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diy solar

Adding more panels

Nevadajim

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May 20, 2021
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I have attempted to add 6 additional solar panels but the system won’t accept them.??
Solar System:
640 S/F Cabin - Off Grid
Currently: 3, 360w solar panels operating.

Midnite Solar / Magnum Energy MNE-MM 1524AE CL150
35a charger and Classic 150 Charge Controller
Pre-wired Panel: 1.5KW, 24v DC-120v AC Power

Batteries: 2 - 12v ‘Sun Xtender’ AGM Deep Cycle
Part # PVX-2580L, Ampere Hour Capacity @ 24 Hour Rate = 258
Bulk/Absorb Charge, 2.37 to 2.40 volts/cell @ 77 degrees F. 14.2 to 14.4 Volts
Float Charge, 2.20 to 2.23 volts/cell @ 77 degrees F. 13.2 to 13.4 Volts

I have attempted to add 3 - 360w panels and 3 - 250w panels.
The system will not currently accept the additional panels even though they are on 3 separate circuits. Please advise. Thanks.
 
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Not enough information here to know what is going on. First, what do you mean saying "The system will not currently accept the additional panels even though they are on 3 separate circuits".

As a general rule of thumb, modern MPPT controllers usually accept the combined power of multiple strings of panels through what's called a combiner box. Each string of panels gets it's own DC breaker, and the combined wattages from all the strings does to +/- output wires connected from the combiner busses to the charge controller.

As a general rule of thumb, all the voltages of each string should match each other within 5% of each other. For your first string of three 330W panels, are each panel putting out 37.5V? In series, three would be 112.5V? The second string of three 250W panels is likely to be 3 X 30.0V = 90VDC. So, the first string is 112.5V and the second string is 90V. NOT a good match. The second string of 360W panels is likely to have a voltage close to the first string, so voltage should be OK, but you need to measure it to actually know.

If your battery is actually full, it won't be accepting power from even one single string. Maybe that is what is causing you confusion? You need to apply a large load to your system that is larger than the combined wattage of your panels. Try a toaster oven. Then you will see what the panels can actually put out.

BTW, if your battery is in fact 258Ah at 24V, then ~970W is optimal for charging that. If you are adding a lot more solar capacity, you should be scaling the size of your battery about the same amount. 258Ah X 1/8C X 25V X 1.2Fudgefactor = 967.5W. So, if you are at least doubling the solar input, you should be doubling your battery capacity.
 
Not enough information here to know what is going on. First, what do you mean saying "The system will not currently accept the additional panels even though they are on 3 separate circuits".

As a general rule of thumb, modern MPPT controllers usually accept the combined power of multiple strings of panels through what's called a combiner box. Each string of panels gets it's own DC breaker, and the combined wattages from all the strings does to +/- output wires connected from the combiner busses to the charge controller.

As a general rule of thumb, all the voltages of each string should match each other within 5% of each other. For your first string of three 330W panels, are each panel putting out 37.5V? In series, three would be 112.5V? The second string of three 250W panels is likely to be 3 X 30.0V = 90VDC. So, the first string is 112.5V and the second string is 90V. NOT a good match. The second string of 360W panels is likely to have a voltage close to the first string, so voltage should be OK, but you need to measure it to actually know.

If your battery is actually full, it won't be accepting power from even one single string. Maybe that is what is causing you confusion? You need to apply a large load to your system that is larger than the combined wattage of your panels. Try a toaster oven. Then you will see what the panels can actually put out.

BTW, if your battery is in fact 258Ah at 24V, then ~970W is optimal for charging that. If you are adding a lot more solar capacity, you should be scaling the size of your battery about the same amount. 258Ah X 1/8C X 25V X 1.2Fudgefactor = 967.5W. So, if you are at least doubling the solar input, you should be doubling your battery capacity.
Thank you for your patience. Solar is not my first language. But I’m working on it.

Yes, I have the combiner box. So, I can eliminate the three - 260w panels that are not compatible I’m to good start.??

Also, I can test, (using a toaster and/ or a microwave ?), to see if I can that solves the issue If the batteries happen to be fully charger.??

Also, comparing the battery size to the number of panels, would suggest: 2 -12v batteries in series (24v system) = 967.5w as you mentioned . Combine this with a total of 2 groups of 3 panels each @ 360w per panels = 360 x 6 = 2,160w. If I’m following your thinking, that would mean at least 2, if not 4, additional batteries??
Thanks and please reply.
 
Really the best way to proceed from here is to get an inexpensive multimeter that you can read voltage and resistance. Here's one I found on Ebay for just 9$

Your 330W panels, are they 72cell panels? That is do they have 6X12 little sub-blocks inside the panel? They are most likely 37.5V panels. The 250W panels I'm guessing are 60cell panels. That is they have 5X10 little sub-blocks. The good news is if you can get one more 60cell panel, most likely it will work for you. Three of the 330W panels in series would give you 37.5V+37.5V+37.5V= 112.5Vmp. Four 60cell panels would give you 29.5V+29.5V+29.5V+29.5V= 118Vmp. So, they differ in voltage by only 4.9%. I routinely do the same matching at my own cabin, linking strings of three 72cell panels with four 60cell panels. The controller will never notice the difference.

Since you already have things wired into the combiner, it is easy to test. Just flip either break #1 or #2 off while running the toaster, and you can measure how much power each array is putting out. Your controller will show how many amps are coming in while the toaster is running.

Yes, you could add more batteries, but that's not the best way for two reasons. First, it's usually not a good idea to mix new batteries with old batteries. The new batteries get dragged down the the level of the old batteries. Secondly, I'm not a fan of multiple strings of batteries. The more and more parallel strings you have, the harder and harder it gets to keep the two or more strings balanced at identical charge levels. I think it's best to have a single string of batteries of the capacity you need. Here's the math for two arrays...
(2000W/25V charging) X .85FF = 68charging amps. Assuming you charge at 1/8C then 68A X 8fold capacity = 544Ah battery.

In fact, for my own 24V workshop system, I have 2000W of panels, and I use this battery which has a capacity of 568Ah.
These are great batteries. They are though very f***ing heavy. You need two grown men to position them.
 
Thank you for your patience. Solar is not my first language. But I’m working on it.

Yes, I have the combiner box. So, I can eliminate the three - 260w panels that are not compatible I’m to good start.??

Also, I can test, (using a toaster and/ or a microwave ?), to see if I can that solves the issue If the batteries happen to be fully charger.??

Also, comparing the battery size to the number of panels, would suggest: 2 -12v batteries in series (24v system) = 967.5w as you mentioned . Combine this with a total of 2 groups of 3 panels each @ 360w per panels = 360 x 6 = 2,160w. If I’m following your thinking, that would mean at least 2, if not 4, additional batteries??
Thanks and please reply.
Really the best way to proceed from here is to get an inexpensive multimeter that you can read voltage and resistance. Here's one I found on Ebay for just 9$

Your 330W panels, are they 72cell panels? That is do they have 6X12 little sub-blocks inside the panel? They are most likely 37.5V panels. The 250W panels I'm guessing are 60cell panels. That is they have 5X10 little sub-blocks. The good news is if you can get one more 60cell panel, most likely it will work for you. Three of the 330W panels in series would give you 37.5V+37.5V+37.5V= 112.5Vmp. Four 60cell panels would give you 29.5V+29.5V+29.5V+29.5V= 118Vmp. So, they differ in voltage by only 4.9%. I routinely do the same matching at my own cabin, linking strings of three 72cell panels with four 60cell panels. The controller will never notice the difference.

Since you already have things wired into the combiner, it is easy to test. Just flip either break #1 or #2 off while running the toaster, and you can measure how much power each array is putting out. Your controller will show how many amps are coming in while the toaster is running.

Yes, you could add more batteries, but that's not the best way for two reasons. First, it's usually not a good idea to mix new batteries with old batteries. The new batteries get dragged down the the level of the old batteries. Secondly, I'm not a fan of multiple strings of batteries. The more and more parallel strings you have, the harder and harder it gets to keep the two or more strings balanced at identical charge levels. I think it's best to have a single string of batteries of the capacity you need. Here's the math for two arrays...
(2000W/25V charging) X .85FF = 68charging amps. Assuming you charge at 1/8C then 68A X 8fold capacity = 544Ah battery.

In fact, for my own 24V workshop system, I have 2000W of panels, and I use this battery which has a capacity of 568Ah.
These are great batteries. They are though very f***ing heavy. You need two grown men to position them.
Michael, I am recuperating from some surgery so my computer time is limited. Don’t think I’m not intersected just only has so much energy.
I have a multimeter by Fluke. No problem there.
My panels (6) are 360w, not 330w. I will count the sub-blocks to verify. So, each group of 3 panels puts out 1,080w = 2160w total.
I also have 3-250w panels. So, like you mentioned, I can add one - 330w panel and make a 4 - group that should put out the same as the 1080w groups, If need be.
Yes, I heard mixing new with old batteries drags the new ones down.
If you run your shop on 2,000w of panels, I suspect I won’t need the 250w panels. I will get the two sets for 3 - panels woking using the toaster method.
Here is the kicker: The current (3) working panels face south and are ground mounted. The (3) panels on the roof face west. (Clever me, thought I could catch the afternoon sun towards dinner time. That might be my whole issue.??) Jim
 
Michael, I am recuperating from some surgery so my computer time is limited. Don’t think I’m not intersected just only has so much energy.

Here is the kicker: The current (3) working panels face south and are ground mounted. The (3) panels on the roof face west. (Clever me, thought I could catch the afternoon sun towards dinner time. That might be my whole issue.??) Jim
So sorry to hear about your situation. Hope you improve soon!

At my own place, I'm going exactly the same thing. I have my main array of panels facing south most of the time, but by about 4:30, they start getting shaded. So, I installed another array facing due West. This helps pick up the late afternoon sun so I can keep my air-conditioner running till about 6pm. Here's a pic of the westward array. This is the array with four 30V panels, which matches quite will to my South arrays with three 38V panels.

In your own case, you might want to still get the fourth 30V panel, and position the four facing East. You'll maximize the watts coming in over the course of the day without overwhelming your controller.
 

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So sorry to hear about your situation. Hope you improve soon!

At my own place, I'm going exactly the same thing. I have my main array of panels facing south most of the time, but by about 4:30, they start getting shaded. So, I installed another array facing due West. This helps pick up the late afternoon sun so I can keep my air-conditioner running till about 6pm. Here's a pic of the westward array. This is the array with four 30V panels, which matches quite will to my South arrays with three 38V panels.

In your own case, you might want to still get the fourth 30V panel, and position the four facing East. You'll maximize the watts coming in over the course of the day without overwhelming your controller.
MichaelK, My health should be back to ‘full power’ in a couple of weeks. Thanks for your well wishes.
So, arrays can face different directions! And, the fourth panel I can add to the east facing array, Great!
We have a week of Dr appointments then can get back to the cabin and start checking options.
I checked out battery you use but couldn’t find the pricing. I’ll look further.
Once all three panels are connected to the combiner box, the run to the controller is 40”. What should the wire size be?
We could use an AC to cool the cabin in late afternoon. What kind do you have? What’s the wattage?
 
MichaelK, Here are some pics of the system. We like to use the cabin much of the year. We are on the Eastern Sierra facing west across the Bridgeport, CA, valley. At 7,000 feet elevation we get warm to hot summers and snow in winters. Being able to add the AC would greatly increase our summer time use. NevadaJim. Our residence is in Smith Valley, NV, about an hour north of the cabin. Makes for easy travel and use.
 

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Wire sizing for your question above: the ABYC wire gage sizing chart is good for 12V. You’ll need to calculate for your system voltage (higher voltage lets you use lighter wire).
Amps are calculated using algebra with the formula Watts=Volts*Amps. So in your case Amperes=W/V and I like to upsize one cable size but that’s not required. You should fuse appropriately.
 
Nice place you have there. Appears to have abundant solar potential.

With panels wired in series, you don't need anything larger than 10 gauge for the distances you are working with. With ground mounts like in the pic above, I run 120VDC solar about 135 feet, without any measureable voltage drop. Once the solar strings get to my combiner box though, I use 4 gauge from the combiner to the charge controller.

Looking at the overhead pic #3, I see lots of places where you could install ground mounts like the ones I made. At the bottom center of pic #3, below the junipers looks ideal. I'd put at least one ground mount there, though if it were me, I'd put two. Then they can be rotated throughout the day to provide maximal power from say 8am till 6pm. What I would do is carefully study the sun around the time of maximal need, and carefully select positions that won't shade each other.

A one-room air-conditioner of about 8000BTU will run with about 750W.

Let's say you start up the air-conditioner around noon and let it run all day long. It will consume 750W at startup, but after a few hours it will cycle on and off, maybe consuming about 400W/hr? By about 6pm, solar resources will drop to the point that it can not support the air-con any more, and you start withdrawing from the battery. How late in the night do you think the air-con needs to run before it can be shut off?

If you say all night, with switch-off at 6am, then that's 12hr X 400Waverage = 4800W, or 4.8kWh. That would be about 1/3 of my battery. I paid ~1000$ each for mine, or 3000$ total. Keep in mind that they are VERY heavy. You will need a stout hand-cart, and a two man lift to get them into their final position. If you want to run more than 1 air-con though it gets tougher. You might want to go with a 48V system instead, or at least expect to completely turn off one air-con after 6pm.

BTW, your Magnum inverter looks like the Modified Sine Wave version. If you are serious about running motor-driven appliances like an air-con, I would replace it with the True Sine Wave version. I'd take a look at their MS4024PAE inverter. It puts out split-phase 120 and 240VAC.

Here is my inverter. It's also 120/240V split-phase AC.
 
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Nice place you have there. Appears to have abundant solar potential.

With panels wired in series, you don't need anything larger than 10 gauge for the distances you are working with. With ground mounts like in the pic above, I run 120VDC solar about 135 feet, without any measureable voltage drop. Once the solar strings get to my combiner box though, I use 4 gauge from the combiner to the charge controller.

Looking at the overhead pic #3, I see lots of places where you could install ground mounts like the ones I made. At the bottom center of pic #3, below the junipers looks ideal. I'd put at least one ground mount there, though if it were me, I'd put two. Then they can be rotated throughout the day to provide maximal power from say 8am till 6pm. What I would do is carefully study the sun around the time of maximal need, and carefully select positions that won't shade each other.

A one-room air-conditioner of about 8000BTU will run with about 750W.

Let's say you start up the air-conditioner around noon and let it run all day long. It will consume 750W at startup, but after a few hours it will cycle on and off, maybe consuming about 400W/hr? By about 6pm, solar resources will drop to the point that it can not support the air-con any more, and you start withdrawing from the battery. How late in the night do you think the air-con needs to run before it can be shut off?

If you say all night, with switch-off at 6am, then that's 12hr X 400Waverage = 4800W, or 4.8kWh. That would be about 1/3 of my battery. I paid ~1000$ each for mine, or 3000$ total. Keep in mind that they are VERY heavy. You will need a stout hand-cart, and a two man lift to get them into their final position. If you want to run more than 1 air-con though it gets tougher. You might want to go with a 48V system instead, or at least expect to completely turn off one air-con after 6pm.

BTW, your Magnum inverter looks like the Modified Sine Wave version. If you are serious about running motor-driven appliances like an air-con, I would replace it with the True Sine Wave version. I'd take a look at their MS4024PAE inverter. It puts out split-phase 120 and 240VAC.

Here is my inverter. It's also 120/240V split-phase AC.
MK,
My plan is to move all the panels past the drive/ gravel to the left of the cabin in pic 3. This position has very good sun exposure for south and west w/o blocking the western view And is fairly close to the cabin.
Our AC needs are for afternoon, to cool the place down just prior to sunset. Once the sunsets the outdoor temp drops fairly fast.
Due to high winds and snow-loads the panels need to very securely anchored. I could make the tilt angel adjustable but the compass heading will remain stationary. So, one set of 3-panels pointed to the west, one to the south and the final set of 4-panels to the east.
Before I move any of the panels, I will first get them all operating in their current locations.
Wish I had known the difference between modified and pure sine wave inverters when getting started!! Add the cost of a new inverter to the 3 new batteries and the bill starts adding up! However, It will be worth the money and effort to get it right And have all the power we need.
Please give a link to your AC model. NJ
 
MK,

Please give a link to your AC model. NJ
Here's what I picked. I just selected what HomeDepot happened to have on salewhen I was making my purchase. I selected through the wall because I want to run the air-con during the day to keep the upstairs cool, but be able to open the windows wide if it happens to cool down later at night.
 
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