Noob here - basic setup question

Snailster
Snailster Registered Users Posts: 18
Hi all! I've been lurking for a while, putting together a simple system for my shack.

Here is the skinny:
Running a 12V system.
Got two 6V 220ah golf cart Costco batteries ($75 + $9 core charge each), in series.
I intend to buy the Sunsaver MPPT 15A controller.

My main dilemma is, what size PV array do I need?

I have heard the rule of thumb that batts should be charged at 5-15% of their capacity (am I understanding the rule correctly?). My capacity is 220 ah. That would suggest an array that can provide 11 - 33A. The closest to that, available in my area is a SunSolar 175W mono ($730 + tax). That should, on a good day provide, after some losses, roughly about 12A - on the lower end of the required panel size, based on the 5-15% rule alone.

However: I feel, given my expected usage pattern, that I could easily get away with an 80-130W array, after accounting for all lossess, inefficiencies, and cloudy days.

My usage: I need only 50W for 4 hrs for THREE DAYS A WEEK (basically just the weekends), and I average 5 hrs of full sun per day in my area (Hawaii).

How set in stone is the 5-15% rule? Could a 220 ah battery bank be brought up to a full charge, and maintained there, by an 80W, or even smaller panel, providing I have less coming out than coming in, of course?

Or is there a certain point where the panel is too small and cannot "top-off" the battery?

Thanks for any and all ideas!

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Noob here - basic setup question

    Roughly, the minimum panel would be 1-2% of the 20 Hour rating (in Amp*Hours). And that would be assuming you have no load.

    The 5% minimum is partially there to ensure that the batteries are capable of equalization (flooded cell battery needing mixing of electrolyte by sufficient gas generation).

    Now, equalization is only a once a month (or so) operation... So, if you have a genset to perform equalization--then you could get away with less panel than the 5-13% or so rule of thumb.

    As batteries age (flooded cell probably more so)--they become less and less efficient--so what worked for charging when new, may not now.

    Also, depending on your loads--you don't want your flooded cell battery to sit below 75% state of charge for more than a few hours or day... Letting it sit below 75% state of charge for days+ causes sulfates to harden and battery capacity to be reduced.

    You certainly can start with your minimum system--and see how it works. Monitor that battery state of charge with a good hydrometer (or, even better yet, a Battery Monitor) and see what the results are.

    You can always add more panels if the battery is not staying properly charged (requiring more generator run-time to be recharged as needed)--or as you increase the loads.

    The Sunsaver MPPT 15 amp controller is a very nice MPPT controller... But you may be better off getting a less expensive PWM controller (like a Xantrex C40) instead--you will be able add more panels without having to add another controller (your SunSaver will be maxed out with current panel at 12 volts).

    Last couple of random points. Get the Remote Battery Temperature Sensor for the solar charge controller. The MorningStar unit really needs it (but it is a good idea for any charge controller for best battery performance--hot charge controllers and cool batteries tend to leave the batteries undercharged). And--are you sure you will not be needing more power (or power during cloudy weather)? Do you have a backup genset (if needed)?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Noob here - basic setup question

    The reason for the 5%-15% rule is that if the batteries aren't recharged at a sufficient rate of charging they become sulphated sooner and that means shorter life. Small charge rates that barely keep up with usage or self-discharge pretty much guarantees you'll be buying new batteries sooner than you should have to.

    Your math is basically correct: 220 A/hr @5% = 11 Amps, and 11 Amps * 12V = 132W minimum panel size. Keep in mind that a panel rating is its peak output, not constant. On average the over-all system efficiency will be more like 50%-80%, so that 175W panel will probably produce about 122W +/-. If you get 5 good hours of sun per day that's ~600 Watt/hrs per day available for use and recharging. Your intended use is only 200 W/hrs per day.

    You could shrink your battery size, but history tells us you'd only end up increasing it later. Better to have the extra capacity now if you can afford to.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Noob here - basic setup question

    the battery bank you have is a bit large for your usage and larger batteries (ah capacity) need more to charge properly. under such circumstances as yours you may get away with about a 3% rate of charge which translates into .03 x 220ah = 6.6a minimally from pv. lower rates i do not advise you to use as the batteries may not charge properly, but on the plus side, you can expand on your power usages some.:D
    btw that rule is 5% to 13% for fla types as trojans and some similar batteries do not allow for charges higher than 13%. some may take higher percentages, but you will also have higher maintenance with it. agms that max figure does not apply to and gels are sensitive to too much current in and out and may be less on the maximum. the minimum stands for normal purposes although you may stretch it downward some under your circumstance.
  • Snailster
    Snailster Registered Users Posts: 18
    Re: Noob here - basic setup question

    Thanks guys for the advice, and clarification of some of the issues!

    I did not realize the minimum amp requirement was mainly for the equalization, but that makes sense. I thought perhaps too small a panel won't generate enough voltage in a large bank for the batteries to reach full charge. But I do have
    a generator, so I could equalize using that, should I go with a small system.

    Temperature sensor - thanks for the idea. My battery and the CC are in the same environment, in the shade, temperature there is pretty stable, usually not more than 95 degrees. Since I will be probably mostly on the lower end of the required current, so the batteries may not run too hot (and please correct me here if I am wrong - I would be running somewhere about 12-14 amp max into a 220 ah bank), I may not need one? But then again, the sensors seem to be pretty affordable, so should probably buy one. After all, this whole project is partly a science exercise for me.

    Remote meter - I was also thinking of getting the Morningstar Remote Meter RM-1, to go with the MPPT cc. However - do I need the meter if I can access the data
    on the MPPT cc with my laptop?

    Thanks again for all the ideas and advice!
  • Snailster
    Snailster Registered Users Posts: 18
    Re: Noob here - basic setup question

    Bill - agree with you that the MS MPPT cc is a bit on the pricey side.
    The main advantage in my situation is the ability to go with a 24V array
    (such as the Solar World 175W ). And I suppose the possibility to monitor things from my laptop is also nice. But hear you about the cost being way high compared to the PWM controllers. Should I decide to go with a 12V array, then
    I will definitely consider a PWM controller.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Noob here - basic setup question

    There are some pretty big "12 volt" panels now. I agree, when you can, to use larger panels. They are usually cheaper per watt and fewer connections/mounts.

    If the panels are a long ways from the batteries--the MPPT controller would be nice (run higher voltage from panels to MPPT charge controller).

    If you think you will be adding a total of 400+ watts of solar panels in the near future--then a MPPT 60-80 amp controller would be worth the investment.

    Otherwise, a nice 40+ Amp PWM controller for your location (12 volt panels, close to battery shed) it not a bad deal (not too hot, not too cold).

    Regarding the Remote Battery Temperature Sensor--the SunSaver MPPT really needs it--for whatever reason, the internal sensor reads high and causes the battery to be undercharged somewhat (according to reports a 1/2 year ago or so).

    If you will be monitoring the battery bank (for experience and predicting loads/charging in the future)--a battery monitor gives you much more useful information than that from a charge controller... Normally, what you need to know is how much charge is in the battery (over 75%, everything OK, 75-50%, think about cutting power usage, perhaps run the genset in the morning if sun is not predicted; if less than 50%, cut loads, start generator within hours or a day to lessen the chances of long term battery damage).

    Reading battery voltage (while under charge or load) is not that accurate. Using a hydrometer is messy--and not even possible with sealed batteries (such as AGM's).

    For the SunSaver, you need to purchase this for almost $40 to connect with a PC--just another cost item:

    Morningstar PC Meterbus Adaptor (MSC)

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Snailster
    Snailster Registered Users Posts: 18
    Re: Noob here - basic setup question

    Thanks Bill, that's another good point - I thought I would just connect the MS MPPT cc to my laptop via a phone cable, obviously not as simple as that. I was hoping to be able to access the current and voltage charts for the day, or several days, both from the array to the cc, and from the cc into the battery.

    I could probably go with the Kyocera KC130TM (comes with a junction box), $450 + 100 SH. Or Kyocera KD135GX-LPU (MC4 connectors), $402 + 100 SH. Those are 12V, and then I could go with a PWM controller, lowering the overall cost of the system considerably.
    Which connectors should I go for, given the choice? I do not expect to increase the number of panels in the near future (a year or so). Thanks.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Noob here - basic setup question

    Regarding logging of controller information--make sure you know, for your make and model, which data and how long (hours, days, etc.) it will log between downloads. Not all logging is the same.

    If you are gong to use MC4 or some other MC connector--usually the panel warranty requires the owner to leave the original connectors on the panels. So--to make things easier and cheaper, just buy a 24" (or whatever length) Male/Female cable set, cut in 1/2, and you plug one end into the panel and hardwire the other end into your system harness.

    Interconnect Cables for Solar

    By the way, there is a pair of tools to unlock the MC4 connectors on the above page too.

    Assuming you are in a humid area--the plugged/pig-tails are probably a better solution than the old wiring box on the back of the panel. Better sealed against weather, easier to connect/disconnect panels.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Noob here - basic setup question

    snailster,
    you said, "I did not realize the minimum amp requirement was mainly for the equalization......"
    now that is not what i said and is not true. you could have a single 2v cell and it would still need a minimum amount in order to charge it and there are no other cells to equalize in that case. there are charge losses and efficiency factors involved here and it just isn't practical to try and charge a battery with less than 3%. 1-2% is about what should be used just to maintain a battery from its losses.
  • Snailster
    Snailster Registered Users Posts: 18
    Re: Noob here - basic setup question

    Niel - actually, the statement regarding equalization is from message #2 written by Bill. I did not say you said that.

    Having said that :-) ... thanks anyway, all info is welcome.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Noob here - basic setup question

    you do not need to eq to go less than the 5-13% rule, but it certainly is good to have all batteries charged equally as that is the purpose of eqing. it is an over charge to the batteries to get the slower ones up to speed as already charged batteries will not go farther than they are capable of and would just boil off a bit of water that can be re-added. care must be given to agms as they are sensitive to overcharge damage.
    bill reached a false conclusion on eqing as a condition for minimally going 5% as an overcharge can be delivered to a battery or battery bank at even for example 4%. 5% of course will deliver the overcharge faster than 4% would, as any percentage even higher up would do also.
    you're welcome anyway.:roll:
  • Snailster
    Snailster Registered Users Posts: 18
    Re: Noob here - basic setup question

    I have not purchased a solar panel or cc yet, and trying to make the decision
    as to what to buy by mid week.

    My general thinking at the moment is this: I will make the system expandable.
    I will go with the locally available SolarWorld 175 Mono for $730 (U.S. made, should be a solid product, right?) which runs at 24V, and I will be able to add additional panels with relatively small wires running the 30' or so to the battery shed.

    The controller: originally I thought the Morningstar Sunsaver MPPT would do the job, but having second thoughts now due to its (for my job) small size. Correct me if I am wrong: should I decide to add a second 175W panel, I would need to pretty much get a second controller in order to harvest efficiently? The MS MPPT is rated up to 400W at 24V, BUT it's rated output is 15A. Both panels running at full throttle in the midday sun would be likely putting around 20A into 12V batteries, right? Please let me know if I am off here, because from this I deducted I may consider buying the OutBack FM60, or the Xantrex XW instead. Not only these could easily handle multiple additional panels, but with multiple additional panels I could also run higher array voltage than with the Morningstar mppt.

    The main question is: am I correct in my assumption that the Morningstar MPPT can basically run efficiently only ONE 175W - 24V panel charging a 12V battery bank?

    So these are my thoughts as of Monday morning.
  • Snailster
    Snailster Registered Users Posts: 18
    Re: Noob here - basic setup question

    This panel thread:

    http://forum.solar-electric.com/showthread.php?t=3021&highlight=Solarworld

    would suggest the SW 175 mono would be a fine panel for my needs.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Noob here - basic setup question

    If you're already looking to expansion, bight the bullet and by the better charge controller now. Something like the MX 60 can grow with your system, allowing you to add panels and change system voltage without having to do a lot of re-rigging. It will also log data that can help you see how your system is performing over time.

    Mine started out on a 12V set-up with 525 Watts of panels and is now on 24V with 700 W. I've still got room to double the array size when I can afford it.
  • Snailster
    Snailster Registered Users Posts: 18
    Re: Noob here - basic setup question

    That's exactly what I was thinking. Just hoping that when I decide
    to expand, the 175 Mono panels will still be around...
  • Snailster
    Snailster Registered Users Posts: 18
    Re: Noob here - basic setup question

    Thanks Tristanscipio, sounds like that would be an interesting project! Unfortunatelly, I am not sure I have the time for a project like that, so for now
    I will stick with the factory made stuff.

    BTW, there is a thread about it here: http://forum.solar-electric.com/showthread.php?t=5290
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Noob here - basic setup question

    snailster,
    for expanding you will need a bigger cc, but the bigger cc will not be as efficient handling the 1 pv in the meantime. the pvs can be any you may want as any on naws' store will be of good quality and you can get pvs cheaper that are good quality. check this 210w kyocera that is cheaper than the solarworld.
    http://store.solar-electric.com/kykd210waaop.html
    do some looking around and try not to rush yourself into a system decision in a few days as this takes planning. pvs, cc(s), batteries, wiring, fuses, etc. all need to be checked and rechecked in the planning stage and is especially true of systems to be expanded on.
    http://store.solar-electric.com/hiposopa.html
  • Snailster
    Snailster Registered Users Posts: 18
    Re: Noob here - basic setup question

    Buying from NAWS is definitely an option which I am considering. However, living in Hawaii comes at a price: All the panels bigger than 135W, or so, ship truck freight, so not really an option for me.

    I am also considering the Kyocera 135: from NAWS the price is about $3.80/w shipped, at least according to the website (the Kyocera 130 purchased in Hawaii will go for about $5.30/w after tax). The Kyocera would be good if decide to go the more affordable route: keep the system 12V would eliminate the need for an MPPT, and I still could add more panels along the way. The downside would be the 30' of thicker pv wiring going to the battery shed, and buying the pvs long distance, which could make potential warranty issues complicated. But I have not ruled out the "Kyocera approach" just yet.

    The SolarWorld pv, when I buy it locally, will cost me $4.30/w after tax, so that's the only thing available here resembling a deal of sorts. Plus with the panel I think it is nice to have local dealer support, shipping it back and forth, albeit unlikely, would be major pain.

    Thanks everybody for the ideas and suggestions thus far, it's a work in progress and I am learning a lot.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Noob here - basic setup question

    understood. if you can get the 2nd pv soon it would benefit you with the larger mppt ccs. if you know it will take a longer time for another pv then go the smaller mppt and sell it later or keep it as a spare as the classic may hopefully come available by the time you'll need it, but if not there are other large mppt controllers out there. use the battery temperature sensor no matter what cc you settle on.