Separate battery banks

[scheek]

Is it possible to run 2 separate battery banks for an off grid system? One bank feeding only the refrigerator and the other for lights, fans, etc. My though was to be able to monitor battery needs per bank. Too, if one bank develops a bad battery and drains the system the refrigerator will be protected or vice versus.

The next question is if this can be done, can I have my backup generator charge just the bank that needs it automatically.

Thanks,

[billvon]

Is it possible to run 2 separate battery banks for an off grid system?

Yes, but it will invariably be more costly, less reliable and give you less overall energy than a similarly sized single battery system.

One bank feeding only the refrigerator and the other for lights, fans, etc. My though was to be able to monitor battery needs per bank. Too, if one bank develops a bad battery and drains the system the refrigerator will be protected or vice versus.

Well, but with twice the batteries/cables/protection etc you've doubled the number of potential failure points, making it less reliable overall - so your odds of losing power to your refrigerator remain the same. (Actually they go up a bit during times of limited generation since you have only half the battery capacity.)

The next question is if this can be done, can I have my backup generator charge just the bank that needs it automatically.

That's also doable - and also a bad idea. Your generator will run twice as often at half the load, thus reducing its lifetime, reducing your fuel economy and increasing the time your system makes noise.

[scheek]

Thanks. Always appreciate your help. I know you have a lot of experience with solar systems. I've read many of your post.

I ran the diy solar system configuration needs and it shows I need 17/ 100 ah batteries and 7 /100 watt panels. What is the best way to go. A more ah battery or stay smaller? 6 volt in series or 12 volt parallel?

Thanks

[Sunking]

You can but a bad idea. Instead use 2 or more battery inverters on a common battery bank to match the load.

[inetdog]

Thanks. Always appreciate your help. I know you have a lot of experience with solar systems. I've read many of your post.

I ran the diy solar system configuration needs and it shows I need 17/ 100 ah batteries and 7 /100 watt panels. What is the best way to go. A more ah battery or stay smaller? 6 volt in series or 12 volt parallel?

Thanks

If the configuration guide comes up with 17 100AH batteries as your energy storage requirement and also gives 7 100 watt panels, it has underestimated the panel requirement by a large amount or is assuming you will be using the generator a lot. If you are using 12 volt FLA batteries, the rule of thumb for minimum available charging current is C/20, which would be 85 amps. Each 100 watt panel will at the very best give you 8.3 amps to your battery bank. That would give a requirement of 10+ panels. And a preferred configuration would be C/10, which would require closer to 2000 watts of panel.

But the bigger issue, IMHO, is that 1700AH at 12 volts is a poor way to go. For that power requirement you should be using a 24 or 48 volt battery bank, That would let you use a smaller number of batteries, of more reasonable size, without having to put batteries in parallel.

1600 AH at 12 volts would correspond to 800 AH at 24 volts or 400 AH at 48 volts. The latter could be done with 8 six-volt 400 AH batteries in series, or a corresponding number of 2, 4 or 8 volt batteries, depending on how you choose to design the system. But the charging requirement issue above remains in any case.

[scheek]

What do you suggest for a nearly 700 watts of panels array? number/ah/volt/ batteries in series?

At the first rate ( 17 /12v batteries parallel) the guide said my daily use was 3500 watt hours. The above according to you would be about half this, right?

Thanks,

[Sunking]

What do you suggest for a nearly 700 watts of panels array? number/ah/volt/ batteries in series?

At the first rate ( 17 /12v batteries parallel) the guide said my daily use was 3500 watt hours. The above according to you would be about half this, right?

What is your location? There is no way a 700 watt panel array can support a 12 volt 1700 AH battery bank, nor can the panels generate 3500 watt hours of useable power year round. There are a lot of things wrong with your system.

Let's start with 700 watt panels. If they are standard 12 volt panels with a PWM controller, the panels can only support a 12 volt 400 to 450 AH battery. If the panels are used with a MPPT controller they can support up to 550 AH battery. But there is now way they can support 1700 AH @ 12 volts.

As for the batteries you have on paper can support up to 2500 watt hours per day usage assuming you can generate that much power, but that is not going to happen with 700 watts of panels. Last note on the batteries if you use them are forced to wire all of them in parallel. If you do that they will be destroyed in a year or two at best.

[scheek]

Location: Mississippi. This is for a weekend cabin by the way. I know I will have to have alternative power source such as a small generator to off-set the power.

Wow! The system guide as <http://www.freesunpower.com/system_sizing.php> is really off then.

My friend uses about 550 watts of panels with a generator that works very little during the month. I do know his batteries are in series. I'm not sure as of the AH they are. They are big batteries though. The tall ones. I will have to ask him what they are. i would really like to have a bit bigger system if possible but not to go over board. AS I said weekends will be about it for now and possibly longer stays when i retire (about 4 more years). I want to be able to add to the system too if need be. I'm about 50% on cabin now and really need to figure this out within the next few weeks.

Thanks for your advice,

[scheek]

Where I'm I not seeing something.

Approx 3000 watt hours used per day or about 25 amps 120 volt = to 250 DC amps that I have to replace.
Charge a day from a 100 PV is a about 8.3 x 4 hours = roughly 32 amps capacity each

1500 AH bank with a loss of 250 amps or 16% daily discharge that I have to regain.

250/32 = about 8/100 watt panels.

This computes out to what the estimate guide suggest.

Thanks

[Sunking]

Where I'm I not seeing something.

Approx 3000 watt hours used per day or about 25 amps 120 volt = to 250 DC amps that I have to replace.
Charge a day from a 100 PV is a about 8.3 x 4 hours = roughly 32 amps capacity each

1500 AH bank with a loss of 250 amps or 16% daily discharge that I have to regain.

250/32 = about 8/100 watt panels.

This computes out to what the estimate guide suggest.

That is because your guide is all wrong. Read this thread to get you on the right track. Here is the data you will need for your area to find the Sun Hours in the calculations.

1st mistake you made and it is a huge mistake using 12 volts. For 3000 wh/day you need to be using 24 or 48 volts. Also never parallel batteries. For 3000 wh day the battery capacity is:

625 AH @ 24 volts
310 AH @ 48 volts

Panel wattage depends on what type of charge controller you use. For your location the December/January Sun Hours is about 3.5 Sun Hours:

For PWM Panel wattage need = [3000 wh x 2] / 3.5 h = 1714 watts, can round down to 1700 watts. Note it would be foolish to use PWM charger at this power level.
For MPPT Panel wattage = [3000 wh x 1.5] / 3.5 h = 1285 watts, round down in your case to 1200 watts.

Charge controller PWM = MPPT = 60 amps @ 24 battery or 30 amps @ 48 battery

Warning buy 6, 4, or 2 volt batteries at the specified AH rating as to make a single battery string. Or else you will be very sorry.

[Naptown]

And here is a calculator you can use to make things easier for you that is based in reality and on Sunking's advice as stated above
Off grid calculator excel 97 version.zip

[scheek]

My friend is using 8/6 volt gel batteries in series at 48 volts. He has 4/125 watt pv run in series (48) with a Magna Sine inverter. Before he got his panels he was just using his generator to charge batteries. He said he avg. about 440 hours a year on his generator. Now that he has panels he runs about 50 hours a year.

They do not use the cabin in July /August because of the heat. They are really conservative. No AC/ even though they have 8 KW backup generator. I may follow his example.

His panels are probably 250 feet from his inverter. However, mine will be less than 50 feet. Housed in a separate building.

I would like to use a cheaper inverter than he used. His was about $3000 for 4000 WATT PSW. I see an Aims 24 volt PSW/5000watt for about $1309 and I see a Aims 3000 PSW for about $799. I'm not sure which to consider. Too, I wonder if I can stack them?

Looking again at the battery bank. What ah batteries should I use for the 625 ah bank at 24 volt. Are we talking 6 /260 ah in series?

[Naptown]

At that AH rating you are probably looking at 4 or 2 volt batteries in series and it's gonna be expensive
At 48V you would need 1/2 the AH rating on the batteries or 312 AH each.
That is assuming you do it right and put the batteries in series and not parallel.

Did you run the calculator in my post above? About the only things you need to know are what your loads are and where and when the system will be used.

[scheek]

I assume you mean 4 pair/ -6 volt batteries? With a total of 312 ah. That would be 320 ah batteries. 8 of them. correct?

[scheek]

At 48 volt, do you think (8 /6volt in series) at 312 ah or about will do the job?

Yes. I did the calculator and that was very interesting. Since my demand is about half of Tuscon's where they needed 6000 wh, I need only 3000 or about. So I need about 8/100 or 9/100 watt panels. correct?

This will throw me into the 80 amp mppt charger too. correct?