Sizing a MPPT charge controller

[Mike90250]

a 2 or 3 inch pipe with a continuous uphill slope will vent without a fan. you can even put a T rain trap on it to help keep stuff out. To vent out, you also need an inlet vent too.

[SunEagle]

Then run the vent from the battery box. The fan is a horrible idea - the day it quits you have the potential for an explosion.

I agree with you. A fan can be used if it is necessary for a "quick air purge" but it is always better to have natural ventilation of explosive gases.

[russ]

Don't vent the whole room, too much heat loss, just the battery box.

Then run the vent from the battery box. The fan is a horrible idea - the day it quits you have the potential for an explosion.

[Amy@altE]

Don't vent the whole room, too much heat loss, just the battery box.

[russ]

If you keep the batteries in a cold room, make sure you factor in the temperature deration. 50F means multiply the size you need by 1.19 to compensate for the capacity loss at cold temps. It may be worth it to make an insulated battery box to help keep in the heat they generate while charging. You can then vent the gasses to the outside with a ZEPHYR POWER VENT.

Amy

If the room is vented then how do you keep the gases (mainly hydrogen) in the room?

A fan is not necessary. H2 is lighter than air and readily goes upward and out if possible.

[Amy@altE]

cold batteries

If you keep the batteries in a cold room, make sure you factor in the temperature deration. 50F means multiply the size you need by 1.19 to compensate for the capacity loss at cold temps. It may be worth it to make an insulated battery box to help keep in the heat they generate while charging. You can then vent the gasses to the outside with a ZEPHYR POWER VENT.

Amy

[Living Large]

Schneider does have their Conext Designer that allows you to figure out with their charge controllers. You should be able to do 2 strings of 8 of those 275s you like with the XWMPPT 80 600. I can run the math later, gotta run now.

Yes, Amy. I have Conext and have run it on the 2 x 8. I like the fact it does the wiring analysis for you - very cool. It was when I saw the drops in voltage, and varied the wire lengths, that the light bulb came on that I could locate the panels far from the controller and batteries. I initially was thinking I had to go to AC before making that 60'-80' run. Then I accepted having FLA batteries in my basement, which I initially resisted, and things got better because I didn't have to worry about keeping the batteries at a temp above 50 or 55 degrees. I am told one can fashion outside air intakes and exhaust to handle the battery out gassing. Thanks.

[Amy@altE]

Schneider does have their Conext Designer that allows you to figure out with their charge controllers. You should be able to do 2 strings of 8 of those 275s you like with the XWMPPT 80 600. I can run the math later, gotta run now.

[Living Large]

OK so what? All you worry about is not to exceed the max 100% limit.

That's easy for people who know the answer to bat around. But Paul realized I didn't know the answer, and explained it. He'd get a "Answered. Best Response" from me if that feature was part of the system. "so what?" would never have occurred to me, but it is an interesting alternative.

[Sunking]

I
To address some of your questions. Yes, if the sun is bright but your battery bank is full, your CC display will read very low watts, only what is required to maintain the batteries. Once the absorb cycle is complete at 56ish V, it will immediately drop to the 52V float voltage. Where does the other 4V go? I believe it is dissipated as heat by the CC.

It does not go anywhere, it is just not being used. It is very rare for a solar system to ever generate its rated power. In fact it never happens. The batteries only take what it needs. In a battery system most of the energy the panels are capable of generating i snever used. Just lost forever because it is never produced.

[Sunking]

Dereck,

I believe the OP is try to ask you about the Charge controller size to match the PV array, He read from somewhere that they believe the charge controller should work at 75% of it's rate power.

OK so what? All you worry about is not to exceed the max 100% limit. Any less than 100% is fine. It is not rocket science. A Midnite Solar classic maximum is:

1250 watts @ 12 volts
2500 watts @ 24 volts
5000 watts @ 48 volts

[Living Large]

I've been away for a bit but I assume you have calculated your loads? This seems to be quite a large system you're putting together.
I designed and built my off-grid system with help from members here and some books, NEC etc. It has been operating problem free for over 2 years now and performs better than I expected it to but I entered it with the mindset of every watt I could save is a watt of capacity I didn't have to build/pay for.

To address some of your questions. Yes, if the sun is bright but your battery bank is full, your CC display will read very low watts, only what is required to maintain the batteries. Once the absorb cycle is complete at 56ish V, it will immediately drop to the 52V float voltage. Where does the other 4V go? I believe it is dissipated as heat by the CC. CC have large heat sinks and fans and they do get hot to the touch.

Hi. Yes, the very first thing I did was calculate my need. I did this by inspecting the monthly usage for my current house over a few years, and predicting some lighter loads where appropriate in the proposed house. All the appliances are currently propane, but I added a load for an electric refrigerator. I watch tv a lot, but LED tvs lower my current tube tv draw. My largest current (both meanings) load is my well pump, which it will be at this place as well. I use a microwave for 2 to 8 minutes some days. Propane cooking and heating and hot water. Hoping to have radiant heat, with electric circulators. Wood heat supplement. In summer, possibly a mini-split on the lowest setting - 300W. I looked at this pretty carefully, and added some but not a lot of headroom. My intent is to have a "grid-like" existence similar to what I have now, with the addition of the monitoring, maintenance, backup power X 2, etc. Since I currently have two heat sources, I have that backup mentality to an extent already. Making this shift is a bit intimidating though. People here are helping with a lot of advice and I am learning more what to expect and how these systems work.

I currently am planning only 2 days of capacity. I have limited area for panels. Right now, I am around 6.9kWh per day, 4000-4400W PV, 730Ah @ 48V, with 6000W backup generator.

[thastinger]

I've been away for a bit but I assume you have calculated your loads? This seems to be quite a large system you're putting together.
I designed and built my off-grid system with help from members here and some books, NEC etc. It has been operating problem free for over 2 years now and performs better than I expected it to but I entered it with the mindset of every watt I could save is a watt of capacity I didn't have to build/pay for.

To address some of your questions. Yes, if the sun is bright but your battery bank is full, your CC display will read very low watts, only what is required to maintain the batteries. Once the absorb cycle is complete at 56ish V, it will immediately drop to the 52V float voltage. Where does the other 4V go? I believe it is dissipated as heat by the CC. CC have large heat sinks and fans and they do get hot to the touch.

[Living Large]

Not all MPPT controllers are made the same so you have to follow manufacturers instructions on sizing and usage. There is no standard/code answer we can give you.

Basic answer is that your array wattage divided by battery voltage has to be less than controller size but some controllers even allow that to be exceeded under the idea that panels seldom operate ate there rated capcity. The other thing to pay attention to is the string VOC of the array. It plus any highlight voltage increase must remain below input voltage of the controller. Here (Wyoming) code requires us to derate the controller input voltage by 1/3 to compensate for the cold temp increase.

WWW

I was starting to believe this as well - that there may be no standard and digging into a specific manufacturer's recommendations may be in order. Looking at a few, I didn't see guidance. I suppose one could contact a manufacturer for advice, if they are willing to discuss this with non-professionals. My principal concern was I am near the maximum for some models, and wanted to understand correct sizing as well as anticipate the cost of an added or larger MPPT.

As far as the VOC, Amy here helped me understand some things about it that I did not, but I was aware it goes up at temperature goes down and you plan for the highest voltage. Is your Wyoming code procedure a substitute for using the temperature coefficients to predict the VOC at the lowest expected temperature, or in addition to it? For example, let's assume a one panel tall array, and a panel has a VOC of 39V, and a predicted VOC using coefficienct of 45V at -20 deg C. Does Wyoming say you need to handle a VOC of 1.33(39) or 1.33(45)?

[Wy_White_Wolf]

Not all MPPT controllers are made the same so you have to follow manufacturers instructions on sizing and usage. There is no standard/code answer we can give you.

Basic answer is that your array wattage divided by battery voltage has to be less than controller size but some controllers even allow that to be exceeded under the idea that panels seldom operate ate there rated capcity. The other thing to pay attention to is the string VOC of the array. It plus any highlight voltage increase must remain below input voltage of the controller. Here (Wyoming) code requires us to derate the controller input voltage by 1/3 to compensate for the cold temp increase.

WWW