question about batteries and most everything

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bcolebuild
bcolebuild Registered Users Posts: 10 ✭
in Solar Beginners Corner #1
So, im in taos and putting together my first solar system, and i thank you in advance for taking time to answer my beginner questions.

i have
2) 210watt 12v panels
1) blue sky solar boost 50
1) aims 2500 watt inverter

im now looking for batteries, i was going to get four used solar deka 375 amp hr L16s and run them all in series for a 24v system. but, my local solar shop is low on the used batteries and so im now looking at buying new.

Deka $225 / 375 amp hr = $0.6/amp hr
vs
Golf cart $83 / 200 amp hr = $0.4/amp hr

does it really matter if i get 4 L16 vs 8 golf cart bats?

this all leads me to a related question, i intended on doing a 24v system because i hear running batteries solely in series wears them more evenly, where as a paralleled connections act as resistors and over time wear batteries unevenly. so if i do go with the 8 golf cart batteries i may as well do a 12v system as there are paralleled connections in both a 12 and 24v system. 12v accessories are cheaper than 24 and 48v

thanks again for your time
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: question about batteries and most everything
    bcolebuild wrote: »
    So, im in taos and putting together my first solar system, and i thank you in advance for taking time to answer my beginner questions.

    i have
    2) 210watt 12v panels
    1) blue sky solar boost 50
    1) aims 2500 watt inverter

    Your 420 Watts of panel will probably handle 350 Amp/hrs of 24 Volt at best. You may be disappointed with that.
    Bluesky charge controllers are generally well received. I've no personal experience with them.
    AIMS inverter - on my personal "waste of money" list, but if you've already got it ...
    im now looking for batteries, i was going to get four used solar deka 375 amp hr L16s and run them all in series for a 24v system. but, my local solar shop is low on the used batteries and so im now looking at buying new.

    Deka $225 / 375 amp hr = $0.6/amp hr
    vs
    Golf cart $83 / 200 amp hr = $0.4/amp hr

    does it really matter if i get 4 L16 vs 8 golf cart bats?

    this all leads me to a related question, i intended on doing a 24v system because i hear running batteries solely in series wears them more evenly, where as a paralleled connections act as resistors and over time wear batteries unevenly. so if i do go with the 8 golf cart batteries i may as well do a 12v system as there are paralleled connections in both a 12 and 24v system. 12v accessories are cheaper than 24 and 48v

    thanks again for your time

    The ol' 12 vs. 24 debate! :p

    If you're going up in Amp/hrs, there's a point where going up in Voltage becomes a really good idea. One consideration is as you mention; using 8 'golf cart' type batteries. Lots of connections between batteries there. More connections = more potential problems.

    But the question is, do you need this much battery capacity? It is generally accepted practice (as well as a smart idea) to design a system based on expected loads. That determines what you need for an inverter and battery bank, and the bank size will tell you how much PV you need for charging it.
    So if you could give us some idea of your intended use?

    As for series vs. parallel connecting of batteries, yes it is easier to keep a large bank balanced if it is of higher Voltage, because having the series only connections makes the whole assembly act as one battery. It's all down to those multiple connections and trying to keep current flowing evenly in and out of all the cells.
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  • bcolebuild
    bcolebuild Registered Users Posts: 10 ✭
    #3
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    Re: question about batteries and most everything

    I'm going to be running a cement mixer and a radio a chop saw once in awhile, im building a small earthship and 420 watts was all i could afford. i guess the root of my question is, does the efficiency of L16s run in series out weigh the savings of using golfcart batteries is series and parallel, and why would anyone buy solar batteries when they are more expensive per amp hr than golf cart ones. thanks a lot
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #4
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    Re: question about batteries and most everything

    What 'coot says, plus,,,

    I would design for loading on a battery bank for ~20% discharge on a daily basis. So you really have to figure in your loads before you can calc in your battery size. That said, once you have the battery bank size, the panel capacity should be in the neighbourhood of 5-10% of that battery ah capacity.

    One of the biggest mistakes folks make is either under batterying (for the load) leading to excess draw on too regular a basis, or in your case "over batterying" for the PV such that you have plenty of draw down capacity, but not enough charge capacity, leading to a battery that is slowly (but surely) killed by undercharging.

    You really need to have a balance between loads, Pv and battery capacity is critical for long term performance. Now with all that said, I would start with the cheaper but possibly shorter lived T-105 golf cart batteries rather than the l-16s. My reasons are two fold. The first is you may prove to be cheaper if they last ~60% as long as the l-16s. (This is the calc I made when I bought my most recent set,, we'll see how it pays off after year 5 in my set,,, I'm in year 3)

    The second is that as a "beginner" you are likely to make some mistakes that will lead to premature failure, so in that case, kill the cheaper ones first, learn what is going on and then with a well designed systems and load history you can make the best choices going forward.

    Tony
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
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    Re: question about batteries and most everything

    A couple of usage warnings: AC induction motors as used in chop saws and cement mixers do not like "Modified Sine Wave" inverters. I'm not sure if the AIMS unit you have is Pure Sine Wave or not.
    Such motors also have substantial start current surges, and could cause the inverter to fault instantly. This does not always happen, but you should be wary of the possibility.

    As for battery comparison:

    Trojan L16's come in two capacities; 325 or 375 Amp hours.
    The T105 is 225 Amp hours.

    You could series connect 4 of the L16A's and get 325 Amp/hrs @ 24 Volts or you could series connect 4 of the T105's and get 225 Amp/hrs @ 24 Volts. If you then parallel another bank of T105's to the first you'd have 450 Amp/hrs @ 24 Volts.

    The 4 L16's would cost (NAWS prices) $1,175
    The 8 T105's would cost $1,120

    There would be additional connectors in the T105 set-up. But you have more Amp/hrs and lower cost. Would there be problems with this set-up? Probably not; it is quite common to use a bank like this (although it is more common to use a 4 T105 12V bank).

    Your panels would not charge the 450 Amp/hr bank efficiently, so that should be considered.

    The "why should I buy this battery over that one" debate rages on. Some people have better service from one type than another. Usually it comes down to "getting your feet wet" with the least expensive deep cycles you can afford, as it is fairly easy to shorten their life inadvertently. Later you buy th expensive ones, which should have a longer cycle life than the cheap ones if looked after properly (long-term investment).
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  • bcolebuild
    bcolebuild Registered Users Posts: 10 ✭
    #6
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    Re: question about batteries and most everything

    so im gathering 12v 24v doesnt really matter when dealing with this small of a system although 12 v inverters, pumps, refrigerators and lights are cheaper, so i will go 12v and my mod sine inverter may blow up my tools, but i cant afford a pure sine right now.

    and if i do go cheap $80 interstate golf cart batteries and parallel 4 12volt stings ill have 900ah?

    I didn’t think it mattered how I configured my batteries volt x amp = watt

    So why would my panels not charge the 450 Amp/hr bank efficiently?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #7
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    Re: question about batteries and most everything
    bcolebuild wrote: »
    so im gathering 12v 24v doesnt really matter when dealing with this small of a system although 12 v inverters, pumps, refrigerators and lights are cheaper, so i will go 12v and my mod sine inverter may blow up my tools, but i cant afford a pure sine right now.

    Under 1 kW of consistent use with occasional surge to 2 kW and there's not much reason to go 24 Volt.

    There are some relatively inexpensive pure sine units now, which might be cheaper than burning out your chop saw. All depends on how much power you actually have to provide. About $500-$600 for a Samlex 12V 1500 Watt unit.
    and if i do go cheap $80 interstate golf cart batteries and parallel 4 12volt stings ill have 900ah?

    Too many batteries in parallel cause problems with keeping current flow even throughout, excess connections can have resistance build-up, et cetera.
    I didn’t think it mattered how I configured my batteries volt x amp = watt

    So why would my panels not charge the 450 Amp/hr bank efficiently?

    Yes, Volts * Amps = Watts.
    But to charge a battery bank you need three things: more Voltage going in than the battery is at, enough time to replace the 'used' Amp/hrs, and sufficient (peak) current to mix the electrolyte and reduce sulphation.

    In simple math form:
    420 Watts divided by 12 Volts equals 35 Amps, enough for a 350 Amp/hr battery.
    420 Watts divided by 24 Volts equals 17.5 Amps, enough for a 175 Amp/hr battery.

    In reality, the panels won't produce 420 Watts, the system Voltage is lower than the panels' Vmp, and the charge rate for a battery should be 5% to 13% of its Amp/hr rating (20 hr rate).

    Don't ask those panels to try and charge 900 Amp/hrs; they won't do it.
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  • bcolebuild
    bcolebuild Registered Users Posts: 10 ✭
    #8
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    Re: question about batteries and most everything

    Ok, say i run my 12v panels in parallel giving me 420watts @ 12v. i will then have 35 amps going to my batteries which should be 350 ah

    how then do i configure 4 L16 350s for a 12volt system
    4 in series will give me 24volts and 2 paralleled strings will give me 12 volts with 700 ah

    or 8 200 ah golf cart batteries 4 paralleled 12 volt strings will give me 800 ah

    none of these situations seem right, id like to go 12volt, but do i have to go 24?

    thanks for hanging in here with me
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #9
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    Re: question about batteries and most everything

    Just FYI,

    Pv will produce ~80% of name plate rating under absolutely ideal conditions. (Seldom more) And even at that, that is only the output of the PV, not the net input out any charge controller. My Rogue Mppt controller will convert at a better than 95% rate. A non mppt controller will only covert that at maybe 70%. so your 420 watts , might only yield net/net ~235 watts net/net into the battery under ideal conditions, and then for only ~4 hours/day or so.

    My rule of thumb is that a battery based system will yield ~ 50-55% of name plate net/net out of the inverter. So your 420 watts might give you ~ 925 whs of power per day given 4 hours of good sun (a pretty good average,, seldom more)

    Tony

    A couple of other "rules of thumb" are that people over estimate their solar capacity and under estimate their loads. One more is that loads WILL grow with time.

    T

    PPS Please define your expected loads and why you need such a big battery bank.

    T
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  • audredger
    audredger Solar Expert Posts: 272 ✭✭
    #10
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    Re: question about batteries and most everything

    It may offed your sensibility's but, for the chop saw and mixer I would suggest a generator! Yes, building an earth-ship using gasoline may sound "earth unfriendly" but, so is killing lead acid batteries!

    I built my off-grid home using a Honda eu200i. I had 12 175 watt panels and a 4000 watt inverter and battery bank that would yield 1950 amps @ 24 volts but, I was unwilling to sacrifice 24 L16h's to the chop and table saw. (let alone the mixer) The mixer is going to run for hours not seconds...
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  • mikeo
    mikeo Solar Expert Posts: 386 ✭✭✭
    #11
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    Re: question about batteries and most everything
    id like to go 12volt, but do i have to go 24?

    What voltage does your inverter run on?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #12
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    Re: question about batteries and most everything
    bcolebuild wrote: »
    Ok, say i run my 12v panels in parallel giving me 420watts @ 12v. i will then have 35 amps going to my batteries which should be 350 ah

    how then do i configure 4 L16 350s for a 12volt system
    4 in series will give me 24volts and 2 paralleled strings will give me 12 volts with 700 ah

    or 8 200 ah golf cart batteries 4 paralleled 12 volt strings will give me 800 ah

    none of these situations seem right, id like to go 12volt, but do i have to go 24?

    thanks for hanging in here with me

    The simplest answer is to not buy so many batteries.
    Whereas a smaller bank can take more than the minimum current, a larger bank can't take less.
    The current calculation I gave is only an approximation; what you actually get out of yours will depend on a number of factors, including your panels' Vmp, their insolation, ambient temperature and weather conditions, length & size of wire runs, controller efficiency .... Gets to be quite a list, eh?
    So we set an arbitrary "10%" target for initial design calculations. That 35 Amps could be 5% of a 700 Amp/hr bank or 13% of 269 Amp/hrs. Chances are the actual current rate will be less - probably only 80%. That would be 28 Amps, suitable for 215-560 Amp/hrs. I know; it's a very wide range to work with. But it also means that either a 225 or a 325 or even a 375 Amp/hr bank would work. It's best to keep the current above 5%, as we don't shut off all loads while charging; net charge rate is gross in minus loads out.

    You can't really have too much PV, except for the expense. Any excess power will simply not be harvested as the charge controller will limit output to what the battery bank needs/can take. Too little panel, however, is an all-too-frequent occurrence.
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #13
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    Re: question about batteries and most everything

    I second what Audreger says.

    To design a system for absolute peak loads, such as shop tools when they may be use only occasionally is expensive and not terribly efficient. That said, if you use X wh of shop tool power every day/week ect. then by all means design to run those off the PV if you can afford to. I chose to keep my Pv costs and system simple, and therefor chose to use a proper size genny for various shop tools.

    Tony
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #14
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    Re: question about batteries and most everything

    I'd agree with the greater practicality of a small gen, but the OP already has panels, CC, and inverter and was looking for info on properly sizing a battery bank to get it all to work.

    Of course, he's going to need a back-up generator at some point anyway. We all do, who live off-grid. :D
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  • bcolebuild
    bcolebuild Registered Users Posts: 10 ✭
    #15
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    Re: question about batteries and most everything

    thanks for the info everyone
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