity. Air conditioning can be a necessity too. Hill Country can be pretty darn hot pretty much around the clock.

we get about a good 8hours of sun
THIS CALCULATOR shows that laying them flat (like on an RV roof) you get this many hours of solar:
Screen Shot 2021-08-17 at 11.00.48 PM.png

At a 45 deg angle and facing south:
Screen Shot 2021-08-17 at 11.03.04 PM.png

So not 8 hours of solar by any means.

Still waiting to hear what solar charge controller (SCC) you have your eye on. We can see if your panels can be configured to make that work for your choice and your batteries.
 
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