Solar panel array /battery sizing

[larrybc1]

I have a question about how to figure out how many AMPS are being produced by a solar array.....and then to determine max charging current for a 900AH/48 volt battery bank..

I have seen a couple of formulas and dont know which one is right...assuming the solar array is 7200 watts is the correct formula:

7200 x 0.77 (efficiency factor). divided by 48 ?...so would be 115 AMPs...or is it:

7200 divided by 58 (voltage charging current from CC)...or is it:

7200 x0.77 (eff factor) divided by 58 ?

On the battery bank side 900 AH bank the max charging would be 13%.. So
900 x .13 = 117 AMPs..

The reason I am asking is that I want to increase my solar array because I have a lot of loads during the day and my current charge rate is a little less than 9% and I am having a hard time getting batteries fully charged because of the loads..so i want to get as close to max charging rate as possible...I have 2 refrigerators and 2 freezers plus lights that run during the day...Also during cloudy days I want to be able to harvest more energy that what I have at present.

Thanks for the help

Thks for the help

[Sunking]

Have no idea where you are getting 77% efficiency from. but that has nothing to do with charging batteries from solar.

Charge current depends on which type of controller you are using being either PWM or MPPT.

For PWM: Input Current = Output Current.
For MPPT Output Current = Panel Wattage / Nominal Battery Voltage.

As for your issue with getting your batteries fully charge is all taken care of in a proper design. So if you are not getting your batteries fully charged, then your design is flawed. The flaw is impossible for me or anyone else to determine because we do not know your daily Kwh usage, location, and time of year use. But your flaw is one or all of those factors.

First Step is to determine maximum daily Kwh usage. Say 5 Kwh. Based on that we can immediately determine battery requirement assuming we know what battery voltage is going to be used. In this case let's say 48 volts. So for 48 volt battery the required Amp Hours = [5000 watt hours x 5] / 48 volts = 520 Amp Hours

Next step is determine panel wattage for worse case month which is usually December so let's say December Sun Hours = 3 Hours. Panel Wattage = [Daily Watt Hours x 1.5] / Sun Hours. So in this example [5000 wh x 1.5] / 3 Sun Hours = 2500 watts.

Next thing is Charge Controller Amps. Already covered than so 2500 watt / 48 volts = 52 amps.

Last is a sanity check to see if charge current falls between C/12 to C/8 for the batteries. 52 amps on a 520 AH battery = 520 AH / 52 Amps = 10 Hours which is perfect charge rate between C/12 to C/8.

Now you can go figure out where you went off track.

[russ]

The 77% is probably the default value from PV Watts - grid tie number that is on the low side.

A battery system is closer to 50% for the derate number.

[mapmaker]

Have no idea where you are getting 77% efficiency from. but that has nothing to do with charging batteries from solar.

I think it has largely to do with STC vs NOCT. The panel wattage is determined by a lab test at Standard Test Conditions. That is done with a flash of light so brief that the panel does not heat up. In the real world, panels get hot in the sun, and their current is less than the STC ratings. The Nominal Operating Cell Temperature rating is an attempt to give a real world rating to the panels. It assumes 800 watts per square meter of irradiance, an air temp of 68° F, a wind speed of 2.24 mph, and the underside of the panel gets some of the breeze. I think the 77% derating is applied to the STC ratings in an attempt to predict the real world energy production of a solar panel. --mapmaker

[russ]

I think it has largely to do with STC vs NOCT. The panel wattage is determined by a lab test at Standard Test Conditions. That is done with a flash of light so brief that the panel does not heat up. In the real world, panels get hot in the sun, and their current is less than the STC ratings. The Nominal Operating Cell Temperature rating is an attempt to give a real world rating to the panels. It assumes 800 watts per square meter of irradiance, an air temp of 68° F, a wind speed of 2.24 mph, and the underside of the panel gets some of the breeze. I think the 77% derating is applied to the STC ratings in an attempt to predict the real world energy production of a solar panel. --mapmaker

Few places receive the 1000 watts/m2 even at solar noon and the rest of the day is less. As you pointed out 800 watts/m2 is used for a more nominal number.

[J.P.M.]

I think it has largely to do with STC vs NOCT. The panel wattage is determined by a lab test at Standard Test Conditions. That is done with a flash of light so brief that the panel does not heat up. In the real world, panels get hot in the sun, and their current is less than the STC ratings. The Nominal Operating Cell Temperature rating is an attempt to give a real world rating to the panels. It assumes 800 watts per square meter of irradiance, an air temp of 68° F, a wind speed of 2.24 mph, and the underside of the panel gets some of the breeze. I think the 77% derating is applied to the STC ratings in an attempt to predict the real world energy production of a solar panel. --mapmaker

The derate in PVWatts is the result of multiplying various things that contribute to system inefficiencies in a simple, and somewhat shotgun approach - wiring losses, panel burn in, inverter losses, system availability, panel fouling (dirt) and other things. The .77 number is a DEFAULT number for the derate factor, and is there for those who don't know or care where the overall derate factor comes from, that it is a combination of other factors, or that those other factors even exist. The fact that the default of .77 is so low, at least for grid tie systems, is more than unfortunate. Being that low serves as a sales tool and dishonest, cynical and phony "proof" and false justification for oversizing systems to customers who are solar ignorant.

The derate factor in PVWatts has nothing to do with STC or NOCT in a direct way. The DEFAULT derate also often has little to do with real world energy production beyond making systems that are probably not optimally cost effective, if cost effective at all, even less so.

Also, as PVWatts states, it's for grid connected systems. I think this thread is about off grid and batteries.

[larrybc1]

Few places receive the 1000 watts/m2 even at solar noon and the rest of the day is less. As you pointed out 800 watts/m2 is used for a more nominal number.

Hi to all that replied...

My point was that a 7200 watt solar array never produces full power--7200 watts....so I wanted to know what the standard derate is so I can figure out how many amps it will deliver to the battery bank so I dont overcharge my batteries. I currently have 5300 watt array to a 900AH /48 volt battery bank.it works great but during the day I have 2 freezers and 2 refirgerators drawing power along with some other things so all the power from the solar does not just charge the batteries. In addition during cloudy days would like more PV to get batteries charged up...does this sound ok? Thks larry

[mapmaker]

My point was that a 7200 watt solar array never produces full power--7200 watts....so I wanted to know what the standard derate is so I can figure out how many amps it will deliver to the battery bank so I dont overcharge my batteries.

Unfortunately you must assume that you will produce full power from time to time. And the system must be designed to handle that full power, even though it rarely occurs. It's sort of like solar panel string voltage... the string voltage that provides good power is much lower than the Voc, early on a cold day, before there is power production... and yet you must design the system so that the cold Voc does not exceed your controller voltage input rating at a time when you are not making power. When you over panel your batteries, there are a number of options.... Options may depend on your controller, What charge controller(s) are you using? --mapmaker

[mapmaker]

Unfortunately you must assume that you will produce full power from time to time. And the system must be designed to handle that full power, even though it rarely occurs. It's sort of like solar panel string voltage... the string voltage that provides good power is much lower than the Voc, early on a cold day, before there is power production... and yet you must design the system so that the cold Voc does not exceed your controller voltage input rating at a time when you are not making power. When you over panel your batteries, there are a number of options.... Options may depend on your controller, What charge controller(s) are you using? --mapmaker

sorry about this being one long paragraph... the forum software keeps collapsing my paragraphs. The previous post was supposed to be three paragraphs. --mapmaker

[larrybc1]

sorry about this being one long paragraph... the forum software keeps collapsing my paragraphs. The previous post was supposed to be three paragraphs. --mapmaker

Hi..Using 3 Outback FM80 CC...Someone suggested using the flexnet DC to maybe manage it..can I program the CC's to prevent too much current?? thks

[Sunking]

Hi to all that replied...

My point was that a 7200 watt solar array never produces full power--7200 watts....so I wanted to know what the standard derate is so I can figure out how many amps it will deliver to the battery bank so I dont overcharge my batteries. I currently have 5300 watt array to a 900AH /48 volt battery bank.it works great but during the day I have 2 freezers and 2 refirgerators drawing power along with some other things so all the power from the solar does not just charge the batteries. In addition during cloudy days would like more PV to get batteries charged up...does this sound ok? Thks larry

Lary again if the system is designed correctly, you should not have to worry. Nothing you can do about Cloudy days, adding more panels is pointless as power is reduced to almost nothing when over cast. That is one reason why a Generator is REQUIRED for OFF-GRID Battery Systems. In addition your panels are not capable of fully charging your batteries and REQUIRES a Generator to run about once a week to apply a SATURATION CHARGE.

If your system is designed properly, you should more than enough panel wattage to run your connected loads and charge the batteries. With 5200 watts running@ 48 volt battery I hope you have a Midnite Solar Classic 150 or 200 as they are the only controllers that can handle up to 5200 watts with a 5200 watt panel input on 48 volt battery. At 5200 watts on 48 volt battery you can generate up to 96 amps but only on a Midnight Solar 150 or 200. Anything else is being clipped and lost.

So if you are looking for improvements you should be looking for a good 7 Kwh Generator, LPG tank, and a 48 volt 150 amp charger. In addition check your daily Watt Hour usage to make sure your battery bank and panel wattage are correct size for your area. Otherwise you are just guessing and taking chances.

[larrybc1]

Lary again if the system is designed correctly, you should not have to worry. Nothing you can do about Cloudy days, adding more panels is pointless as power is reduced to almost nothing when over cast. That is one reason why a Generator is REQUIRED for OFF-GRID Battery Systems. In addition your panels are not capable of fully charging your batteries and REQUIRES a Generator to run about once a week to apply a SATURATION CHARGE.

If your system is designed properly, you should more than enough panel wattage to run your connected loads and charge the batteries. With 5200 watts running@ 48 volt battery I hope you have a Midnite Solar Classic 150 or 200 as they are the only controllers that can handle up to 5200 watts with a 5200 watt panel input on 48 volt battery. At 5200 watts on 48 volt battery you can generate up to 96 amps but only on a Midnight Solar 150 or 200. Anything else is being clipped and lost.

So if you are looking for improvements you should be looking for a good 7 Kwh Generator, LPG tank, and a 48 volt 150 amp charger. In addition check your daily Watt Hour usage to make sure your battery bank and panel wattage are correct size for your area. Otherwise you are just guessing and taking chances.

Hi derek...yes i have 2 outback fm80 cc. I have a 30 kw genset to charge and run the resort when it is cloudy days..I am in the Philippines and completely off the grid so no other options..i have added more loads then i originally had planned for and that is why i have added more panels and another charge controller for those panels ...the loads are running during the day -- 2 freezers and 2 refrigs and seem to draw current that would othwise be going to charge the batteries...thks

[larrybc1]

Lary again if the system is designed correctly, you should not have to worry. Nothing you can do about Cloudy days, adding more panels is pointless as power is reduced to almost nothing when over cast. That is one reason why a Generator is REQUIRED for OFF-GRID Battery Systems. In addition your panels are not capable of fully charging your batteries and REQUIRES a Generator to run about once a week to apply a SATURATION CHARGE.

If your system is designed properly, you should more than enough panel wattage to run your connected loads and charge the batteries. With 5200 watts running@ 48 volt battery I hope you have a Midnite Solar Classic 150 or 200 as they are the only controllers that can handle up to 5200 watts with a 5200 watt panel input on 48 volt battery. At 5200 watts on 48 volt battery you can generate up to 96 amps but only on a Midnight Solar 150 or 200. Anything else is being clipped and lost.

So if you are looking for improvements you should be looking for a good 7 Kwh Generator, LPG tank, and a 48 volt 150 amp charger. In addition check your daily Watt Hour usage to make sure your battery bank and panel wattage are correct size for your area. Otherwise you are just guessing and taking chances.

Hi derek..wouldnt a 48 volt 150 amp charger be tooo much for a 900 AH battery bank?.. Being that 13% is max charge rate?... (900x13%= 117).. Just wondering if i am figuring it right..i have a 48 volt 100 amp charger now..thks larry

[Sunking]

Hi derek..wouldnt a 48 volt 150 amp charger be tooo much for a 900 AH battery bank?.. Being that 13% is max charge rate?... (900x13%= 117).. Just wondering if i am figuring it right..i have a 48 volt 100 amp charger now..thks larry

What battery chemistry? If FLA lead calcium not enough to be concerned with. General rule for FLA lead Calcium is C/8 max. 900/8 = 113 amps. 150 amps is roughly C/6 but not high enough to worry about. You can charge them at 900 amps if you can keep the battery temp below 50 C.

The issue with lead calcium is internal resistance is on the high side which causes heating when either charging or discharging to high. C/8 is just a general rule to start making you stop and think about over heating. As long as the batteries do not over heat or go above Gassing Voltage you can charge them up to 1C. Most of the better battery chargers you can limit the current, so if 150 amps were causing heat problems just back the current off. With a genny you want to charge at high of a rate as possible to minimize fuel burn.

If you have a genny you should be running it at least once a week to saturate your batteries as it is almost impossible for solar to saturate a battery unless it is not used frequently. If you use it everyday, then the batteries are never fully charged up. Adding more panels will not change that.

However 100 amp will work, just takes longer to get to the Absorb Phase. Absorb is the Saturation Charge that takes 6 to 8 hours to complete. Only a genny or commercial power can do that.

[Mike90250]

using loads in the daytime, subtracts that power from battery charging, so if you had 100 A from the PV array, and 600 watts of loads at the same time, you are only putting 88 A intro the batteries. (50v used for battery voltage)
And the arrays produce less than nameplate when they warm up in the toasty sun.

Either reduce loads, or add panels.

You could put the fridges on timers to shut off nights, from 10pm- 8am. That would shift power from battery used, to solar provided, and up your efficiency a little bit.