New member in Alabama

[Sunking]

Why did you wire the panels in parallel?

By doing that you just threw away all the benefits of your controller, and forced you to use fuses and combiners that cost you big money. Additionally the wire between the panels and controller looks way to small making it dangerous and very inefficient. Wirre all the panels in series or 2S2P, and all those problems and expense goes away.

[SunEagle]

Hello,

Joined today. I'm interested in feedback for 48V off-grid mobile marine application that I'm working on. Unfortunately I don't see any controls to create a new post with details. What am I doing wrong?

Thanks

Jack

New members need to be vetted by the Admin before they can make a new post. Hang in there and hopefully Solar Pete will work on your access.

[AllenSco]

Why did you wire the panels in parallel?

By doing that you just threw away all the benefits of your controller, and forced you to use fuses and combiners that cost you big money. Additionally the wire between the panels and controller looks way to small making it dangerous and very inefficient. Wirre all the panels in series or 2S2P, and all those problems and expense goes away.

I was wondering when you were going to come in LOL Thanks for taking the time, I'm always up for learning how to make everything more efficient.

Okay, it's a small system, so I figured wiring in parallel would be the way to go to keep the charge current up. 10 AWG wire is plenty big to handle the current (20-23 amps) to keep up two 100 ah batteries, also wired in parallel. I'm still learning...and reading this forum when I have time. By wiring the panels 2S2P, the voltage doubles (24 volts) and the current is halved (10-13 amps). If I understand correctly, the Victron MPPT will drop the extra voltage to match the battery and jack up the amperage, yes? This will be enough to keep up the battery?

Thanks again
Allen

[jangr]

New to this, but I believe 2S2P will double BOTH Vmp and Imp, while 4P will quadruple Imp. The issue with 4P is that the MPPT will not have any over-voltage to boost current when the panels are delivering less than Imp, which is most of the day. If your energy draw requires significant bulk charging, your total charge time will be significantly longer with 4P than with 2S2P. As noted, 2S2P also has fewer bits and pieces to worry about, which is always a good thing in terms of $$ and time.

[AllenSco]

Okay gentlemen, I have changed the panel wiring to 2S2P. It's working fine Thanks SunKing and jangr for the suggestion/correction.

[AllenSco]

That's crazy! You guys were right on...less time to charge. Sky here is mostly cloudy with a few breaks and is in the process of clearing. Here's what the Victron says...

[Mike90250]

That's crazy! You guys were right on...less time to charge. ....

Happy Thanksgiving , enjoy the power

[AllenSco]

Happy Thanksgiving , enjoy the power

Happy Thanksgiving to you too Mike!

[Sunking]

That's crazy! You guys were right on.

Duh! You are welcome.

Although you cannot take advantage of the savings it offers, they make Charge Controller with 600-Volt Input to charge 12, 24, 36, 48, 70, 72 up to 120 volt batteries. They are expensive but save you a lot of money.

Example lets say you want to make a 3750 watt 48-volt battery system using 15 x 250 watt panels. You can use either say a 150 VOC 80 amp controller ($600) or a 600 VOC 80 amp controller ($1000). Most folks like you would run away from a $1000 controller. By doing so you just lost a lot of money.

If you went with the 150 Voc controller forces you to wire the panels in 3S5P. With 5 parallel strings now you are required to use a very expensive Combiner with fuses and breakers. However that is not the end of added expense using lower voltage. From that combiner you will need a Feeder circuit to go to the Controller. With 5 parallel stings means the current will be roughly 38 amps and that will require a minimum 8 AWG up to 15 feet, larger is longer. Wire is expensive.

Now if you had chosen the 600 volt controller, you would wire all the panels in series, thus no combiners or fuses, and can use cheap 12 AWG up to 75 feet to the controller saving you a lot of cash and a very efficient system. When the 1000 volt controllers come out will be even better.

[jangr]

Sinking - Does this approach scale down to a 1200 watt 48V system? With 4 x 300 watt panels, that would seem to suggest that 4S would be a better configuration than 2S2P. There must be a tipping point - # panels, VOC, # parallel connects, etc - where a pure series configuration is always better than a balanced serial/parallel configuration. Thoughts?

[Sunking]

Sinking - Does this approach scale down to a 1200 watt 48V system? With 4 x 300 watt panels, that would seem to suggest that 4S would be a better configuration than 2S2P. There must be a tipping point - # panels, VOC, # parallel connects, etc - where a pure series configuration is always better than a balanced serial/parallel configuration. Thoughts?

Maybe, maybe not. Depends on location and controller/panels Voc rating. Lets get you going, so far you are on the right track.

MPPT controller Voc input varies from on the low side say 50 volts up to 600 volts. Controllers with lower voltage are restricted to 12 and 24 volt battery systems of up to 400 watts with 20 amp output. Larger controllers for 48 volts start at 30 amps output. So what you do with that info works out to is:

0 up to 500 watts is 12 volt battery territory with an output of 5 to 40 amps as a general rule.
500 to 1500 watts is 24 volt battery, with an output of 20 to 40 amps.
1500 to 4000 watt sis strictly 48 volt battery up to 80 amp output.

What size controler is simple to figure out. Output Amps = Panel Wattage / Battery Voltage. Examples:

4000 watts / 48 volts = 80 amps
2000 watts / 24 volts = 80 amps
1000 watts / 12 volts = 80 amps.

So if you had 1000 watts of panels, you could use a 12 volt battery, but a very poor choice because at 24 volts all you need is 1000 watts / 24 volts = 40 amps which is a lot less expensive than an 80 amp controller. With me so far and understand?

Now lets get into Voc ratings. Voc means Voltage Open Circuit aka disconnected. Solar panels have a negative temperature coefficient meaning as temps go down, the panels Voc goes up. You cannot exce3ed the controller Voc rating. There are two ways to calculate the Voc parameter. If you live in the I-40 corridor or south of Interstate 40, you can use the NEC Correction Factor because that reagion of the country does not get extremely cold in winter. Real simple look at the panels Voc rating and multiply by 1.25.,

Example say you have a panel Voc rating of 40 volts and a 150 Voc controller. How many panels can you put in series? First you take the panel Voc (40 volts) x 1.25 = 50 volts. 3 panels in series is 150 volts right at the limit and too close for comfort IMO so you might get away with 3, but I would only go 2 in series because I wear a belt and suspenders. You will not see my tighty whities or my butt. If you live north of I-40, then you would need to use the panel manufactures temp correction factor for Voc as panels are rated at 75 degrees.

Bottom line is you have to do your homework to find the right numbers. I can help and guide you, but you need to do the work yourself so you can learn how. I will point you in the right direction, and check your work.

Now get to work, yes you are on the right track. Keep asking good questions and clarifications.

[jangr]

OK, that makes sense, and I guess I understand the difference between Voc and Vmpp now, which I didn't before

My 1200 watts @ 48V yields 24A, so a 30A controller will work with some upside buffer. I'm not south of I40, but my off-grid application is seasonal April-Nov. So my 300W panel with 39.7 Voc @ 1.25 correction is pretty much the same as your illustration above.

So I could go with a 150Voc 30A controller in 2S2P. A 4S configuration would be 200Voc, (250Voc to be safe) and current would drop to 12A? 2 questions, 1) is there such a thing as a 250 Voc controller, and 2) if so would I break even with the extra cost of the controller vs cheaper 12A wiring, no combiner, and overall fewer bits and pieces.

Is this how the calculus works?

[Sunking]

My 1200 watts @ 48V yields 24A, so a 30A controller will work with some upside buffer. I'm not south of I40, but my off-grid application is seasonal April-Nov. So my 300W panel with 39.7 Voc @ 1.25 correction is pretty much the same as your illustration above.

Be careful because it takes a minimum 16 volts Vmp for each 12 volts of battery, and that yields minimum efficiency. For a 48 volt battery you realistically want to be at 90 to 120 volts vmp on a 150 Voc controller.

1) is there such a thing as a 250 Voc controller, and 2) if so would I break even with the extra cost of the controller vs cheaper 12A wiring, no combiner, and overall fewer bits and pieces.

Is this how the calculus works?

Yes, Midnight Solar makes them at 150, 200, and 250 Voc and they call it hyper Voc which means Voc plus battery voltage. So a 150 Voc controller with 48 volt battery can run as high as 198 volts. For you I would recommend looking at the Midnite Solar KID product line, a 30-amp controller. The KID controller at 48 volts will allow you to grow to 1500 watts. One of the models is Marine Rated which means it is tough as nails.

[jangr]

Now I'm really confused. I had assumed all along that 4 x 300 watt in a 2S2P configuration & MPPT 30A controller would work charge my 48V battery pack. You're saying that the 70 Vmp I get with 2S2P is not enough, or not efficient. So 4S is basically my only option. Is that right?

Unrelated, holiday shopping discounts this weekend are pretty amazing. Canadian Solar 300W 5 bus mono panels are going for $193. While I'm still learning as I go, should I just take advantage of the holiday market, pull the trigger, and assume these panels will be adequate. Mike made a great suggestion to just assume panels will get eaten up in a couple of years of marine exposure and replace them. At this price that seems fair.

Thoughts?

[Sunking]

Now I'm really confused. I had assumed all along that 4 x 300 watt in a 2S2P configuration & MPPT 30A controller would work charge my 48V battery pack. You're saying that the 70 Vmp I get with 2S2P is not enough, or not efficient. So 4S is basically my only option. Is that right?

In order to perform a EQ charge takes up to 2.66 vpc or 16 volts on a 12 volt Pb battery. The key is using a controller with a high enough Voc input rating. That ranges from 40 to 600 volts. Jump on your thread to discuss.