Battery selection for large off grid system

[Joe Silver]

OK guys, I lurked around the site about a year ago, and was helped by the information/ideas that are here. This last year I got away from the solar and haven't thought much about it. I think solar is fantastic, but I like my hassle free, grid power.

Saying that, about a year ago I was able to purchase a system (new), a bit cheaper than normal, so decided to get it. In about a year from now, I plan on installing it in a off grid location in Central America. If grid power was available, I would not mess with the solar (at least at this time).

I am looking at sizing my batteries, and this is where I could use some help. I know the question is often asked how much power do you need. For me, the answer is that I will not ever have as much as I would like.

Question number one for me is what battery bank size and type (lead acid, lithium, 2volt--high amp, forklift, etc.) will give me a well balanced power storage for the amount of panels that are pushing power to them?

The house will have the normal stuff; led lights, gas water heating, gas cooking and gas clothes drying, an electric frig and freezer and washing machine. A few computers, ceiling fans, and would really like a whole house exhaust fan (not attic type, the kind that sucks outdoor air in the windows and to the attic--about 6 amps). The things I wonder about is what else can I power? Can I put in a split A/C with a high seer rating? If so, can I put in 2 or 3?

I'm jumping ahead now, let say that I purchase a battery bank and design it with 5 days of autonomy.
Solar isolation of 4.5
54 of a 280 watt panel (15,120 watts)
Feeding through 3 Midnight 150v charge controllers.
Triple Magnum MS4448PAE powering the loads

Yes, I know the battery bank is going to be expensive. I just do not have the experience to know in my "gut", what is the best way to approach the design. Any help or suggestions are appreciated.

Joe

[Wy_White_Wolf]

Figure the amp rating that the array will push to the batteries. FLA (your only real choice) need to charge between C/8 and C/12. So your array at 48Vs:

15120W/48V= 315A

315*8=2520AH
315*12=3780AH

You battery bank should be between 2520AH and 3780AH at 48V.

A single string of batteries is preferred so I think you will be limited to 2V cells.

WWW

[jflorey2]

Question number one for me is what battery bank size and type (lead acid, lithium, 2volt--high amp, forklift, etc.) will give me a well balanced power storage for the amount of panels that are pushing power to them?

All of those will work.

Question - how long do you want them to last? That question, more than any other, will determine LiFePO4 vs lead acid.

[SunEagle]

That is huge system. Even a battery bank rated in the middle of that 2520 & 3780Ah, lets say 3000Ah at 48V can safely produce 36kWh a day without being discharged more than 25%.

A system that size might cost close to $100k depending on what you get those 54 x 280w panels for.

The problem I see is you will need about 300 charging amps and those 3 Midnights will only get you about 250 amps so you might have a little too much panel wattage unless you point some East and West to limit the load on the CC's. They will also limit your battery bank to the lower number of 2520Ah which is still a big system.

[Sunking]

FLA last longer than Lithium and less expensive.

[NEOH]

Joe Silver,
You really need to do a Google Search of the yearly KWh usage for your appliances and then divide by 365 to get Average Daily KWh usage.
Or just multiply the wattage by the number of hours used per day.
YOU need to do this.
Without a daily loads calculation you have no clue if you have 5 days autonomy or 1 day autonomy.
Your answer of "... I will always want more ..." is not an acceptable answer.
You cannot design anything given "I want more".

Your appliances might consume a large portion of the Solar Power that you generate Daily.
Your appliances might consume ALL of the PV Power generated in the morning hours and evening hours.
You may only have 4 to 5 hours, near Solar Noon, to recharge the Battery Bank.

Do you plan on turning OFF every appliance in your house (especially the A/C) when the batteries need recharged?
If the A/C is ON then you will NOT have 15,120 watts x the 4.5 Sun Equiv Hours to recharge your batteries.

Here are two examples, but you have no clue what you need for "5 days of autonomy"

Trojan 1,849 AH @ 2 Volt battery for $900 each = $24,000 for 48 Volt Bank.

Rolls 2,430 AH @ 2 Volts battery for $1,200 each = $28,000 for 48 Volt Bank

And you have to factor in MAX 50% DOD.
Prefer not to parallel banks.

[Joe Silver]

Thank you all for taking the time to post that information. I will spend a day digesting some of your responses. I did have a few immediate thoughts that I hope to use as learning points for myself.

Figure the amp rating that the array will push to the batteries. FLA (your only real choice) need to charge between C/8 and C/12. So your array at 48Vs:

15120W/48V= 315A

315*8=2520AH
315*12=3780AH

You battery bank should be between 2520AH and 3780AH at 48V.

A single string of batteries is preferred so I think you will be limited to 2V cells.

WWW

If I am not mistaken, that assumes that I am not using any of the power as it is produced. Don't get me wrong, I am to blame for giving incomplete information, but lets assume I am diligent and use my heavy loads when the sun is shining. I then might have a battery bank that has 1500AH. I think you will tell me that does me no good for the 5 rainy/cloudy days in a row that I will experience. If so, I agree, and at that point no attic fan goes on and the split a/c units are certainly not used. Is that good logic, or poor logic? Now that I am experiencing a few rainy days and I turn on that expensive generator. Another question is how large should it be in relationship to the battery bank? Not just for charging the batteries during rainy periods, but for equalization, etc? I do not have a good understanding of that.

Thanks,

Joe

[Joe Silver]

All of those will work.

Question - how long do you want them to last? That question, more than any other, will determine LiFePO4 vs lead acid.

This is something I don't have a good handle on. I hear about rapid advances in battery technology and wonder if I should just try to get by cheaply for a few years and then purchase that great battery (Lithium or whatever), that surely will be available shortly. It reminds me of a time 25-30 years ago, I thought it might not be too long before many of us were flying our helicopters to work.
Also, even though self driving technology is almost here, things always take longer than they should. Maybe I am wrong, but I wonder if most of us will still be driving our own cars 25 years from now. So if a battery were to come out in 3 years that was twice as expensive but lasted 10 times longer, then I would jump on it. Anyone know what the future holds? Right now I figure what Sunking said fits me well, "FLA last longer then Lithium and is less expensive". Without information contrary to that, I will likley go with some big ol 2V FLA batteries. Probably the Rolls that NEOH mentioned (Rolls 2,430 AH @ 2 Volts battery for $1,200 each = $28,000 for 48 Volt Bank). Makes me appreciate the grid!

[Joe Silver]

Joe Silver,
You really need to do a Google Search of the yearly KWh usage for your appliances and then divide by 365 to get Average Daily KWh usage.
Or just multiply the wattage by the number of hours used per day.
YOU need to do this.
Without a daily loads calculation you have no clue if you have 5 days autonomy or 1 day autonomy.
Your answer of "... I will always want more ..." is not an acceptable answer.
You cannot design anything given "I want more".

Your appliances might consume a large portion of the Solar Power that you generate Daily.
Your appliances might consume ALL of the PV Power generated in the morning hours and evening hours.
You may only have 4 to 5 hours, near Solar Noon, to recharge the Battery Bank.

Do you plan on turning OFF every appliance in your house (especially the A/C) when the batteries need recharged?
If the A/C is ON then you will NOT have 15,120 watts x the 4.5 Sun Equiv Hours to recharge your batteries.

Here are two examples, but you have no clue what you need for "5 days of autonomy"

Trojan 1,849 AH @ 2 Volt battery for $900 each = $24,000 for 48 Volt Bank.

Rolls 2,430 AH @ 2 Volts battery for $1,200 each = $28,000 for 48 Volt Bank

And you have to factor in MAX 50% DOD.
Prefer not to parallel banks.

I don't disagree with you about not being able to accurately design the system with "I want more". I am just wanting to get your feedback as to how you all think about these things. I have never had an off grid system, and am used to living on 2000kwh per month. I like that. I hear the national average is 900kwh in a month (maybe that has changed). My real goal is to find the maximum amount of amp hours that the battery bank should be sized at based on the existing panels and equipment. I will adjust the loads accordingly. I am not sure how to accurately predict loads anyways as life has many unexpected turns. (eg. wife says she it too hot, mother-in-law moves in and won't turn off the ceiling fan, etc). I don't have an endless pot of money, but the pot is not empty either. I would like to not waste dollars in finishing this system and I realize you all have more "back of the napkin" expertise, compared to me figuring and refiguring again.

So thank you, and I will re-read and consider all of your inputs!

Joe

[AzRoute66]

Absolutely refreshing to see someone besides myself working from the supply side. It seems crazy to some people to adjust your living to conform with the resource that is available when it comes to solar power although those same people tout major lifestyle changes and huge investments to lower an electric bill a few bucks a month. I don't have enough expertise to even beak at the answers you're looking for, but will remind you that there is a maintenance surcharge on the FLA batteries and you will have a lot of caps to pull. Here's to the campfire coffee percolator, should it come to that.

EDIT: Again, I don't know, but for some reason Central America seems a good place to seriously consider SunEagles suggestion to orient some east and some west although it might also be one of the best places for a totally flat install. Even here in southern AZ, my optimal tilt angle in the summer is actually 3 degrees to the north to catch the morning and evening sun.

[NEOH]

I don't disagree with you about not being able to accurately design the system with "I want more". I am just wanting to get your feedback as to how you all think about these things. I have never had an off grid system, and am used to living on 2000kwh per month. I like that. I hear the national average is 900kwh in a month (maybe that has changed). My real goal is to find the maximum amount of amp hours that the battery bank should be sized at based on the existing panels and equipment. I will adjust the loads accordingly. I am not sure how to accurately predict loads anyways as life has many unexpected turns. (eg. wife says she it too hot, mother-in-law moves in and won't turn off the ceiling fan, etc). I don't have an endless pot of money, but the pot is not empty either. I would like to not waste dollars in finishing this system and I realize you all have more "back of the napkin" expertise, compared to me figuring and refiguring again.
Joe

Joe Silver,
2,000 KWH Gross PV production per month = 15,120 PV Watts x 4.5 Sun Equiv Hours x 30 Days
I don't see much a of a lifestyle change here?
There is your "Supply Side" answer.

I will now repeat what I wrote in previous message ...
YOU really need to do a Google Search of the yearly KWh usage for your appliances and then divide by 365 to get Average Daily KWh usage.
Surely you can do this.
Simply ADD up all of the Average Daily KWH's used per appliance that you plan to use ( disregard mother-in-law issues ).
You should do this before replying about mother-in-law, wife, endless-pot-money ( blah-blah-blah ) issues.

You need to understand that your appliances ( A/C ) could consume +/- 50% of the PV Power generated during the 12 hours of daylight.
Therefore, you do not have ALL of the PV Power available to recharge your batteries.
And that is why the AVG Daily KWH for House Loads number is required.
You cannot size the Battery Bank based solely upon MAX / GROSS Daily PV Wattage, without knowing the House Loads.
The NET amount of PV KWH available to recharge the Battery Bank is = ( PV KWH generated - House Loads )

My real goal is to find the maximum amount of amp hours that the battery bank should be sized at based on the existing panels and equipment.
I will adjust the loads accordingly.
Joe

That answer was already given to you in message #2.
And it assumed that you would adjust your House Loads to 0 KWH from sunrise to sunset, to allow the PV Panels to recharge the batteries.
Is that a reasonable / livable "Supply Side" solution for you?

[Wy_White_Wolf]

Thank you all for taking the time to post that information. I will spend a day digesting some of your responses. I did have a few immediate thoughts that I hope to use as learning points for myself.




If I am not mistaken, that assumes that I am not using any of the power as it is produced. Don't get me wrong, I am to blame for giving incomplete information, but lets assume I am diligent and use my heavy loads when the sun is shining. I then might have a battery bank that has 1500AH. I think you will tell me that does me no good for the 5 rainy/cloudy days in a row that I will experience. If so, I agree, and at that point no attic fan goes on and the split a/c units are certainly not used. Is that good logic, or poor logic? Now that I am experiencing a few rainy days and I turn on that expensive generator. Another question is how large should it be in relationship to the battery bank? Not just for charging the batteries during rainy periods, but for equalization, etc? I do not have a good understanding of that.

Thanks,

Joe

So what happens on the days that you are not diligent on watching the loads and array output? Sitting there all day every day watching loads and output is about as much fun as watching chrome rust. I have better things to do.

What happens on the day you have to go to town and it's cloudy in the morning so you don't turn the loads on, then before you get home the sun comes out? That bank is not going to like a C'4 charge rate.

How about the opposite when you have to leave and it's sunny so you have the loads on and then it turns cloudy running your battery below the 50% level?

Doing either of those once a week will destroy you battery bank within a year.

As for the 5 days autonomy, I already figure you have zero with the "I'll always want more" attitude.

[Sunking]

EDIT: Again, I don't know, but for some reason Central America seems a good place to seriously consider SunEagles suggestion to orient some east and some west although it might also be one of the best places for a totally flat install. Even here in southern AZ, my optimal tilt angle in the summer is actually 3 degrees to the north to catch the morning and evening sun.

Well I live in Panama which is about as much Central America as you can get. Panels here are pretty much pointed straight up with some tilt to the north to keep rain water off. Now while you might think Central America is a great place for solar might I remind you its a Tropical Jungle down here which brings on two challenges. Your home is surrounded by Jungle and you get rain and clouds almost every day. We get some awesome rainbows.

[Sunking]

Yes, I know the battery bank is going to be expensive. I just do not have the experience to know in my "gut", what is the best way to approach the design. Any help or suggestions are appreciated.

You have no idea Joe what you are asking for. Example you will have a new full time job with no days off or vacation watching and protecting your investment Once of twice a month you will spend a day cleaning and watering your batterries. Once a week measuring specific gravity every 15 minutes on every cell while you charge them up on your very large thirsty generator.

Then in about 5 years the big expensive job of replacing those batteries. You are talking about 9000 pounds of batteries costing around some $20,000. Are you crazy rich?

[ButchDeal]

Even here in southern AZ, my optimal tilt angle in the summer is actually 3 degrees to the north to catch the morning and evening sun.

UM, where did you get that?

I did a little simulation in Nogalas AZ with 3 degree north vs 3 degree south. and using one 300W module each.
It gets really close and in June you get 300watts more with the north but for the summer (June to September) south is better by 3kWh

Month	Energy Production [kWh] North	Energy Production [kWh] South
Jan	23.04	27.18
Feb	27.33	30.48
Mar	39.73	42.23
Apr	49.39	50.85
May	55.72	56.02
Jun	55.59	55.29
Jul	49.01	49.09
Aug	46.36	47.33
Sep	40.34	42.51
Oct	34.68	38.29
Nov	25.74	30.13
Dec	21.6	25.95
Total	468.53	495.35
Summer	191.3	194.22