Sizing solar watts to battery bank

mountainman · November 2018

I'm going to upgrade my 12 volt battery bank to 416 amp hrs. 416x14.4÷.77×.13=1011 watts. 400 watts on a 30 amp and 600 watts on a 50 amp cc. Winters i stay close to myrtle beach With 4.08 sun hrs in Dec. Daily useage 25%. After a rainy day at 50 % soc and 13% charge rate. wouldn't I come close to full charge in 1 day with clear sky's on mppt?

Estragon · November 2018

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.

mountainman · November 2018

Panels would be on a pole and adjustable.

bigbillsd · November 2018

I have found that due to the way batteries accept a charge getting them to 80% SOC works well with solar, but getting them to 100% on solar alone when they are used every day is almost impossible even with very long sun hours. I am a newbie so I am hoping I am doing something wrong, but what I found is that once the batteries hit 80% and the cc switches to absorb the amperage charging the batteries starts to drop fairly quickly making my 640 watts of panels put out way less than what they will do when bulk charging, no matter where the sun is.

My original thinking was I might get 50amps to the batteries for 4 or so hours, but in reality the 50 amp of charge only occurs when the batteries are being charged in Bulk mode. I have seen the charge at 10 amps at solar noon in ABQ after watching 45 amps going into the battery at 10am.

Basically I have to run the genny in my RV to get them to 100%. I found that running it in the morning to bring them up to 80% screws up my Victron CC by making the starting voltage the cc see's higher and shortens the absorb charge to maybe an hour or two when it should have been on absorb for 6 hours. Still a work in progress for me. Hope your's works better, but with just 4 hours of sun I would like to hear how it works. From what I am seeing, adding more panels isn't as effective as I once thought it would be.

-Bill

BB. · November 2018

In my humble opinion (I am not a battery engineer), you do not need to reach 100% every day--And in fact, it is bad for lead acid batteries to do this. Basically "Equalizing/Gassing" FLA batteries every day is pretty hard on them (plate corrosion, plate erosion, loss of water, elevated temperature).

The goal should be >90% State of Charge at least once per week. As long as the batteries are cycling (say somewhere in the range of 50% to 80%) during the week, they will be fine.

Batteries sitting below 75% state of charge (day/days/weeks) uncycled--That is when the batteries begin to sulfate more quickly.

On battery mfg had suggested to one of our members that you only needed to get the battery >90% SoC once a month...

-Bill

Photowhit · November 2018

bigbillsd said:

... but what I found is that once the batteries hit 80% and the cc switches to absorb the amperage charging the batteries starts to drop fairly quickly making my 640 watts of panels put out way less than what they will do when bulk charging, no matter where the sun is....

This is NOT a function of the charge controller. It's a chemical reaction within the battery. The rising voltage allows the current to freely flow into the battery. When the battery reaches the absorb voltage, all the controller does is limit the voltage. If allowed to continue rising the battery would heat up and out gas tremendously and be damaged. The restriction of the reaction as a by product reduces the current flowing into the battery. Hence the battery is limiting the current flowing into it at that voltage.

Estragon · November 2018

IMHO, 6 hours is too long for absorb. With a grid connected charger, maybe a longer, lower voltage absorb would make some sense for some batteries. Off-grid charging with generator/solar and healthy (lead acid) batteries though, voltage should be high enough to get current down to 1-2% of capacity in a couple of hours.

If it's taking 6hrs, I'd wonder if the absorb voltage is too low (so it goes into absorb too early), or too high (batteries are full and current is heating the bank instead of charging it). Is charging voltage adjusted with a remote temperature sensor?

bigbillsd · November 2018

I wasn't trying to hijack this thread, just wanted to point out to the OP that solar charging batteries may not work as he was thinking. I will call Crown, my battery manufacturer to get their take on how often I should take my batteries to 100%. I had always heard our LA batteries want to be at 100% all the time... Yes, 6 hours might be too long, but 15 or 20 minutes seems way too short if it went for hours at Bulk then less than an hour in Absorb when it dropped to float..

mountainman · November 2018

I looked at pvwatts at 37° gives me 4.31 hrs in dec So back to my question. Bulk charging stage 1000 watts roughly 55 amps 416 amp hrs @12 volts from 50% soc to 80% with losses 135 amp hrs needed about 2.5 hrs. Leaving 1.8 hrs in absorb 80% to 90 % is around 42 amp hrs. Would 1.8 hrs get me to 90%?

Estragon · November 2018

Keep in mind 4.3hrs is an average full sun equivalent. Some days will be cloudy, with little/no sun, others will be longer. Also, once you get to absorb current drops off anyway, so even the low afternoon sun might be enough to keep absorbing. Bottom line is I think some days you should get to full, and others you won't.

bigbillsd · November 2018

@mountainman If I am reading your post correctly, you will be using 25% of your 416AH battery bank. You roughly have to replace 104 AH of power, probably more like 114+ AH. If it were all in bulk charging you could reasonably expect to accomplish that. But in reality you will probably only be in Bulk for an hour or so then the panel output will throttle down quite a bit once it hits absorb. My guess is you will not be able to get to 90% SOC in your 4 hours of sun from 75%. And from 50% SOC you will be very lucky to get to 80% in 4 hours. Also, when using 2 controllers you should check with the vendor to see how they will interact. Some do not play well with others.

mountainman · November 2018

@bigbillsd from 75%to 90% is only 15%. 62 amp hrs. If a 1000 watt array in 4 hrs wouldn't produce 62 amp hrs, then I need to spend the cash on a bigger fuel container for the generator.

Estragon · November 2018

My 1000w array would do close to the 15a needed even in light overcast conditions.

mountainman · November 2018

If I understand it correctly from 50 to 80 fla will accept what ever amps you put in. At 80% the battery starts accepting less amps at a constant voltage. Close to the end of absorb 3 to 5% roughtly on a 416 amp hour bank 2 or 3 hundred watts would surfice. no matter how large your array is the rest is not producing anything. After many hrs of research on line, rolls has a formula for absorb time on there batteries at 10% rate of charge. From 80 to 100% Time=0.42×416/41.6=4.2 hrs. bulk from 50 to 80% soc. 125 +10% 137 amp hrs needed ÷41.6=3.3 hrs. . So 7.5 hours from 50.to 100%. Conclusion lead acid takes more than 4.2 hrs to complete a full charge even with perfect sun from 75% soc.

Photowhit · November 2018

Give up and quit now... Just a suggestion.

Flooded lead acid will accept about 13% of it's capacity in bulk, (When 50-80% SOC) More in some batteries dependent on design and thickness of plates. I was told 15% with my forklift battery, I suspect Rolls high capacity batteries are made with similar thick plates.

Of course the array can and will produce more than is used for battery charging, that is the reason most people living off grid load shift to use energy when the sun is shining and the batteries don't need the extra... Also starting 'opportunity loads' in my case I run a water heater.

mountainman said:

Conclusion lead acid takes more than 4.2 hrs to complete a full charge even with perfect sun from 75% soc.

It's your conclusion, from 75% SOC using 1.5% capacity 'end amps' it takes less than that for me. As for 'perfect sun' since the batteries need much less current as they approach fully charged, they don't need 'perfect sun'. At 2% nearing the end of absorb, my 16 kWh battery is only taking in about 13 amps at 28.8 volts or 375 watts, my 4500 watt array doesn't need perfect sun to produce that.

Estragon · November 2018

To me, a 10% rate of charge is the bank capacity rating (20hr room temp) x 10%. Most FLA are okay at, and will take 15% or a bit higher rate (if available) at under 80% SOC, and are quite efficient in that range. This implies roughly 3hrs for 50-80% SOC at a 10% rate.

I consider my bank full when absorb current drops to a bit over 1%. This generally takes 2-3hrs. Total time from 50-100%SOC ~5-6hrs with good sun. From 75%, it would be roughly 3hrs.

4.2 hours on pvwatts is full sun equivalent hours. That would be 4200watts/sq.meter on average daily. Even full sun at noon may not be "full" (defined as 1000w/sq.m), but rather (eg) 500w/sq.m, so it would take two hours at that insolation to make one full sun equivalent hour.

BB. · November 2018

One reason I use rules of thumbs is they are "good enough" to design and run systems that will meet most people's needs (if you have low current drain 24x7, that is a different system that runs 5 hours in the evening, or runs high power during the day for irrigation, etc.).

Here is a nice article that shows the "complexity" of trying to understand a Lead Acid Battery:

http://www.scubaengineer.com/documents/lead_acid_battery_charging_graphs.pdf

People are (generally) very inconsistent about their use of energy... Some base load systems (like refrigerators) and more variable like tools, when to run the washing machine/pump water, kids inside playing on the computer/watching TV. Toss the variability in solar radiation (seasonal, time of day, weather, haze, growth of the trees next door, etc.)--And it is much easier to (conservatively) design your solar power system so that it will support most of your needs and you can use a genset (or stop using optional power) to carry you through when the sun isn't shining or you use more power than average.

-Bill

mountainman · November 2018

In my original post I had planned to use a 30 amp mppt cc with 400 watts.And 600 wattson 50 amp cc. but on second thought. Someone please correct me if I'm wrong. With 400 watts on cold days I could possibly see 27-30 amp peaks? 600 watts on 50 amp cc again on coldest 25f days 43-45 amppeaks 75 amps on a 12 volt 416 amp hour battery? Wouldn't it be better to run 2 30 amp controllers 500 on each. To limit charge amps in winter to a more reasonable 13 or 14% limit? My thoughts are in the heat of summer. 500 *.77=385/14.5=26.5 x 2 arrays 53 amps close to 13%. Any suggestions or criticism would be appreciated.

Estragon · November 2018

Personally, I'd run two 50a controllers. Most can limit output in settings.

IMHO it's better to not run controllers at near max capacity for long periods.

mountainman · December 2018

Well since i already have a 30 amp pwm and a 30 amp mppt, 4 100 watt panels and 4 6 volt 208 batterys this is what im trying. I bought a 265 and a 260 watt 60 cell panel. On mppt for a 12 volt bank (not sure if I should wire them in parallel or series?) I'm thinking 525*.77=405 watts /14.5 volts 29 amps+ 400 watts on pwm 22 amps. 51 amps total. Roughly a c/8 charge rate on a 416 amp hr @ 12 volts with a 925 watt array. If and when my controller crashes and burns I'll get a bigger one while limping on the other one.

OldMan · December 2018

All I'm reading here makes lithium look like a GREAT choice.

mountainman · December 2018

By cariboucoots calculations even on pwm 525/17.5=30 amps 400/17.5 =22.85 (52.85) close to 13%. What if I put 1 panel on 170°and 1 on 190° wouldn't that bring the peak down and give more over all production?

mountainman · December 2018

Lithium? Very often in winter we have 15f temperatures.

mike95490 · December 2018

mountainman said:

Lithium? Very often in winter we have 15f temperatures.

You are going to have to keep them indoors and heated. Below 41F and you need to start limiting charging amps. Below 32F you must stop charging.
https://batteryuniversity.com/index.php/learn/article/charging_at_high_and_low_temperatures

Li ion can be fast charged from 5°C to 45°C (41 to 113°F). Below 5°C, the charge current should be reduced, and no charging is permitted at freezing temperatures because of the reduced diffusion rates on the anode.

mountainman · December 2018

$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?

mountainman · December 2018

Is it possible to aim 2 panels in series in slightly different positions on a single mppt controller?

Estragon · December 2018

Generally speaking, you want to avoid putting different oriented arrays on a single mppt, as they can have different maximum power points much of the time.

OldMan · December 2018

mountainman said:

Lithium? Very often in winter we have 15f temperatures.

Well, I'm not limiting my options. Arizona in winter, the rest of the west spring summer and fall until Thanksgiving, then back to the desert. I mean, seriously, cold is morally reprehensible.

OldMan · December 2018

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.

BB. · December 2018

Ease up on the "mind reading"... Personally, whether or not I watch Fox News--I cannot, simply because I am too cheap to pay for cable.

Same thing with statements how/where/why folks store their batteries... Provide answers (temperature range for active batteries) and/or ask the questions.

There is no body language with forums, or instant one on one feedback for jokes and snark (and I love snark). Just be upfront and treat all visitors/members with honor and love.

Take care and happy holidays,

-Bill

mountainman · December 2018

Thanks all merry Christmas

Sizing solar watts to battery bank

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