Need Help/Advice

Maybe the system was design to be 24V, and a 12V inverter was accidentally listed or ordered.
At 24V, the Classic 250 charge controller is good for about 1700 to 1800W.
3100W (STC) of panels might produce 2700W under actual conditions. That is just reasonably over-paneled, makes up for days and seasons of less sun.

If the two series strings of 5 PV panels were oriented differently, like 9:00 AM and 3:00 PM sun, then just about all power produced could be used. And battery charge rate would approach optimum more hours of the day (63A from charge controller, 85A target for battery).

Perhaps a modification of the ground mount could change angle of the panels in groups of 5. Different times of day difficult to do with those two rows of 5, and don't want to tilt toward cabin which would shade it. But could tilt 5 optimal for winter and 5 optimal for summer.


I don't believe having all the panels connected as they are now will damage the SCC. They provide 200 Voc, about 160 Vmp, which is OK. (Midnight 250 allows 250 Voc)
They can deliver 15A, which the SCC could handle fine (Midnight can deliver 63A.)
They won't ever deliver 3000W. The most that comes out of Midnight to the battery is 63A x 15V or so, 945W.

Excess watts won't be dissipated in Midnight either; a "Buck" circuit simply turns on a transistor connecting 180V PV (and a paralleled capacitor) to battery through an inductor, waits until current rises to 60A (comes from capacitor not PV), then turns off transistor. 60A continues to flow through inductor (that's what inductors do) and through a diode to negative. The transistor is on for maybe 1 millisecond, off for 11 milliseconds. 15 A at 180V flows from PV panel to capacitor for 12 milliseconds. 60A at 15V flows to battery for 12 milliseconds. 60A at 180V flows from capacitor to inductor for 1 millisecond.

The only somewhat higher than normal stress is that transistor is switching between 15V and 180V at 60A. (most of transistor's power is dissipated during switching.) If used with a 48V battery where this PV array would be right sized, transistor would switch between 60V and 180V at 60A. With a 12V battery the power dissipation of the transistor is about 1/3 higher than with a 48V battery. Not a big deal.

upnorth said:

So here is my new challenge for all of you very knowledgeable people....how can I get more use out of this system while putting out the least amount of money? Hook batteries up in series and try to exchange current inverter for a 24v one? Not even sure I would be able to do this? And buy another charge controller?

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Your batteries can be wired as 12V (like presently) or 24V.

If you replace the inverter with a 24V inverter, and keep just the one charge controller, you can get a bit over half the PV during full sun. We call this "over paneling", letting power production be clipped in the middle of a sunny day. During partially cloudy days or other seasons, clipping will be reduced; you'll still get the same 1700W peak.

If you keep the 12 inverter and add a charge controller (need wires from array so each charge controller has its own 5 panels), that works too.

But the 24V inverter is more attractive. You can get a higher wattage one, and you can get one with charger built in.

The drawing from the seller shows 1 string of 5 panels going into a charge controller, x2. So I interpret that as do that twice, which means 2 sets of that, 1 string per charge controller. But the mistake is that he wired the batteries in the drawing as 12V, not 24V, and sold you a 12V inverter and only 1 charge controller. I suggest you call the seller, explain the problem, and tell him you want to exchange the 12V inverter for a 24V, and get a discounted price for a 2nd controller. That is what I would do to make it right if I sold a system based on a bad design I did.

Problem to describe to seller: The Midnite Classic 250 is capable of 63A output. At 12V, that is less than 3 panels. He sold you 10 panels and one controller. If you switch it to a 24V system, which you can easily do by rewiring the battery bank, you can use 5 panels per charge controller. You need to exchange the 12V inverter for a 24V inverter. Don't let him say he can't do it because it is used. It is used based on their mistake, not yours. They need to make it right.

Are there 2 wires coming to the cabin (and charge controller) from the solar panel array? or 4 wires?
That is, where is the connection made to parallel the two series strings of 5 panels?

That could be a "Y" connector at the array, with only 2 wires (maybe plus a 3rd wire for ground). In that case, you will need to pull an additional pair of wires if you add a second charge controller.

It could be 4 wires all the way from array to charge controller (or a combiner box). In that case, just connect two to one charge controller, two to the second new charge controller.

The two charge controllers can be wired together on the battery side (both charge one battery bank)
They can't be connected to the same PV panel wires. Each SCC needs its own wires to its own PV panels.


Another option besides 24V inverter would be 48V inverter.
You probably don't want to do that because it requires doubling the number of batteries.
At 48V, the single Midnight Classic 250 would be just fine for your 3100W of PV panels.
(not likely what you want to do because batteries cost more than everything else. Just listing this option for completeness.)

Here is an updated photo of display from about 20 minutes ago. Very cloudy day currently.

I think they are only 2 wires coming from the solar array to the cabin. The combiner box is mounted on the array and then the 2 wires come into cabin from there.

From what everyone is saying I am guessing the best option would be to try and swap the 12v inverter for 24 volt inverter, preferably an Evo model so I could plug the generator into the inverter and this in turn would charge the batteries if needed? And I would need another midnite 250 combiner box?

If the system is left as is for now until I have everything sorted then will it charge all the batteries up but it will just take longer??

upnorth said:

I think they are only 2 wires coming from the solar array to the cabin. The combiner box is mounted on the array and then the 2 wires come into cabin from there.

From what everyone is saying I am guessing the best option would be to try and swap the 12v inverter for 24 volt inverter, preferably an Evo model so I could plug the generator into the inverter and this in turn would charge the batteries if needed? And I would need another midnite 250 combiner box?

If the system is left as is for now until I have everything sorted then will it charge all the batteries up but it will just take longer??

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Interesting, very cloudy, so you are getting only 292W out of the 3100W array. It's good to be overpaneled on a cloudy day like today. I am curious what you get tomorrow when it is sunny. If you start seeing full output from the array, I would personally recommend turning off one of the breakers in the combiner box so you only have 1 string going into the controller. I'm sure Hedges will disagree and say leave it. Your choice.

With the second charge controller, you can still use the existing combiner box. Just remove the finger bus bar from the top of the breakers, and bring your 2 positive lines into the 2 charge controllers without combining them. Just PV to breaker to charge controller. This requires pulling a new positive line from the combiner box to the controller. I think you can use the existing negative wire from the combiner box negative bus bar for both controllers, but not 100% sure, I'm going to check with Midnite, standby for their answer. If not, you'd need to pull another one of them too. The good news is, the wire doesn't need to be as big as the wire already running, because they are carrying half the current, since you are not combining the two strings.

upnorth said:

I think they are only 2 wires coming from the solar array to the cabin. The combiner box is mounted on the array and then the 2 wires come into cabin from there.

From what everyone is saying I am guessing the best option would be to try and swap the 12v inverter for 24 volt inverter, preferably an Evo model so I could plug the generator into the inverter and this in turn would charge the batteries if needed? And I would need another midnite 250 combiner box?

If the system is left as is for now until I have everything sorted then will it charge all the batteries up but it will just take longer??

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Charging 292W, 23A is an improvement over 100W. As you get sun it should keep charging. But at this rate it will take a week (300W x 5 hours = 1500 Wh/day, 7 days for 10,000 Wh

If you bring generator and charger that would speed things up.

Midnight 250 is a charge controller, not a combiner box.
To add a second Midnight 250, you will have to pull 2 more wires from the array. If there is conduit, should be easy enough. If direct burial, would require digging another trench.

Just having one Midnight 250 isn't so bad with 24V inverter. You'll get a most a bit over 50% of peak power from the array. It would be "over paneled", so you continue to get that peak 1700 to 1800W for more hours of the day, and more seasons.
If there is a way to tilt one string of 5 panels differently from the other string of 5, that will reduce the power lost (or not harvested) due to overpanelling.

Sure, shutting off one string on a sunny day as SolarQueen suggests is no harm, and reduces stress on the charge controller.
Getting 10% on a cloudy day is to be expected. I get 5% to 50% depending on how thick the cloud cover.

To pull wires, shoving a fishtape through is sometimes difficult. But (assuming you have conduit), there are already at least two wires in it. You can use one wire as a pulling line, pull it out while pulling three in. (If you like, pull one of those back out to pull the original one back in, rather than cutting another wire off another spool.) Just be sure to secure the wires together well so they don't pull apart. Spread the knots/twists for each wire apart so it isn't too thick. You can get wire pulling lubricant or any other water-based glycerin product if you happen to have something else available.

Here is an updated screenshot from right now. Still cloudy I guess.

Hedges said:

Midnight 250 is a charge controller, not a combiner box.

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When you are combining over 150V, you need the Midnite PV6-250 combiner to hold the higher voltage 300V breakers. He's probably referring to that. https://www.midnitesolar.com/produc...e=Combiners&productCat_ID=9&sortOrder=6&act=p

I checked my invoice and I have the MNPV3 combiner box. Would this have to be changed?

SolarQueen said:

When you are combining over 150V, you need the Midnite PV6-250 combiner to hold the higher voltage 300V breakers. He's probably referring to that. https://www.midnitesolar.com/produc...e=Combiners&productCat_ID=9&sortOrder=6&act=p

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That would put the panels in parallel

upnorth said:

Here is an updated screenshot from right now. Still cloudy I guess.

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you are MAXED out of solar charger the rest of your panels are being unused....

Midnite confirmed you can share the existing negative wire, as long as it is properly sized to carry all the current. Since it was designed to do so with the strings combined, you should be good. That's good news, you only need to pull 1 wire. He also said if it were him, he'd turn off the breaker for the second string until you get it fixed, but you can get away without if you want the advantage of cloudy days getting more power. You'll just be stressing the controller a bit on sunny days.

I'm really mad at your supplier. He's got you running over 200V through breakers that are designed for 150V. Add that to the list to tell him to replace. You can get away with keeping the PV3, it's just the box, and you no longer need the finger bus bar that combines the strings (it is different in the 150V vs 250V unit, as the 300V breakers are double the width), but you need to replace the 150V breakers with 300V 15A breakers.

GSXR1000 said:

That would put the panels in parallel

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Exactly, which is how he currently has it, and we are working out how to change that. We're just figuring out what equipment he has, and what changes are needed.

upnorth said:

Here is an updated screenshot from right now. Still cloudy I guess.

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Perfect.
800W, 61.7A at 13.4V. That's the most you can get. You're on a roll, just need 12 hours like that to fully recharge.

If you convert to a 24V inverter, you get twice as much power.

As SolarQueen said, if your breakers don't already have two-pole 150V breakers wired in series (for 300V), you need that because the PV panels are putting out 200V or more.

I have been hoping for a full sun day but hasn't happen. I am just trying to decide the best route to take so wanted to confirm I understand what the options might be.....

1) Keep the 12V inverter and add another Midnite 250 charge controller. In this case I should get double what I am getting now but will be over-panelled. So approx. 1600 watts?

2) try to exchange for 24V inverter and with current set up (just one Midnite CC) the system will generate approx. 1700-1800 watts as per Hedges?

3) If I went with 24V inverter and another Midnite 250 CC what could I expect for Watts to be generated?? Would this boost it above the 1800 Watts?

A couple of questions. When talking about the size of breakers, do I just look in the combiner box and see what size of breakers are in there? Is it simply stamped on them?

If I did upgrade to 24V inverter, what size would I need in terms of Watts? Still 3000 Watt or smaller/larger?

Thanks everyone for their help and advice.

upnorth said:

I have been hoping for a full sun day but hasn't happen. I am just trying to decide the best route to take so wanted to confirm I understand what the options might be.....

1) Keep the 12V inverter and add another Midnite 250 charge controller. In this case I should get double what I am getting now but will be over-panelled. So approx. 1600 watts?

2) try to exchange for 24V inverter and with current set up (just one Midnite CC) the system will generate approx. 1700-1800 watts as per Hedges?

3) If I went with 24V inverter and another Midnite 250 CC what could I expect for Watts to be generated?? Would this boost it above the 1800 Watts?

A couple of questions. When talking about the size of breakers, do I just look in the combiner box and see what size of breakers are in there? Is it simply stamped on them?

If I did upgrade to 24V inverter, what size would I need in terms of Watts? Still 3000 Watt or smaller/larger?

Thanks everyone for their help and advice.

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Ill answer question 3 and 4.

3. 24V inverter & midnite 250. Midnite can charge 62amps at 24v each. So if you had 2 of them you'd be producing 124amps total. 124ax24v = 2976w. So you would be using all of your panels power.

The inverter size it totally up to you. It depends how much power the cabin is drawing. You should do an energy audit to know what you need + a little leg room.

Really hope they trade in the inverter for you.

A key problem to fix is that the inverter drained the battery too low. You need an inverter with suitable low battery shutdown, somewhere around 80% DoD.

I'll answer the breaker question, I think Ryang did a good job with #3. If you have a Midnite PV3, you most likely have 150V breakers. They look like this. If they gave you 300V breakers, they will be double breakers, and end up looking like 4 breakers in a box designed for 3 breakers, so modification would have had to have been done. The 300V breakers look like this.

why leave the inverter on when you are not there?

Ok thanks for the info. To be honest just left the inverter on because I didn’t know to turn it off and didn’t realize it would drain the batteries.

upnorth said:

Ok thanks for the info. To be honest just left the inverter on because I didn’t know to turn it off and didn’t realize it would drain the batteries.

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i would have thought the same. that is a lot of battery reserve.... at least it has shown the other design issues with this system

SolarQueen said:

I'll answer the breaker question, I think Ryang did a good job with #3. If you have a Midnite PV3, you most likely have 150V breakers. They look like this. If they gave you 300V breakers, they will be double breakers, and end up looking like 4 breakers in a box designed for 3 breakers, so modification would have had to have been done. The 300V breakers look like this.

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How about low battery shutdown? I read spec for his model inverter as saying doesn't shut off until 10.2V.
Do you have a retrofit shutdown solution? Does Midnight charge controller have a signal which could be used to control inverter or a relay?

Hedges said:

How about low battery shutdown? I read spec for his model inverter as saying doesn't shut off until 10.2V.
Do you have a retrofit shutdown solution? Does Midnight charge controller have a signal which could be used to control inverter or a relay?

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If we switch to a 24V system, the Victron BatteryProtect could do the trick. Available with or without Bluetooth. https://www.victronenergy.com/battery_protect/battery-protect