Is my math off on the size of array I need?

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So I copied this formula, provided by BB, on how to determine the size of array necessary to charge your required AH.

“Size of solar array--Two calculations, first based on minimum rate of charge for battery bank. 5% for weekend/seasonal usage, 10% to 13% for full time off grid--Your system may be better off with the 10% rate of charge--Basically you are running the fridge 24x7, so you are "stealing" some charging current during the day--The 5% minimum rate of charge assumes no concurrent loads:

AH x 14.5(12V) or 29(24V) x 1/0.77(panel efficiency) x % rate of charge(5% for weekend/seasonal use, 10% to 13% for full time off grid, 10% was suggested for our situation)= Wattage of panel array”

According to the specs on my panels, they are rated at only 16% efficiency. When you plug that in to the formula using the 430AH that I will need for my loads, I get.

430 x 29 x 1/0.16 x 0.10 = 7,793.73 Watts

I already have 6 panels for a total of 1860 Watts. That comes out to a 2% rate of charge. Am I in trouble?

1860 W (6) Rensola JC310M, Classic 150, [email protected] (8) Deka DSGC15 FLA's, Victron Phoenix Multi 24/2500 Inverter
Powering-20.5 CuFt. Fridge, 1 HP Submersible water pump, UV, Washing Machine, Gas Dryer, Gas Stove, Lights and Receptacles
Off Grid Cottage on lake in Northern Ontario 46˚N

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rp3703 said:

So I copied this formula, provided by BB, on how to determine the size of array necessary to charge your required AH.

“Size of solar array--Two calculations, first based on minimum rate of charge for battery bank. 5% for weekend/seasonal usage, 10% to 13% for full time off grid--Your system may be better off with the 10% rate of charge--Basically you are running the fridge 24x7, so you are "stealing" some charging current during the day--The 5% minimum rate of charge assumes no concurrent loads:

AH x 14.5(12V) or 29(24V) x 1/0.77(panel efficiency) x % rate of charge(5% for weekend/seasonal use, 10% to 13% for full time off grid, 10% was suggested for our situation)= Wattage of panel array”

According to the specs on my panels, they are rated at only 16% efficiency. When you plug that in to the formula using the 430AH that I will need for my loads, I get.

430 x 29 x 1/0.16 x 0.10 = 7,793.73 Watts

I already have 6 panels for a total of 1860 Watts. That comes out to a 2% rate of charge. Am I in trouble?

The "Panel efficiency" you are referring to is not the same as Bill is referring to... You panel efficiency has to do with the amount of available power the panel is converting to electric per square foot. Bill is referring to the Normal Operating Cell Temperature value (NOCT). This is the difference the panel exhibits from Standard Test Conditions (STC). In essence a panel rated at 100 watts under STC, will only produce 77% (I think 75% is a better number, I haven't seen one above 74% in quite a while)
Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
I usually use the term "controller+panel deratings"... Nominally I use ~81% derating for NOC (normal operation condition?) and 5% for charge controller efficiency (technically for an MPPT type charge controller.

0.81 * 0.95 = 0.77

The 81% is because "cool" (STC standard test conditions) the Vmp of the panel is higher than under actual operating conditions (cells heating from direct sunlight, hot summer weather on a roof, etc.).

MPPT controllers (math point of view) operate Power=Vmp-operating * Imp-operating (where Vmp can fall to ~81% of Vmp-std in summer weather).

PWM controller have different operating conditions... Their formula can be estimated as Power=Imp*Vbattery

For Solar panels Imp changes very little with temperature (in fact, Imp rises slightly as the panels get hot). As long as Vmp-hot is higher than Vbattery-charging, then temperature has little do to do with the "math" for PWM+Solar power calculations.

I have no problem using Photwhit's 75% derating for solar panels+controller... More or less, the numbers, amount of sunlight, weather conditions, etc. -- If you are within 10% of theoretical numbers, you are doing fine. Solar is just not that accurate (and measuring power closer than 5% accuracy for solar is almost impossible without laboratory equipment and other testing gear).

I would have done much better in grade school if my teachers did math more like an "English Sentence":
• 430 Amp*Hour battery bank x 29.0 Volts battery charging x 1/0.77 panel+controller derating  x 0.10 rate of charge = 1,619 Watt nominal array
Note that Battery AH ratings depend on how fast they are being discharged... If you pull lots of current out of a battery bank (very fast discharge), the batteries are less efficient and have a "lower AH rating". For our rule of thumbs, we use the C/20 or 20 hour discharge rate:
• 430 AH / 20 hours = 21.5 amp discharge rate (battery from 100% to 0% state of charge).
Of course, we recommend that the battery be discharged to 75% to 50% state of charge in normal usage, and avoid going below 20% state of charge if at all possible (if you have a weak cell and it "reverses polarity" (goes below zero volts because the cell is full discharged), most battery chemistries are permanently damaged/ruined if this happens.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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So 1619 watts is what I need. I have 1860 Watts. Even though the Midnite String calculator says my Classic 150 can handle all six panels wired in three parallel strings, is there any danger that I may have too much power from my panel array?
1860 W (6) Rensola JC310M, Classic 150, [email protected] (8) Deka DSGC15 FLA's, Victron Phoenix Multi 24/2500 Inverter
Powering-20.5 CuFt. Fridge, 1 HP Submersible water pump, UV, Washing Machine, Gas Dryer, Gas Stove, Lights and Receptacles
Off Grid Cottage on lake in Northern Ontario 46˚N
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On a 24v system voltage, I think you're fine.

IIRC, the classic is good for 90-odd amps. 90 x 28v charging is ~2500w, and I think Midnite suggests no more that 20% overpanelling.
Off-grid.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
A standard "Classic 150" controller is rated:

 Max Current Output at 25°C (77°F) 96A @ 12V 94A @ 24V 83A @ 48V
• At 94 amps * 29.0 volts max charging voltage = 2,726 Watts output
If you assume that solar panels and charge controllers have a derated output (from solar panel to battery bank) in mid-summer of 77%, then the controller would support a maximum array of (approximatly):
• 2,726 Watt (battery charging power) * 1/0.77 panel+controller derating = 3,540 Watt STC rated solar array "cost effective" maximum
Since MPPT controllers control their output current as a matter of standard operation, the charge controller will simply draw less power (current/voltage) from the solar array if the array exceeds the controllers output current rating.

The 77% derating is a "typical" hot/clear non-winter day "derating". And it means that the controller will rarely "clip" its output current on a typical day--Or if it does clip output current (i.e., you are not harvesting all available current), it will be only a small amount around solar noon.

You can install even a larger array (check with Mfg.) and all will be fine--The controller will simply "clip" more current for longer times in the middle of the day.

And there is the other concern--One of the enemies of electronics and heat and thermal cycling. Keeping the controllers in a cool/shaded place with good air circulation will extend their life.... And some folks would prefer to run their controllers at less than full power (i.e., only run at 80% of rated power/current) to keep them running cooler. Not a bad idea--But adding a second controller costs extra money too.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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What If I reduced my array to only two strings. That would be 1240 watts at 7% rate of charge. Would running less power through the charge controller and ultimately to the batteries(8- 6V 215 AH FLA wired in two strings of 4)be more advantageous than overpanelling?
1860 W (6) Rensola JC310M, Classic 150, [email protected] (8) Deka DSGC15 FLA's, Victron Phoenix Multi 24/2500 Inverter
Powering-20.5 CuFt. Fridge, 1 HP Submersible water pump, UV, Washing Machine, Gas Dryer, Gas Stove, Lights and Receptacles
Off Grid Cottage on lake in Northern Ontario 46˚N
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edited June 13 #8
No, Makes no sense, under panel your system in hopes of saving a little heat in the charge controller?

FWIW - I've been running 2000 watts through each of my 2 Classic 150 lite's for 6 years. No problems yet with charging... I do have some communication issues with one unit, but it works fine and does 'follow me' but sometime is difficult being brought up on the single screen.

Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
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What about the C/rating for the battery bank? Do we need to limit the output of the Midnite to not charge the batteries too fast?

Full disclosure- RP3703 are working on the same system.
No... Just keep good air circulation around the charge controller (and battery bank if the batteries run hot).

In engineering, you can drive yourself nuts trying to take into account all of the deratings, and customer requirements... Designing a phone system (voice mail in my case) to run at 50C at full rated capacity--There are reasons that they pay millions of dollars for a piece of gear that is the size of two household freezers.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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AYHSMB said:
What about the C/rating for the battery bank? Do we need to limit the output of the Midnite to not charge the batteries too fast?

Full disclosure- RP3703 are working on the same system.
If you exceed the battery manufacturers maximum charge current, you can very easily shorten the battery life!
"we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
http://members.sti.net/offgridsolar/
E-mail [email protected]

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Exactly my point. Most FLA like a C/7 or better from what I understand. So I need to limit the output of the Classic 150 to prevent charging too fast correct?
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AYHSMB said:
Exactly my point. Most FLA like a C/7 or better from what I understand. So I need to limit the output of the Classic 150 to prevent charging too fast correct?
You can if you'd like, but it's not likely to happen. It would take a very cold day to exceed the 13% rate most flooded Lead acid batteries like.

Under Normal Operating Cell Temperature NOCT from your PDF on your solar panels. Your 310 watt panels are likely to produce 230 watts. So the math goes 230 x 6 panels = 1380 watts 1380 watts at 29 volts (near the end of bulk) will receive 1380watts/29volts= 47.6 amps into your 430 amp capacity battery bank for a rate of 47.6/430= 11%

The charge controller will maintain the charging voltage once it reaches absorb and the battery will slowly accept less current. The charge controller actually doesn't limit the current once it reaches absorb, it's the battery slowly accepting less current. You can set the absorb stage for it to time out after 1-5 hours or you can have the controller switch over to float using 'end amps' usually set to 1-2% of the battery capacity. Using end amps the charge controller will switch once the battery only accepts this minimal amount.

Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
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It seem to me, a worst case (max current) situation would go something like this; bank is discharged to ~22v. It's a gloomy, cold, breezy morning. The clouds part around noon, and the panels put out something close to STC for a while.

1860w panels ÷ 22v = ~85a into the batteries, or about C5 less losses. On such a day, I'd probably be flipping on a pump or something anyway, so I'd just as soon have the classic take what it can get.
Off-grid.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
Also, lead acid batteris at less than 80% state of charge can take >C/8 charging current (they are very efficient).

As you get into the 80-90+ state of charge, the battery bank starts to gas (lead acid chemistry). And even a C/20 (5%) or C/40 (2.5%) rate of charge (>95% SoC), and you can overheat your battery bank if left charging/Equalizing for many hours (something like 1/2 the charging current goes into gassing, the other 1/2 goes into heat--Roughly, as I recall).

As long as the Absorb charging voltages are reasonable, generally, you will not overheat the battery bank. Installing the Remote Battery Temperature Sensor is a good way to keep the battery bank from overheating/too much charging voltage/too much charging current.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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Of the damaged batteries I have seen, the owner had the solar right but never limited the generator charge source.
There are at least 2 chargers in most systems.
"we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
http://members.sti.net/offgridsolar/
E-mail [email protected]

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Dave, do you know if the damage happened in bulk?

I limit charging current to avoid overloading the generator, not to protect batteries. In theory, the sun could come out and get bulk charging current from both pv and genny. Unlikely, but not impossible.
Off-grid.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
• Posts: 4,092Solar Expert ✭✭✭✭
It could have but the one I remember well was the owner had a small 4kw genset for years and was fine until he bought a 7kw genset. Batteries failed after that winter. The same scenario you suggest could just as easily happen in absorption. I do not miss my genset
"we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
http://members.sti.net/offgridsolar/
E-mail [email protected]