Adding more panels

Nevadajim

New Member
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.
 
Last edited:

MichaelK

Solar Addict
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.
 

Nevadajim

New Member
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.
 

MichaelK

Solar Addict
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.
 

Nevadajim

New Member
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
 
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