Sizing solar watts to battery bank

bigbillsd · December 2018

@Mountainman, its December, I was wondering what you are experiencing on the charging front? Are you able to charge those batterys fully in 4 hours of sun? -Bill

mountainman · December 2018

14 days into this month and only 2 or 3 days of sun. My generator ran so much it needs an oil change. Pv watts says 4 hrs the only way to get 4 average sun hours per day this month is if the next 2 weeks brings 8 hrs a day. If I have a couple days of sun maybe I can do some testing.Rolls formula says not.. absorb time 80 to 100% 416×0.42÷55 amps=3 hrs 17 mins?

mike95490 · December 2018

When you talk solar, nobody thinks bad weather !

mountainman · January 2019

Probably a crazy question. A 200 watt 30 vmp 6.66 imp panel vs a 200 watt 18.9 vmp 10.58 imp panel. Mppt cc @ 12 volts would would there be an amp advantage with the grid tie panel?

Estragon · January 2019

Maybe. If the 12v panel got hot, or the gt panel got cold, assuming the mppt works as it should?

mountainman · January 2019

But at 77f 200 watts is 200 watts.no matter how you slice it?

Estragon · January 2019

True, but how often will a panel be 77°f?

When it sits in the sun, a 12v panel may drop enough in voltage that mppt controller could have trouble bucking to battery voltage.

Depending on distance and wire size, there may also be a bit of difference in line losses with higher current / lower voltage into the controller.

BB. · January 2019

With an MPPT charge controller you want a minimum of 1.3x max battery voltage or 1.3x15v=19.5 Vmp-array suggested minimum MPPT controller voltage.

Running a Vmp~18 volt panel (the "12 volt" solar panel spec) on an MPPT controller will not gain you much of anything, and could cost you energy (hot/sunny day, 18 Vmp-std becomes ~Vmp-hot~14.4 volts). If the panel was exposed to sub freezing temperatures in full sun, perhaps a MPPT controller might give you a bit of better performance (something like ~10% +/- -- Pure Guesswork).

The minimum array would be 2x Vmp~18 volt panels in series, or a single Vmp~30 volt panel would be better for MPPT. I don't think you would "see" any difference between 2x100W "12 volt" panels vs a single 1x200W "31Vmp" panel) on a MPPT controller.

-Bill

littleharbor2 · January 2019

mountainman said:

But at 77f 200 watts is 200 watts.no matter how you slice it?

A solar panel, correctly oriented, on a sunny 77 degree day will have a considerably warmer surface. Cell temp and air temp. are two different things. Have you ever picked up a crescent wrench that has been lying in the sun for a while? Same idea.

Estragon · January 2019

It's pretty amazing watching steam rise off panels with ambient <0°f temp air when the sun hits and melts remnants of ice and snow, even in weak winter sun.

mountainman · February 2019

@Estragon what are your nominal and peak amps from a 1000 watt array with 12 volt bank?

Estragon · February 2019

IIRC, the best I've seen is ~80a on a cool breezy day with broken cloud. In normal conditions more like 50-60a.

mountainman · February 2019

Thanks So normally 55 amps 10 % charge rate on a 550 amp hour 12 volt bank with around c/7 when it's putting out 80. But On a 450 ah @ 12 volts 55 amps = 13% with a max of 80 roughly c/6. A 624 bank @55 amps puts me at 9℅ And with 80 amps c/7.8 charge rate. looks to me a 550 is the best match for a 1000 watt array. Question is could I squeak by on a 624 ah bank by doing some bulk if needed. I only have a 300 watt inverter for TV dvd and charging phones. So I normally crank the generator each morning for making toast and using the Microwave and the S.O. blow drying etc. Or would it be a better match with 550 or the 450 battery bank. Suggestions?

mountainman · February 2019

Being that im in RV and want to stay 12 volt. With 600 + ahs im thinking for 30 minutes running a 800 watt washer. 5 or 10 minutes of Microwave 900 watts. and a 830 watt toaster 4 slices 5 min. Does it Sound doable ? I'd have 1200 watt hrs available with a 3 day reserve. With 5 avg sun hrs from April - aug would a 1000 watt array produce 3500 watt hrs per day ? Anyone

Estragon · February 2019

3500wh/(sunny)day is roughly right for a 1kw array most of the year in many locations, IMHO.

I wouldn't run more than one of the heavy (toaster/microwave) loads at the same time on a 12v system, but one at a time should be doable. Running the genny for heavy loads in the morning sounds like a good plan to me.

mountainman · April 2019

Screenshot_20190421-193553.png

Trying to figure temp compensation With this panel. at 86c ambient temp. What would you expect the panel temp to be if it were pole mounted. Full sun at noon.

BB. · April 2019

Mountainman,

Just to be clear, what are you asking about temperature compensation? Generally we temperature compensate battery charging voltages (hot batteries, lower charging voltage; cold batteries, higher charging voltage).

With solar panels, their Vmp and Voc temperatures are cell temperature dependent... As the temperature rises, Voc and Vmp fall... However, we generally do not compensate for this other than looking at worst case cold temperature (Voc-cold vs Vpanel-max for MPPT charge controller, and Vmp-hot for hot panels on MPPT charge controllers) and that those numbers fall within MPPT controller specifications. And remember it is cell temperature (Tambient+Trise due to sunlight on cells).

For PWM charge controllers, we generally look at Vmp~17.5-19 volts as "optimum" for solar panels on a 12 volt Lead Acid battery bank (i.e., panels will generally work OK when hot and not "waste" too much energy when cold (due to the way PWM controllers work).

More or less, there is almost a 2:1 range from Voc-cold (subfreezing weather) to hot/no wind/sunny summer days.

Are you looking for the equation for Voc and Vmp vs temperature(s)?

-Bill

mountainman · April 2019

Yes voc and vmp at

correction from last post 30c 86f (not 86c.) Open field 5 mph or so wind. Roughly what temp would the panel be. And how much would it drop the voltage.

mcgivor · April 2019

The temperature of the panel is what matters, the ambient temperature along with wind will have a cooling effect, test with an inferred thermometer if needed. Checked mine yesterday, 82°C (180°F) on the back of the panels, 44°C (111°F) ambient with no wind, the output was 1240W from 1890W STC , or 65% . Sure they have reduced output but not something to be overly concerned with it is what it is.

billybob9 · April 2019

mcgivor

Ok, If the light waves from the sun are causing the panels to heat up but some of those waves were being converted to particles ( wave function collapse ) and becoming Photons, then wouldn't it be better to have the panels doing a continuing output to keep them cooler ?

mcgivor · April 2019

https://forum.solar-electric.com/discussion/comment/397458#Comment_397458

They are running an air conditioner, not sure about wave function collapse and photons, but the UV index of 16 yesterday may have some effect, today panel temperature is 65°C, ambient 40°C, UVI 12 with light breeze.

mountainman · June 2019

Ok from what I've read on here. You should bulk charge early and let solar finish. My situation. Being on the north west side of a mountain in a valley. This time of year I get 11 to 3:30 sun on the array. Im Thinking of putting another array closer to the rv where it would get sun (when it shines) from 2-6. 1 array on mppt and one on pwm?? Thoughts??? Any way this time of year at 3:30 or so I crank the generator for ac about 3-4 hours. What if instead of morning charging. I just charge the batteries in the afternoon while the ac is going?? I have the new type Iota with the iq 4. In the Virginia mountains it's either foggy in the morning. Cloudy mid day or raining in the afternoon. Weather forecast says the next full sun day is July 27. Last September had 2 days that it wasn't raining. Lastly If im charging every day at 13℅ or higher with the generator. is it doable to use a 5℅ solar array? This is a Summer only 3-4 month vacation stationary rv.

BB. · June 2019

You can try the PVWatts program to search for weather stations near you--And then play with orientation/tilt/etc. and see what you can do to optimize your harvest.

https://pvwatts.nrel.gov/

You can also download hourly data and see what only having 11am to 3:30pm of non-shaded sun has on your output.

The PVWatts and other data is actual 20+ year averages... So if you summer thunderstorms that is in there... However, if you are in a valley with local AM fog... Probably not (and you would have to do your spreadsheet download to do your own estimates).

-Bill

mountainman · June 2019

Yes im in a valley. when I bought the property I didn't consider that I'd need Solar power. because power lines are on my property .Here most days solar just isn't working. @Estragon seems I've read where you charged with a generator and no solar for a while? And that you run 2 45 amp Iotas off a 2000 Honda?

Estragon · June 2019

Yes, I can charge a ~700ah 12v bank with a pair of 45a Iotas with the little honda. Fortunately, I don't need to do it often, but there are ~6 weeks with little solar in winter (low sun angle shading from a treed ridge to the south). If charging to nearly full, eco mode reduces the fuel that would otherwise be wasted. If running much in the way of other loads in bulk, I just unplug one Iota and bulk slower.

I also sometimes charge the ~350ah 48v bank via a pair of Outbacks with the honda by setting the ACin and/or charging amp limit lower. Had to do that recently when company ended up drawing the bank low, there wasn't much sun forecast, and the diesel was acting up.

A higher rate of bulk charge is mainly so you can get charged in a day with available sun. With a generator, you can charge at a lower rate for longer if you're okay with it running longer. I've gone as high as ~20-25% using solar and diesel generator at the same time, down to ~5% using just the little honda on the 48v bank.

bigbillsd · September 2019

OldMan said:

mountainman said:

$2600.00 for a 200 amp hour battery.You got that right about bringing it inside.@Oldman does it pull power from the night air?

Oh, PLEASE. People who BUY Battle-Borns, thinking they had to pay that price, are MORONS!!! It is SO not necessary. You can even get prismatic 12v 100AH batteries from China. Every lithium cell in the WORLD is made in china. I can get two 12V 200AH battery from this vendor for about the price of your one Battle-Born. You just have to quit watching Fox News and do some work researching the topic. If you just want batteries and don't care about getting your money's worth, lead-acid is for you.

Anybody using those "cheap" Li batteries from "this vendor"? 400 Ah for ~$1000 seems way to good to be true? But I sure wish it is true!! -Bill

scrubjaysnest · September 2019

Estragon said:

Seems to me that could work, as long as array can be tilted (~30°) and faced south. If mounted flat, it looks like pvwatts has Dec at more like 2.5hrs full sun equivalent.

Sept 2019 getting 2 and 1/2 hours of sun on 3 100 watt panels. We were near Marion VA. This was fine for our 210 aH bank. Panels were 35 degrees and this brought from about 90% soc to between 101 and 102.

powersurge · September 2019

September in bluefield Virginia mountains
380 watts on our 208 ah bank
panels at 36°.
With just 4 hours sun 10:30 to 2:30. If I cut 1 more tree I'd get sun till 5:30
With batteries at ~75%. Sg at 8 am ~1.230.
By 2:30 im at 1% charge rate.
Sg at the end of the day showing 1.270 and 1.275. Funny that I haven't had to add any water?
My only day loads are a 3 amp DC fan and 1 amp lp fridge.
I've been here since July 1st.
haven't had to run the 2000 genny for charging yet
Almost 3 months and only 1 day of rain for about an hour.
Leaves are so dry I'm afraid some idiot's gonna throw out a cigarette.

BB. · September 2019

Some batteries use more water than others... The Trojan Solar type seem to not use much (3+ months?), and forklift batteries a lot more (every 1-2 months?).

If you are lightly cycling the batteries, they probably won't use as much either (really depends on your charging--longer absorb times/higher absorb voltage, more water usage).

What are your absorb voltage setpoint and absorb charging time? Have you done any EQ charging yet?

-Bill

powersurge · September 2019

20 to 25% daily usage. 14.8 volts and 3 hours absorbing. I equalized before I started using them. Haven't lately because of only .005 difference. I'm sure when it rains and my soc is 50-60% it will take a couple days at 4 hours sun to charge back up. Or maybe I'll have to bulk charge to give them some help.

Sizing solar watts to battery bank

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