Discussion of 12 volt / large AH system design

Cariboocoot
Cariboocoot Banned Posts: 17,615 ✭✭✭
in Off Grid Solar & Battery Systems #1
Note: I have split the discussion of Nu2solar's 12 volt / high AH solar PV system to its own thread. N2S wanted that thread to have people discuss/post pictures of their homes/systems and not a discussion of her and her husband's specific solar PV design choices. I have moved that discussion to here.

The original thread is:

Show Me Yours And I'll Show You Mine:)
nu2solar wrote: »
My husband and I are coming up on our 3 year anniversary of building our off grid berm home. We are now in the stages where we can tweak and hopefully perfect what we have done.

...

We have a 1620 watt 12 volt system and 1540 amp hours of batteries...we need to add more batteries. Right now we are producing more electric than we can use or store....this is not a bad thing, lol.

We have twelve 135 watt Kyocera 12 volt panels, two Outback 80 amp charge controllers, one Outback 12 volt inverter, fourteen 6 volt golf cart batteries and a back up Honda generator which is only used for pumping our well water into an underground storage tank. My husband did the installation. Our system works flawlessly.

When my husband designed our system he wanted to have 2 redundant systems so that if something happened to one we would still have power from the other. We also went with two larger charge controllers for future upgrading which we are now in the process of doing....

-Bill B. Moderator

That looks very nice indeed! I know someone else around here who will find that interesting and probably want to ask you a few thousand questions. :p

I don't want to be Mr. Negative but ... you've already got way too many batteries for the amount of solar you have. 3080 Amp hours? And on a 12 Volt system that's really not a recommended configuration. If you really need that kind of power (12 kW hours) you should be thinking about upping the system Voltage.

The system works flawlessly now, but if you're really trying to keep that much battery charged off that little panel the battery life is going to be much shorter than what you could expect.

I'm putting caveats in this because fourteen 6 Volt golf cart batteries in a 12 Volt configuration would be 1575 Amp hours by my reconing, not 3080. Still, seven parallel 12 Volt banks can be problematic for keeping current distribution even; you could be losing batteries without noticing - until it's too late.

Boy, I sound like gloom-n-doom! :roll: :p
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  • nu2solar
    nu2solar Solar Expert Posts: 28 ✭
    #2
    Re: Show Me Yours And I'll Show You Mine:)
    icarus wrote: »
    Welcome to the forum, looks like a nice set up, nice area.

    (Nice Sprinter too!)

    Tony

    Thank you.
    Cariboocoot wrote: »
    That looks very nice indeed! I know someone else around here who will find that interesting and probably want to ask you a few thousand questions. :p

    I don't want to be Mr. Negative but ... you've already got way too many batteries for the amount of solar you have. 3080 Amp hours? And on a 12 Volt system that's really not a recommended configuration. If you really need that kind of power (12 kW hours) you should be thinking about upping the system Voltage.

    The system works flawlessly now, but if you're really trying to keep that much battery charged off that little panel the battery life is going to be much shorter than what you could expect.

    I'm putting caveats in this because fourteen 6 Volt golf cart batteries in a 12 Volt configuration would be 1575 Amp hours by my reconing, not 3080. Still, seven parallel 12 Volt banks can be problematic for keeping current distribution even; you could be losing batteries without noticing - until it's too late.

    Boy, I sound like gloom-n-doom! :roll: :p

    Thank you for the correction. I did change it for people to see the correct numbers.

    My husband is the one who plans the system changes. I don't get all of it but I used this sample to see that we need a lot more batteries. Do you see a flaw in these numbers?

    http://www.freesunpower.com/example_systems.php

    Thanks
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
    Re: Show Me Yours And I'll Show You Mine:)

    I can see two problems with that web site without even looking at it:

    1) "free" sun power? Not if you understand you have to buy all the equipment. About $1 per kW hour over the typical lifespan of a system. Not anyplace near "free", nor even competitive with utility power.

    2) If you got your battery bank sized from that and came up with 1540 Amp hours @ 12 Volts they're either handing out bad advice or aren't clear in their explanations.

    You size a system based on expected usage. How many Watts maximum at once? How many Watt hours per day? Those numbers will determine what system Voltage to use, what size inverter to get, and how big a battery bank you need. The battery bank size is the basis for what you need in solar panels and the controller(s) to handle it.

    Personally I wouldn't go for a 12 Volt system larger than 1 to 2 kW of inverter and 400 to 500 Amp hours of battery. When you get into the upper end of that configuration its definitely time to think about going to 24 Volts.

    NOTE: I use a Linux netbook which does not work well with some web sites, including the one linked to in the previous post.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #4
    Re: Show Me Yours And I'll Show You Mine:)

    welcome back to the forum as it has been awhile. i'm glad it worked out well for you.

    i have to ask what happened to the wind turbines you had? did they fail you?

    also, what future controller are you referring to?

    i have to admit the last time i visited your running thread on going off grid in east tennessee was around january or so and there hasn't been too much activity on the thread. forgive me if i've overlooked what may have already been written on that thread and tell mike, good job and that goes for you too.
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  • nu2solar
    nu2solar Solar Expert Posts: 28 ✭
    #5
    Re: Show Me Yours And I'll Show You Mine:)
    Cariboocoot wrote: »
    Personally I wouldn't go for a 12 Volt system larger than 1 to 2 kW of inverter and 400 to 500 Amp hours of battery. When you get into the upper end of that configuration its definitely time to think about going to 24 Volts.

    Why?


    Thank you for the other input.
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  • nu2solar
    nu2solar Solar Expert Posts: 28 ✭
    #6
    Re: Show Me Yours And I'll Show You Mine:)
    niel wrote: »
    welcome back to the forum as it has been awhile. i'm glad it worked out well for you.

    i have to ask what happened to the wind turbines you had? did they fail you?

    also, what future controller are you referring to?

    i have to admit the last time i visited your running thread on going off grid in east tennessee was around january or so and there hasn't been too much activity on the thread. forgive me if i've overlooked what may have already been written on that thread and tell mike, good job and that goes for you too.

    Thanks.

    Funny you should ask about the wind turbines. The first year they did supplement our system but not by much. The second year we had hardly any wind....so we sold them. Now this year the wind is back and we have no wind turbines, lol. We like the solar panels better. Less maintenance and upkeep. We like things simple.

    Mike installed 2 M-80 charge controllers for future possible system expansion.

    There isn't much activity on our thread in the winter but it is up and running now. Just at a slower pace...we should be done by the end of this year...finally.
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #7
    Re: Show Me Yours And I'll Show You Mine:)

    Higher system voltages have a number of advantages. The first is that you reduce wiring loses as the voltage gets higher, and it allows you to use smaller wire for the same amount of loss. Second, for any given wattage load, the current (amps) would be cut in half if you double the voltage, which would allow the batteries to produce that current easier.

    Perhaps the biggest reason however is that it would reduce parallel strings of batteries, making battery management better and more efficient. Same number of batteries, same net KWH capacity, but more efficient.

    T.
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  • nu2solar
    nu2solar Solar Expert Posts: 28 ✭
    #8
    Re: Show Me Yours And I'll Show You Mine:)
    icarus wrote: »
    Higher system voltages have a number of advantages. The first is that you reduce wiring loses as the voltage gets higher, and it allows you to use smaller wire for the same amount of loss. Second, for any given wattage load, the current (amps) would be cut in half if you double the voltage, which would allow the batteries to produce that current easier.

    Perhaps the biggest reason however is that it would reduce parallel strings of batteries, making battery management better and more efficient. Same number of batteries, same net KWH capacity, but more efficient.

    T.

    OK...I agree with the first part.;)

    On the second part do you mean, You would prefer to run your batteries in series rather than in parallel? And why is it more efficient?

    Thanks
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #9
    Re: Show Me Yours And I'll Show You Mine:)
    nu2solar wrote: »
    OK...I agree with the first part.;)

    On the second part do you mean, You would prefer to run your batteries in series rather than in parallel? And why is it more efficient?

    Thanks

    Aside from the Voltage loss reasons Tony mentioned, parallel banks make current distribution more difficult. There is every possibility that with seven sets in parallel most of the current will be going in and/or out of some but not all, or at least not equally.

    This means that some of the batteries will be discharged 50% while others only 10%, and likewise with the recharging. So some will be worked more and others less, both conditions leading to premature battery death. You won't even notice this until it's too late.

    Whereas in a higher Voltage series configuration there is only one path for the current, and that is evenly through all batteries. This helps keep them in the same state of charge and condition.

    That's quite a lot of power potential you have there, btw.
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  • nu2solar
    nu2solar Solar Expert Posts: 28 ✭
    #10
    Re: Show Me Yours And I'll Show You Mine:)
    Cariboocoot wrote: »
    Aside from the Voltage loss reasons Tony mentioned, parallel banks make current distribution more difficult. There is every possibility that with seven sets in parallel most of the current will be going in and/or out of some but not all, or at least not equally.

    This means that some of the batteries will be discharged 50% while others only 10%, and likewise with the recharging. So some will be worked more and others less, both conditions leading to premature battery death. You won't even notice this until it's too late.

    Whereas in a higher Voltage series configuration there is only one path for the current, and that is evenly through all batteries. This helps keep them in the same state of charge and condition.

    That's quite a lot of power potential you have there, btw.

    You would be correct if our batteries were running in parallel and the positive and negative were taken off of one end only. But our battery set up takes the positive off one end and the negative off the other. Also our 2nd charge controller charges the middle of the bank positive and negative. This system works excellent because upon testing the batteries they are all at a full state of charge after 3 years so far.

    This is from my hubby. You guys already lost me, lol.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #11
    Re: Show Me Yours And I'll Show You Mine:)

    Yes; diagonal wiring.
    It's really only good for two parallel banks, though.

    The concept of attaching a second charge controller in the middle somewhere ... My personal opinion would be "oog". I wouldn't do it.

    With that much battery I wouldn't expect to see any ill effects after only three years, no matter how the connections were aligned. In fact if you're using a small enough percentage of it you may not see any symptoms until that fateful day when battery capacity sinks below usage. Even SG readings aren't sure evidence of everything about the internal state of the battery.

    But then it's not my money, so everyone is free to ignore my advice as they see fit. :roll:
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  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    #12
    Re: Show Me Yours And I'll Show You Mine:)
    nu2solar wrote: »
    You would be correct if our batteries were running in parallel and the positive and negative were taken off of one end only.

    Something is lost in the terms here, if you have one battery bank of 12volts 1500amphours... then the batteries must be run 2 in series with 7 runs(in parallel) of 12 volts each
    nu2solar wrote: »
    But our battery set up takes the positive off one end and the negative off the other. Also our 2nd charge controller charges the middle of the bank positive and negative.

    If the batteries are in a single bank, regaurdless of where you put the leads it charges the whole bank, you may in fact have seperate banks for the redundancy but you would then have 2 different battery banks.

    I'm glad it's worked out well so far, the problem will not be apparent when fully charged, if you have a string of cloudy days and run your battery bank down to 70-80% then disconnect them all from each other for a couple hours, my bet is you'll see voltage differences between the strings of batteries, specific gravity readings would confirm this, but the voltage check likely would be easy to do. It starts off as a slightly more resistance in a string than another, one string contibutes more and it tries to raise the other battery, over time the differences grow, when measured as a group, your not checking individual strings, and the strong bring up the weaker, so you have to seperate them and let the strings settle to see the differences, the reason for the couple hours.

    As the the costs "...nor even competitive with utility power" There are a lot of ways you can be competitive with grid power, I am, with the grid running in front of my house, likely you had a long run to get to your hill top?

    I'm off to check out your blog/web site again for the up date, Looks very nice and much neater than I could ever keep a place, I how to post something on growing a photovoltaic system ( just can't say solar system...lol) w/some photos, and perhaps a new cost projection.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #13
    Re: Show Me Yours And I'll Show You Mine:)

    Post Scriptum:

    Sorry I rather ruined this thread, everyone. :blush: I'm rather horribly cynical and pessimistic at times.

    I really do like the house, though. :D
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #14
    Re: Show Me Yours And I'll Show You Mine:)

    It is difficult to notice a bad cell (open or shorted) or bad connection in a bunch of parallel connected batteries. I like to recommend using a DC Clamp Current Meter (such as this inexpensive one) so you can check for balanced current flow in each battery string (under load, under charge) to make sure everything is OK.

    Also, when you have lots of parallel batteries--if flooded cell--you will have lots of cells to check for electrolyte levels (more maintenance).

    And, you should have a large fuse for each parallel battery string to protect against short circuits (i.e., a 100-300 amp fuse per string)... Adding the extra hardware gets expensive too.

    So, my personal recommendation--I like larger AH cells all in series (raise from 12 to 24 or 48 volt bank to increase power).

    However, if you have to move batteries by hand (no crane/fork lift access)--you will probably have to parallel for larger banks... And when this happens--I like to recommend a maximum of three parallel strings. It gets difficult to have good current sharing with more parallel strings (plus more maintenance and expenses).

    Lastly, regarding charge controllers--The are rated at 60 amps maximum output (example) at 12, 24, or 48 volts... Basically, if you run the controller at 48 volts, you can manage 4x the solar array vs a 12 volt battery bank (1 controller does the job of 4--save money):
    • 60 amps * 14.5 volts * 1/0.77 derating = 1,130 watt array
    • 60 amps * 58.0 volts * 1/0.77 derating = 4,519 watt array
    However--that being said, there are certainly people with 4 or more parallel battery banks that are perfectly happy with them.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    #15
    Re: Show Me Yours And I'll Show You Mine:)
    nu2solar wrote: »
    We have a 1620 watt 12 volt system and 1540 amp hours of batteries...we need to add more batteries. Right now we are producing more electric than we can use or store....this is not a bad thing, lol.

    Personally I would use what you have for now, if you don't need more don't try to store more. I would wait until your batteries are ready to retire regaurdless, I would not add new batteries to 3yr old ones. Particularly if your not taxing the ones you have, perhaps they'll last a bit longer.

    I'm currently finally at the stage to replace my 5 1/2 year old golf cart batteries, just a single string of 4, in a 24volt string and they get abused over the summer when I run a window AC off of them, I've grown out my array while I was waiting, and currently have 1300 watts hooked up and can add the other 700 watts on cloudy days if needed.

    I'll get a real test to see if there is any 'umph' left in the batteries as temps here this week will be in the 90's the next couple days.

    I too like to have single strings of batteries and will likely purchase a 24volt fork lift battery. NAWS has some limited selection, and I'd be glad to PM you a link to a distributer if you like for a greater selection.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #16
    Re: Discussion of 12 volt / large AH system design

    I have moved the discussion of Nu2solar's system design to its own thread.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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