Help me scale my panels / batteries correctly

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stereoman405
stereoman405 Solar Expert Posts: 56 ✭✭✭✭
Hi all,

I am in the process of building a small PV setup to use for backup power at my North Idaho cabin. Unfortunately I have to do this backwards... basically letting the cost of the components dictate my setup instead of starting with my power use. The cabin is grid tied and this is purely to be used as a 'worse case' backup that doesn't have me relying on my genny. (Wind and snow can be brutal up there.)

I basically want it to be able to power a standard energy star fridge / freezer unit, some basic lighting, and a small HAM setup.

The gear I currently have:

- Two Suntech 165w 24v panels. (Different ages, so specs aren't 100% match.)

- Xantrex 12v modified sine inverters (XM 1800 Pro) I got very cheap. 8) Plus a couple 1000 watt'ers.

- I plan on picking up two new REC 210w 24v panels this week. For a grand total of 750 watts @ 24v.

Other useful info:

- Cabin is at 48.5 degrees latitude.

- Roof is southern exposed with minimal if any shade.

- Cold winters with a lot of snow. (At least this would minimize refrigerator use during cold months. Just put the food in the shed.)

- PV should be within 75' of battery location.

So that means I need to keep the battery bank 12v to work better with the inverters I already have, and to easier integrate with some other 12v gear. Does this sound like a reasonable setup for my requirements? I could potentially get more of the REC 210w's, but that might dig into the wallet too much. I do plan on getting a quality controller like a Tri Star MPPT 45amp to allow for a little future growth.

Maximum battery longevity is a priority here, so how many AH battery array would you suggest so that I am not draining too much (keep the 50% - 80% full), but so that the PV's can keep them topped off properly? I definitely don't want to over battery this and have them only able to get to 90% charge.

PS - I forgot to mention that my backup genny is a Honda EU2000 (2,000w) running on LP. Puts out a fixed 7.6A on its 12v hookup. I really don't want to have to rely on this too much, but I figure it might help with battery sizing.

Comments

  • stereoman405
    stereoman405 Solar Expert Posts: 56 ✭✭✭✭
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    REC Panels - Opinions?

    Have a local guy selling brand new REC 210's for $615 a pop. Seems pretty good. I'm getting the 'bulk discount' even though I'm only getting a couple.

    What are your experience with REC's? These will be going in the North Idaho wilderness so they need to be able to handle the snow and cold.
  • stereoman405
    stereoman405 Solar Expert Posts: 56 ✭✭✭✭
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    Need some charge controller advice

    I'm gong to be running 700w - 1000w of 24v panels into a 12v battery bank. I need something that's:

    - MPPT (Needs to efficiently go from a higher voltage (48v) into 12v bank.
    - Can float charge
    - Can equalize charge
    - Check battery temp
    - Charge a smaller aux bank
    - Display that gives real time stats.
    - Can scale a little, maybe to 1500w.

    I like the looks of the Morningstar Tri Star 45amp MPPT, but are there other models out there that could give me better bang for the buck? Or do I need to step up to a 60amp model, which would open up a few more posibilites like the Outback Flexmax, or the Xantrex. I've been doing my digging, but it gets a little overwhelming. I get confused on what the amperage rating refers to. Coming in, or going out?
  • stereoman405
    stereoman405 Solar Expert Posts: 56 ✭✭✭✭
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    Battery advice in this environment... AGM vs Flooded

    I'm looking to put a battery bank at my cabin in North Idaho. Summers can get toasty, and winters can get pretty cold. The bank will most likely sit in a crawl space under the cabin. This isn't under the frost line so it most likely will get below freezing regulalry, but will stay cool during the summer.

    The cabin is used heavily during the summer, and only occasionally the rest of the year (can go months during the winter with no visits) so the batts have to be very low maintenance. I am grid tied and plan to leave a Xantrex Trucharger 10 on the bank to keep things topped off when the PV isn't enough. The PV array will use a quality PMMT controller as well. I plan to size the bank to the array to avoid heavy discharges.

    I want these batts to last 8+ years in these conditions and I'm wiling to invest a little more money now to do it. So what would you recommend? Can some relatively cheap T-105's cut it or should I really get some quality AGM's? I don't mind checking fluid level on some cheaper flooded's occasionally, but with the long periods away this might not be the best idea. I could ditch the Trucharger for something better if it helps keep these alive longer.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: Need some charge controller advice

    stereoman,
    be careful that you don't start multiple threads all being similar as they apply to the same proposed system you are developing.
    with that said i am confused as to exactly what pv system voltage you plan on. is it 48v to 12v by putting the 24v pvs with 2 in series or 24v to 12v with all parallel? do you know the current rating (imp) of the pvs?
    the nec in its ever wondrous wisdom limits the controllers to 80% of their capability on the output side. so for example a flexmax 60 rated at 60a is held back to 48a. if the output voltage is 12v this becomes 12v x 48a = 576 watts. at 24v the ampere rating is still at 60a and forced back to 48a to be 24v x 48a = 1152w. at 48v output it's the same story and becomes 48v x 48a = 2304w. says allot being you want up to 1500w. this is what the input would basically be too if the pvs are for the same battery voltage, ie 12v pvs to 12v batteries roughly speaking as even then the mppt will recover some current lost due to voltage differences. what happens when the pv input voltage is say doubled to 24v? the output voltage from the controller the controller will act like a transformer of sorts and double the amps when it halves the voltage. similar thing for 48v to 12v also but amps is x4 while voltage is /4 and is roughly keeping the same wattage plus a bit recovered through mppt. this will not happen without an mppt downconverting controller and some have voltage limits that can work with. you must be careful that you don't waste you pv current by overfeeding the mppt controller as the controller will autolimit at the output to its respective rating. that means that if say you have 35a of 24v pvs feeding a flexmax 60 to output 12v at about 66a that does 2 things,
    1 violates nec rules by exceeding the 48a output.
    2 exceeds the limits the controller will output and it will hold it to 60a wasting the extra 6a.
    if you can't satisfy your wattage requirements with a controller you either need a larger capacity controller or more than one of them with each being fed from their own array of pvs.
    charging another battery bank can be accomplished with a battery switch or that bank can have its own set of pvs and controller.
    all of the bigger mppt ccs that i know of can float and eq charge and many have meters that readout too. some prefer a battery monitor to watch over those stats as the ccs are not always that accurate nor the main purpose of the cc as they consider that a secondary bell and whistle added.
    this is enough here to make you figure out what it is you need, but if you don't quite get it in some respects ask again in the same thread, thanks.

    ps i see bb beat me to the combining stage.:D
  • BB.
    BB. Super Moderators, Administrators Posts: 33,477 admin
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    Re: Help me scale my panels / batteries correctly

    Note: I moved all of SteroMan's questions into one thread--they are all related to his one Idaho Cabin--so hopefully we can keep track of everything in one thread.

    -Bill
    Hi all,

    I am in the process of building a small PV setup to use for backup power at my North Idaho cabin. Unfortunately I have to do this backwards... basically letting the cost of the components dictate my setup instead of starting with my power use. The cabin is grid tied and this is purely to be used as a 'worse case' backup that doesn't have me relying on my genny. (Wind and snow can be brutal up there.)

    I basically want it to be able to power a standard energy star fridge / freezer unit, some basic lighting, and a small HAM setup.

    The gear I currently have:

    - Two Suntech 165w 24v panels. (Different ages, so specs aren't 100% match.)

    - Xantrex 12v modified sine inverters (XM 1800 Pro) I got very cheap. 8) Plus a couple 1000 watt'ers.

    - I plan on picking up two new REC 210w 24v panels this week. For a grand total of 750 watts @ 24v.

    Other useful info:

    - Cabin is at 48.5 degrees latitude.

    - Roof is southern exposed with minimal if any shade.

    - Cold winters with a lot of snow. (At least this would minimize refrigerator use during cold months. Just put the food in the shed.)

    - PV should be within 75' of battery location.

    So that means I need to keep the battery bank 12v to work better with the inverters I already have, and to easier integrate with some other 12v gear. Does this sound like a reasonable setup for my requirements? I could potentially get more of the REC 210w's, but that might dig into the wallet too much. I do plan on getting a quality controller like a Tri Star MPPT 45amp to allow for a little future growth.

    Maximum battery longevity is a priority here, so how many AH battery array would you suggest so that I am not draining too much (keep the 50% - 80% full), but so that the PV's can keep them topped off properly? I definitely don't want to over battery this and have them only able to get to 90% charge.

    PS - I forgot to mention that my backup genny is a Honda EU2000 (2,000w) running on LP. Puts out a fixed 7.6A on its 12v hookup. I really don't want to have to rely on this too much, but I figure it might help with battery sizing.

    OK--What we really like to do is get a handle on the electrical loads, and then size the system from there.

    Assuming a 356-400 kWhr per year Energy Star Fridge is one possibility. Another is to use a 100 kWhr per year 7 cuft chest freezer to refrigerator conversion.

    So--just some starting power numbers--1kWhr per day for refrigerator 9 months of the year, and 1 kWhr per day for all other loads (HAM, lights, TV, laptop, etc.).

    So, 4 months of winter assume 30 kWhrs per month (pretty minimal amount of power use--depends on your life style), and the rest of the seasons assume 60 kWhrs per month. Feel free to adjust to your actual needs.

    Use the PV Watts Website to predict average solar power collection for your site. Use Kalispell Montana--seems to be closest to your cabin. Use 1 kW of solar panels to start with (even number, smallest program accepts). Use a derating of 0.52 for power (flooded cell batteries and 85% efficient inverter). Fixed array (although, for winter you will probably want to tilt to near vertical for snow shedding and picking winter sun/snow refection:
    "Station Identification"
    "City:","Kalispell"
    "State:","Montana"
    "Lat (deg N):", 48.30
    "Long (deg W):", 114.27
    "Elev (m): ", 904
    "PV System Specifications"
    "DC Rating:"," 1.0 kW"
    "DC to AC Derate Factor:"," 0.520"
    "AC Rating:"," 0.5 kW"
    "Array Type: Fixed Tilt"
    "Array Tilt:"," 48.3"
    "Array Azimuth:","180.0"

    "Energy Specifications"
    "Cost of Electricity:"," 7.9 cents/kWh"

    "Results"
    "Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
    1, 2.14, 34, 2.69
    2, 2.96, 42, 3.32
    3, 4.02, 63, 4.98
    4, 4.70, 69, 5.45
    5, 5.42, 81, 6.40
    6, 5.77, 80, 6.32
    7, 6.24, 87, 6.87
    8, 6.19, 88, 6.95
    9, 5.23, 72, 5.69
    10, 4.08, 61, 4.82
    11, 2.16, 31, 2.45
    12, 1.46, 21, 1.66
    "Year", 4.21, 730, 57.67

    So, for "winter"--your solar array should be (remember, these are 20 year average numbers--any season may be more or less by 10% or more depending on weather). For December:
    • 30 kWhrs per month * 1;/(21 kWhrs per month per 1kW of panels) = 1.43 kW = 1,430 Watts of solar panels for December
    And we assume that the other three seasons will need ~60 kWhrs per month. So, outside of November-February (I am assuming those are your "cold" months), the next lowest monthly power number is October at 61 kWhrs per month:
    • 60 kWhrs per month * 1;/(61 kWhrs per month per 1kW of panels) = 0.98 kW = 980 Watts of solar panels for October
    Of course, you will have to decide the trade off between a smaller array and generator run time (and winter/non-winter electrical loads).

    You also asked what does the "45 amps" for a MPPT charger mean--that is the maximum output current to the battery bank. For your 24 volt battery bank, that would be roughly:
    • 29 volts charging battery * 45 amps maximum = 1,305 watts into your battery bank (assuming enough solar panels and good sun).
    MPPT type controllers are actually setup to limit power and current on their output--so you can put a larger array on the controller and it will limit it self to 45 amps maximum.

    Notice, that as the battery bank voltage goes up--the same 45 amp controller can handle larger and larger PV arrays with a single controller (a 48 volt bank and 45 amp controller would control >2,610 watts of solar panels). So--if you are not in love with the 12/24 volt battery bank, higher voltage banks do have their advantages (plus you can use smaller gauge wiring for the banks and the inverter's DC input--P=I*V -- double V, then I goes down by 1/2 for same power).

    And shade is a killer for solar panels--at the very least no shade from ~9am to 3pm is a good idea for a south facing roof.

    Battery sizing--we like to do 3 days of "no sun" and 50% maximum discharge for long life. for 60 kWhr per month or 2kWhrs per day and 85% efficient inverter and 24 volt bank:
    • 2,000 watts * 1/24 volts * 1/0.85 inv eff * 3 days * 1/0.50 batt disch = 588 Amp*Hours at 24 volt battery bank.
    For an Inverter--A True Sine Wave is really good for an off-grid cabin. MSW inverters tend to damage some electronics (like wall mount transformers, some computer power supplies, and even some motors like for a refrigerator). MSW does work for many people--but if you can swing the TSW--much better choice in the long term.

    For the Honda eu2000i genset--Do not use the 12 volt output for battery charging... Not regulated and not very much power for the gasoline consumed. Get a "real" 24 volt battery charger (depending on lots of things--around 20 amps or so maximum for a eu2000i). Use the generator in the mornings (when needed) to charge the bank to 80-90% capacity--then let the solar panels take care of the rest--saves fuel and run-time. The Honda is pretty fuel efficient down to ~25% of rated output power (400 watts)--much below that it starts to waste fuel (that last 10-15% to charge a battery bank is at lower currents).

    For 45 amp - 60 amp chargers--At this time, the Morning Star and Xantrex MPPT are the first models to look at. Get the remote battery temperature option (better battery charging--faster and fuller). Most people tend to add loads to their off-grid systems--so designing to allow for a bit of growth is not a bad thing.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • stereoman405
    stereoman405 Solar Expert Posts: 56 ✭✭✭✭
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    Re: Help me scale my panels / batteries correctly

    Thanks for the help guys. Sorry for the rookie mistake of getting things in too many threads. :blush: Other forums I'm on like things spread out.

    I was hoping to keep the battery bank at 12v since I already have inverters and the Trucharger charger that's already 12v.

    With that said I was planning on running the 165w suntechs in a 48v series, put the REC's in a 48v series, then parallel them for 48v into the controller. Sounds like this could be much even for a 60 amp controller?

    I might be able to trade out some of my 12v gear for 24v stuff, but there are still some items that I would need 12v for. Are the DC to DC converters out there very efficient?

    Good info on the Honda charging items. I had always planned on just using 12v Dc out because it seemed more efficient. Sounds like I might need to add even another controller to the mix....

    Either way, it sounds like my 750w might be a little lean here.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,477 admin
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    Re: Help me scale my panels / batteries correctly

    You can use 12 volts for the battery bank--But it does limit you to a maximum of around 1,200-1,500 watts of inverter output as the cables/fusing/switches on the DC has to handle a lot of current:
    • 1,200 watts *1/10.5 volt minimum * 1/0.85 inv eff * 1.25 NEC safety factor = 168 amp minimum circuit
    So, you are looking at wiring for a 175 amp circuit just to use a 12 volt / 1,200 watt inverter. At 48 volts, it would be 1/4 the current/circuit size.

    Also, for a 12 volt battery, you have 10.5 volts for a "dead battery" cutout--giving you ~1.5 volts for wiring drop, etc.

    For a 48 volt bank, the same rating assumptions would be 48v-42v=6v wiring drop allowed...

    For a 12 volt bank, just double the AH numbers from my previous calculations.

    And, regarding 750 watts of solar panels--It really depends on your personal power usage... Tony/Icarus here has a remote home/cabin in the great white north and ~400 watts of solar panel. Sig:
    Signature:
    300 watts Siemens/BP panels,plus a Sun 90,, making ~300. ~30 amps into Rogue MPT-3024 controller,into 450 ah of Trojan T-105, powering a Morningstar ts300 inverter, and monitored with a Tri-Metric meter.a collection of antique generators, plus 2 Honda eu-1000i's (also a BS2512 IX controller) and some assorted other stuff!
    And lives a very nice life (computer, sat internet, electric lights, rados)... However, Propane powered fridge, gas engine washer, etc... minimize electrical use.

    If you are living in the cabin 9 months or more a year--the electric fridge is probably worth the extra costs. If less than 9 months a year (or weekend use, etc.)--then a propane fridge is probably more cost efficient.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
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    Re: Help me scale my panels / batteries correctly

    re DC-DC converters,
    How much 12V power do you need ? if under 100W, there are several 48-12V dc-dc converters >85% , many >90% efficient. fleabay often has many listed, I just got one for $13 there.

    Or you could use a "trickle charger" off your 120VAC, to keep a 12V battery topped off.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • bryanl
    bryanl Solar Expert Posts: 175 ✭✭✭✭✭
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    Re: Help me scale my panels / batteries correctly

    re "I want these batts to last 8+ years in these conditions" --

    Plan on about a pound of battery per watt of solar and vice versa.

    Plan on sufficient battery to last three days of use without going below 50% DoD in nominal conditions.

    8 years will be a bit of a stretch. That means prompt and full recharging when needed, storage with a good desulfation inhibition or prevention method, cool temperatures, avoiding excessive discharging but still providing appropriate exercise,

    For monitoring system status, direct methods (specific gravity or voltage) are usually better than indirect (electron counters).

    When cost is a concern, adapt your lifestyle to fit. That means reducing energy consumption and battery use profiles to suit what battery you have rather than buying battery to suit your lifestyle preference.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,477 admin
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    Re: Help me scale my panels / batteries correctly

    Regarding your AGM question...

    AGM's are probably close to the ideal lead acid battery -- At 2x the cost as flooded cell.
    • Little cleaning of battery tops/connections required
    • No distilled water to add (can be gallons per month for a large bank)
    • More efficient-- very roughly 90% efficient vs 80% for flooded cell
    • Some AGM brands (Concord Sunextender brand?) recommend down to 20% state of charge for normal cycling, very high current recharging, not supposed to sulfate (so they say--I don't know)
    • Downside is very sensitive to overcharging--You overcharge once and they vent--they are on a countdown to failure.
    • High recommend Remote Battery Temperature Sensor for chargers.
    • I am not sure of AGM vs Good Quality Flooded Cell lifetime--I am guessing that high quality flooded cells, properly maintained, will outlast high quality AGMs--I would like to hear from others their experiences.
    Regarding Battery Monitors--I know that "bryanl" does not favor them for battery bank monitoring--whereas I think they are very useful--especially when the Off-Grid bank is being used by people without the detailed knowledge of proper bank operation (spouse, kids, guests). "You" paid the high cost for that pile of lead and acid--so you are probably very careful in how you mange your battery bank--others; not so much.

    Specific Gravity measurements (with temperature correction) are the gold standard for battery capacity estimates... But that is a physical operation (pop cell cap, pull electrolyte, read/temp compensate). You should check all cells once in a while (monthly when you check electrolyte levels)--and check one cell for a quick reading. Not something you will want to do multiple times per day--let alone require a guest to do that. Also--there is a chance of contamination and electrolyte loss with every measurement.

    And, for AGM/sealed batteries--you cannot measure specific gravity at all with a hydrometer.

    Battery voltage measurements can work well--but you are limited to accurate results when the battery bank has been resting for 3 hours or so--Estimating capacity using voltage measurements when the bank is under load or charging--is not very accurate at all.

    So, now you are looking at the Battery Monitor (a shunt installed on the negative battery bank before the negative bus--so that you are monitoring the total current flow into and out of the battery bank.

    Lead Acid batteries are just about 100% efficient at storing/releasing electrons (coulomb counting). So--if you take out 10% of the electrons, then you need to replace 10% of the electrons. As you near 100% charge--it does break down a bit because electrolysis--out gassing hydrogen/oxygen--is a direct loss.

    A battery monitor (as I understand their operation) will "reset" to 100% state of charge when you approach "full charge" (high voltage, low current flow, measured amount of time)--so that tends to "restore" accuracy over time.

    Also, when you read "84% capacity"--that is an assumption that the value of XXX AH was somewhere correct--Absolute capacity does change over time, aging, cycling, and temperature--and some monitors have a remote battery temp sensor option). Many offer further battery bank insight (such as estimated bank energy efficiency--possibly useful for identifying a dieing battery bank).

    So--the battery monitor estimates are just that--estimates. But--for day to day operation and catching the major battery killer of "deficit charging"--I think they are the cat's meow.

    As always read the battery FAQs that we frequently post here ("know your enemy" ;)) :

    Deep Cycle Battery FAQ
    www.batteryfaq.org

    Your mileage may differ (with all due deference to Bryanl).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • stereoman405
    stereoman405 Solar Expert Posts: 56 ✭✭✭✭
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    Re: Need some charge controller advice
    niel wrote: »
    stereoman,
    ...do you know the current rating (imp) of the pvs?

    Dang, you have to burst my bubble with all this math and logic don't you??? :p

    For starters here are the specs on both panels since I'm sure it has relevance here.

    Suntech 165's: (Lists Vmp and Imp as 'Optimum' voltages)
    Voc: 43.6v
    Vmp: 34.8v
    Isc: 5.04A
    Imp: 4.74A


    REC 210's: (Lists Vmp and Imp as 'Maximum' voltages)
    Voc: 36.1v
    Vmp: 28.2v
    Isc: 8.1A
    Imp: 7.5A

    So operating off the assumption of two of each of these panels is series, then paralleled, what would my effective current be at the controller? (V and A?) I've never understood how you calculate voltage when you have two dissimilar legs and you go into an MPPT. Do you average them, or does the 'lowest win'? Or is this a horrible idea in general to have these two mis-matched arrays?

    Let me see if I'm starting to get it here:

    Basically is sounds like if I want to keep my battery bank at 12v, regardless of what my PV voltage is, I'm going to need a minimum of an 80 amp controller like the Outback for a 768w array 12v ceiling. (80A * .8 * 12v = 768W)

    The maximum watts that these 4 panels can put out would be 750w, so the Outback 80 would work for a 12v bank, but I'd give up any expansion until I switch to a 24v bank, which would then double to 1,536w. (80a * .8 * 24v = 1536W) with the 80 amp Outback.

    Add this to what Bill said:
    BB wrote: »
    You can use 12 volts for the battery bank--But it does limit you to a maximum of around 1,200-1,500 watts of inverter output as the cables/fusing/switches on the DC has to handle a lot of current:

    * 1,200 watts *1/10.5 volt minimum * 1/0.85 inv eff * 1.25 NEC safety factor = 168 amp minimum circuit

    So, you are looking at wiring for a 175 amp circuit just to use a 12 volt / 1,200 watt inverter. At 48 volts, it would be 1/4 the current/circuit size.

    Also, for a 12 volt battery, you have 10.5 volts for a "dead battery" cutout--giving you ~1.5 volts for wiring drop, etc.......

    ...For a 12 volt bank, just double the AH numbers from my previous calculations.

    I'd have a max inverter threshold of 1,200 - 1,500 watts. This would be good as I was planning on running a 1800watt Xantrex 1800 pro at 60% - 70% max load. (Seemed like it was general consensus to not run these at 100%.)

    Last I'd double the 588ah rating of my bank to 1,176ah @ 12v? Or can I dial this number down since my PV input numbers have dropped from where Bill started from? (1KW)?

    Sounds like the big disadvantage of this route would be the ammount of batts needed to hit the 1,176ah, and that I'd be running thick cable at the batts to the inverter.

    Am I getting on the right page here?
  • stereoman405
    stereoman405 Solar Expert Posts: 56 ✭✭✭✭
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    Re: Help me scale my panels / batteries correctly
    BB. wrote: »
    Regarding your AGM question...

    AGM's are probably close to the ideal lead acid battery -- At 2x the cost as flooded cell.
    • Little cleaning of battery tops/connections required
    • No distilled water to add (can be gallons per month for a large bank)
    • More efficient-- very roughly 90% efficient vs 80% for flooded cell
    • Some AGM brands (Concord Sunextender brand?) recommend down to 20% state of charge for normal cycling, very high current recharging, not supposed to sulfate (so they say--I don't know)
    • Downside is very sensitive to overcharging--You overcharge once and they vent--they are on a countdown to failure.
    • High recommend Remote Battery Temperature Sensor for chargers.
    • I am not sure of AGM vs Good Quality Flooded Cell lifetime--I am guessing that high quality flooded cells, properly maintained, will outlast high quality AGMs--I would like to hear from others their experiences.
    Regarding Battery Monitors--I know that "bryanl" does not favor them for battery bank monitoring--whereas I think they are very useful--especially when the Off-Grid bank is being used by people without the detailed knowledge of proper bank operation (spouse, kids, guests). "You" paid the high cost for that pile of lead and acid--so you are probably very careful in how you mange your battery bank--others; not so much.

    Specific Gravity measurements (with temperature correction) are the gold standard for battery capacity estimates... But that is a physical operation (pop cell cap, pull electrolyte, read/temp compensate). You should check all cells once in a while (monthly when you check electrolyte levels)--and check one cell for a quick reading. Not something you will want to do multiple times per day--let alone require a guest to do that. Also--there is a chance of contamination and electrolyte loss with every measurement.

    And, for AGM/sealed batteries--you cannot measure specific gravity at all with a hydrometer.

    Battery voltage measurements can work well--but you are limited to accurate results when the battery bank has been resting for 3 hours or so--Estimating capacity using voltage measurements when the bank is under load or charging--is not very accurate at all.

    So, now you are looking at the Battery Monitor (a shunt installed on the negative battery bank before the negative bus--so that you are monitoring the total current flow into and out of the battery bank.

    Lead Acid batteries are just about 100% efficient at storing/releasing electrons (coulomb counting). So--if you take out 10% of the electrons, then you need to replace 10% of the electrons. As you near 100% charge--it does break down a bit because electrolysis--out gassing hydrogen/oxygen--is a direct loss.

    A battery monitor (as I understand their operation) will "reset" to 100% state of charge when you approach "full charge" (high voltage, low current flow, measured amount of time)--so that tends to "restore" accuracy over time.

    Also, when you read "84% capacity"--that is an assumption that the value of XXX AH was somewhere correct--Absolute capacity does change over time, aging, cycling, and temperature--and some monitors have a remote battery temp sensor option). Many offer further battery bank insight (such as estimated bank energy efficiency--possibly useful for identifying a dieing battery bank).

    So--the battery monitor estimates are just that--estimates. But--for day to day operation and catching the major battery killer of "deficit charging"--I think they are the cat's meow.

    As always read the battery FAQs that we frequently post here ("know your enemy" ;)) :

    Deep Cycle Battery FAQ
    www.batteryfaq.org

    Your mileage may differ (with all due deference to Bryanl).

    -Bill
    Thanks Bill.

    I forgot to mention that I did already pick up a Xantrex Link 20 dual bank monitor. (The guy I got a smoking deal on my inverters from gave me a new unopened one for $50.) I figured that this would be a good way to keep an eye on battery health.

    I've poured over those FAQ's I was hoping to see some real life results and opinions based on my scenario. Sounds like AGM would be my best route since there will be months at a time that I won't be around to check fluid and acid levels. How do you go about avoiding the dreaded overcharge on them? If I have a quality charge controller like the Outback 80 and a quality float charger like the Xantrex Trucharge 10 should I be fine?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,477 admin
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    Re: Help me scale my panels / batteries correctly
    I've poured over those FAQ's I was hoping to see some real life results and opinions based on my scenario. Sounds like AGM would be my best route since there will be months at a time that I won't be around to check fluid and acid levels. How do you go about avoiding the dreaded overcharge on them? If I have a quality charge controller like the Outback 80 and a quality float charger like the Xantrex Trucharge 10 should I be fine?

    If the batteries are not being cycled (just stored)--you should be fine with flooded cell batteries too. Just make sure that the charger is set to float and no equalization. Also you can look at using water miser caps to reduce acid misting from any bubbling during charging.

    For AGM's--same thing, make sure that the charger is set to float for storage and never ever set to equalize (at high voltage/current) for AGM/Sealed batteries.

    For both types of batteries, get the Remote Battery Temperature Sensor for proper charging (especially if the battery bank and charge controller are not in the same environment).

    I have a question about the Xantrex TC 10--it is OK for maintenance charging a 100-200 amp battery bank (flooded or AGM)--but pretty small for charging a larger bank and/or running on generator power (you want to load down the genset much more to quickly and fuel efficiently). If you are going to use an AC charger with your genset and a > 100 AH battery bank--I would go with a much larger charger (exact size depending on bank and generator you end up using).

    Are you planning on using the TC 10 with a genset or at home on utility power?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • solarvic
    solarvic Solar Expert Posts: 1,071 ✭✭✭✭
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    Re: Help me scale my panels / batteries correctly

    B. B. Referring back to #12 post. Isn,t these panels he wants to use going to be to disimilar. I know I don,t know nearle as much as you do. In grt setup I have kyocera kc 158 on one string with 1 inverter a1 string of sharp 167 on other inverter. I was told by a reputable solar dealer not to mix them because of them bieing unbalanced. The spec. for what I have is a lot closer than what sterioman wants to use. Solarvic
  • stereoman405
    stereoman405 Solar Expert Posts: 56 ✭✭✭✭
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    Re: Help me scale my panels / batteries correctly
    solarvic wrote: »
    B. B. Referring back to #12 post. Isn,t these panels he wants to use going to be to disimilar. I know I don,t know nearle as much as you do. In grt setup I have kyocera kc 158 on one string with 1 inverter a1 string of sharp 167 on other inverter. I was told by a reputable solar dealer not to mix them because of them bieing unbalanced. The spec. for what I have is a lot closer than what sterioman wants to use. Solarvic

    I'm curious about this as well. I got the 165's for $600 together. It was such a good deal that I'm nervous to get rid of them. BUT, I could grab two more of the REC210's for $600 each if its warranted and sell the other ones. But is it worth potentially $500 - $600 out of pocket in difference to ditch the 165's for the sake of symmetry? If it is I need to know ASAP as he only has 10 left....
  • BB.
    BB. Super Moderators, Administrators Posts: 33,477 admin
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    Re: Need some charge controller advice
    Suntech 165's: (Lists Vmp and Imp as 'Optimum' voltages)
    Voc: 43.6v
    Vmp: 34.8v
    Isc: 5.04A
    Imp: 4.74A


    REC 210's: (Lists Vmp and Imp as 'Maximum' voltages)
    Voc: 36.1v
    Vmp: 28.2v
    Isc: 8.1A
    Imp: 7.5A

    So operating off the assumption of two of each of these panels is series, then paralleled, what would my effective current be at the controller? (V and A?) I've never understood how you calculate voltage when you have two dissimilar legs and you go into an MPPT. Do you average them, or does the 'lowest win'? Or is this a horrible idea in general to have these two mis-matched arrays?

    In general--
    • if you are connecting panels in series--the Imp should match within 10%.
    • if you are connecting panels in parallel--the Vmp should match within 10%
    • If the panels mismatch by more than 10%--you run the risk of losing much more than 10% of their output because of the mismatch (as much as 100% of a string--worst case).
    It is difficult to predict how this will affect the MPPT controller... Mismatching won't hurt the controller (and as long as you don't exceed the Vmax/Imax for the panels, you won't hurt them either)--However, it is difficult to predict how well a MPPT controller will collect power. They are not tested or designed to operate on a non-homogeneous set of panels with wide variations in Imp and Vmp for the strings... No idea if the controller will lock on the "true maximum" (which is still not the total panel output) or it will lock on a local maximum (the first peak from zero)--or what happens as clouds go by and such.

    It is not so much as a bad electrical mistake and more of a $$$$ mistake to not collect all of the power your array is capable of.

    If you have panels which are so far apart--your best/optimum solution is probably to purchase a separate MPPT controller for each group of panels.

    The above panels are pretty far apart to try to mix and match currents/strings.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • stereoman405
    stereoman405 Solar Expert Posts: 56 ✭✭✭✭
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    Re: Need some charge controller advice
    BB. wrote: »
    In general--
    • if you are connecting panels in series--the Imp should match within 10%.
    • if you are connecting panels in parallel--the Vmp should match within 10%
    • If the panels mismatch by more than 10%--you run the risk of losing much more than 10% of their output because of the mismatch (as much as 100% of a string--worst case).
    It is difficult to predict how this will affect the MPPT controller... Mismatching won't hurt the controller (and as long as you don't exceed the Vmax/Imax for the panels, you won't hurt them either)--However, it is difficult to predict how well a MPPT controller will collect power. They are not tested or designed to operate on a non-homogeneous set of panels with wide variations in Imp and Vmp for the strings... No idea if the controller will lock on the "true maximum" (which is still not the total panel output) or it will lock on a local maximum (the first peak from zero)--or what happens as clouds go by and such.

    It is not so much as a bad electrical mistake and more of a $$$$ mistake to not collect all of the power your array is capable of.

    If you have panels which are so far apart--your best/optimum solution is probably to purchase a separate MPPT controller for each group of panels.

    The above panels are pretty far apart to try to mix and match currents/strings.

    -Bill

    Well you are making a good case to just sell the 165's and just get 2 more 210's. That would make mounting the whole thing much easier.... Besides after doing all the math after assuming even a 10% array loss I'd essentially be paying $600 for an additional 175watts, not too bad of a price...

    Bill - What's your opinion if I did have (4) 210's running to a 12v battery bank as far as charge controller is concerned? Should I worry about that 80% overhead drop and get an 80 Amp model, or is it safe to assume that my 840 watts of panels would never be at 100% to over saturate a 60 amp model? The Flexmate 60's spec sheet does claim 900 watt support, but then claims that NEC recommendation is 750w. So what does that mean exactly? It dumps the extra, or just a precautionary warning?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,477 admin
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    Re: Need some charge controller advice
    Bill - What's your opinion if I did have (4) 210's running to a 12v battery bank as far as charge controller is concerned? Should I worry about that 80% overhead drop and get an 80 Amp model, or is it safe to assume that my 840 watts of panels would never be at 100% to over saturate a 60 amp model? The Flexmate 60's spec sheet does claim 900 watt support, but then claims that NEC recommendation is 750w. So what does that mean exactly? It dumps the extra, or just a precautionary warning?

    Several issues--As it stand right now, from reviews posted here, the FM XX family of Outback controllers still have some major issues to resolve--The MPPT's to look at right now--Morning Star good for 15-60 amp range (just introduced), Xantrex for 60 amp, and the Rogue looks pretty good for the 30 range (both have been running well for a while now). [please note--just my interpretation--I don't use off-grid -- So I have not direct experience with any of them--please do your own research]

    Regarding "ratings"... I have a couple massive issues with some the stuff in NEC regarding solar PV arrays--And one of them is the derating of already derated / Listed components.

    [note: I am using 60 amps here--but subsitute the maximum ouput/input current limits for the controller you choose--30 amps, 15 amps, etc.]

    So to require solar panel current derating by 1.25 or 1.25^2 and to derate the output (or the input) of the 60 amp regulator by 60a/1.25=48v is just a total misunderstanding of the MPPT type charge controllers--and/or conflating older controller technologies that do not have power/current regulation technologies.

    It would be like requiring your laptop power brick to be rated to handle 120 VAC and 15 amps (1,800 watts) of the branch circuit on the output for a 30 watt power brick for a laptop computer... When, in reality, all the power brick has to handle is a "short circuit" on the input and ouput and to pop the 120 VAC 15 amp branch circuit breaker and not cause a fire if the laptop end of the power brick is shorted (hope that made sense)...

    So, from my two cents--follow normal branch circuit design rules. If the MPPT controller is rated for 60 amp output (and you will have near that amount in solar panels)--then the circuit needs to be rated at:
    • 60 amps * 1.25 = 75 amps minimum (80 amps is usually the next larger size for wiring and breaker).
    Follow the manual for the input limitations... But from my point of view--if the input is rated at 60 amps (like the Xantrex is--just checking the manual)--then the solar array wiring should be rated at 80 amps (if you are planning on Isc > 48 amps for the panels). If you want to be really code compliant (meeting the actual requirements of the code)--then you could put an 80 amp breaker/fuse on the PV array output to limit the maximum short circuit current available from the array (if Isc of > 80 amps--the NEC requires a 1.25 current derating for very cold panels--which is stupid because solar panels output less current when they get cold (Vmp/Voc increases when panels get cold).

    So--practically speaking (i.e., $$$)--the maximum panel size for a reasonable high noon for various battery banks:
    • 60 amps * 14.5 volts charging * 1/0.77 panel/charger deratings = 1,300 watts
    • 60 amps * 29 volts charging * 1/0.77 panel/charger deratings =2,600 watts
    • 60 amps * 58 volts charging * 1/0.77 panel/charger deratings =4,520 watts
    The 0.77 derating is a typical value used by PV watts and accounts for wiring, controller, useful panel power at operating temperatures, etc. The peak power I have seen from my GT system would give me a derating of 0.85--but I see that only a few times a year for a short period of time... The rest of the time 0.77 is a day to day maximum value.

    And, the only "safety caveat" is that you you use a minimum of 80 amp wiring on the PV input and an 80 amp fuse/breaker if the Isc rating of the panels is over 80 amps...

    The "extra power" goes nowhere if the panels exceed the 60 amp output rating of the controller--The MPPT charge controller simply "converts" the maximum allowed power and no more. And the "excess" available energy from the solar panels is simply not used. Just like your computer supplies "don't blow up" if the computer uses a bit less power than is available from the wall outlet.

    -Bill

    PS: Add standard warning--I am not connected with any manufacturer, vendor, wholesaler, or retailer in any business--solar or otherwise.

    The above interpretations are my own based on equipment I used to design in the computer industry and how we interpreted codes, NEC, and other requirements.

    In general, you cannot go wrong if you follow the manufacturers requirements... And if those manufacturers have UL/NRTL Listings-then following their recommendations is a safe bet.

    However, even the NEC and UL make mistakes and/or have misunderstandings and sometimes even change code based on further refection.

    Please read, understand, and make your own decisions. Using adequate wire size and protective devices (fuses/breakers) when working with the high energy available from large battery banks is critical to help prevent fires and injuries.

    Solar panels are pretty tame regarding their energy output (they are natural current and voltage limited by physics). However, using series protection fuses/breakers/combiner boxes in each string, when 3 or more strings are connected together is very important to ensure that a short/failure in the array does not start a fire in the array or the wiring for the array.

    Enough of the warnings tonight. ;)

    -BB
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • stereoman405
    stereoman405 Solar Expert Posts: 56 ✭✭✭✭
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    Re: Need some charge controller advice
    BB. wrote: »
    Several issues--As it stand right now, from reviews posted here, the FM XX family of Outback controllers still have some major issues to resolve--The MPPT's to look at right now--Morning Star good for 15-60 amp range (just introduced), Xantrex for 60 amp, and the Rogue looks pretty good for the 30 range (both have been running well for a while now). [please note--just my interpretation--I don't use off-grid -- So I have not direct experience with any of them--please do your own research]

    Regarding "ratings"... I have a couple massive issues with some the stuff in NEC regarding solar PV arrays--And one of them is the derating of already derated / Listed components.

    [note: I am using 60 amps here--but subsitute the maximum ouput/input current limits for the controller you choose--30 amps, 15 amps, etc.]

    So to require solar panel current derating by 1.25 or 1.25^2 and to derate the output (or the input) of the 60 amp regulator by 60a/1.25=48v is just a total misunderstanding of the MPPT type charge controllers--and/or conflating older controller technologies that do not have power/current regulation technologies.

    It would be like requiring your laptop power brick to be rated to handle 120 VAC and 15 amps (1,800 watts) of the branch circuit on the output for a 30 watt power brick for a laptop computer... When, in reality, all the power brick has to handle is a "short circuit" on the input and ouput and to pop the 120 VAC 15 amp branch circuit breaker and not cause a fire if the laptop end of the power brick is shorted (hope that made sense)...

    So, from my two cents--follow normal branch circuit design rules. If the MPPT controller is rated for 60 amp output (and you will have near that amount in solar panels)--then the circuit needs to be rated at:
    • 60 amps * 1.25 = 75 amps minimum (80 amps is usually the next larger size for wiring and breaker).
    Follow the manual for the input limitations... But from my point of view--if the input is rated at 60 amps (like the Xantrex is--just checking the manual)--then the solar array wiring should be rated at 80 amps (if you are planning on Isc > 48 amps for the panels). If you want to be really code compliant (meeting the actual requirements of the code)--then you could put an 80 amp breaker/fuse on the PV array output to limit the maximum short circuit current available from the array (if Isc of > 80 amps--the NEC requires a 1.25 current derating for very cold panels--which is stupid because solar panels output less current when they get cold (Vmp/Voc increases when panels get cold).

    So--practically speaking (i.e., $$$)--the maximum panel size for a reasonable high noon for various battery banks:
    • 60 amps * 14.5 volts charging * 1/0.77 panel/charger deratings = 1,300 watts
    • 60 amps * 29 volts charging * 1/0.77 panel/charger deratings =2,600 watts
    • 60 amps * 58 volts charging * 1/0.77 panel/charger deratings =4,520 watts
    The 0.77 derating is a typical value used by PV watts and accounts for wiring, controller, useful panel power at operating temperatures, etc. The peak power I have seen from my GT system would give me a derating of 0.85--but I see that only a few times a year for a short period of time... The rest of the time 0.77 is a day to day maximum value.

    And, the only "safety caveat" is that you you use a minimum of 80 amp wiring on the PV input and an 80 amp fuse/breaker if the Isc rating of the panels is over 80 amps...

    The "extra power" goes nowhere if the panels exceed the 60 amp output rating of the controller--The MPPT charge controller simply "converts" the maximum allowed power and no more. And the "excess" available energy from the solar panels is simply not used. Just like your computer supplies "don't blow up" if the computer uses a bit less power than is available from the wall outlet.

    -Bill

    PS: Add standard warning--I am not connected with any manufacturer, vendor, wholesaler, or retailer in any business--solar or otherwise.

    The above interpretations are my own based on equipment I used to design in the computer industry and how we interpreted codes, NEC, and other requirements.

    In general, you cannot go wrong if you follow the manufacturers requirements... And if those manufacturers have UL/NRTL Listings-then following their recommendations is a safe bet.

    However, even the NEC and UL make mistakes and/or have misunderstandings and sometimes even change code based on further refection.

    Please read, understand, and make your own decisions. Using adequate wire size and protective devices (fuses/breakers) when working with the high energy available from large battery banks is critical to help prevent fires and injuries.

    Solar panels are pretty tame regarding their energy output (they are natural current and voltage limited by physics). However, using series protection fuses/breakers/combiner boxes in each string, when 3 or more strings are connected together is very important to ensure that a short/failure in the array does not start a fire in the array or the wiring for the array.

    Enough of the warnings tonight. ;)

    -BB

    Thanks Bill. So it does sound like I would be fine to use a 60amp rated controller as long as my other wiring / fusing is up to par. I think that if I can get just one controller that meets my needs for now I'll be fine. For future expansion I'll just plan on an additional controller.

    So to sum up as of now I'm looking at:

    - 4 x REC 210w panels (2 series strings paralleled for 56.4v IMP (840w) at the charger). Going into 12v bank.

    - Fusing / breakers for 80 amps.

    - Xantrex 60amp MPPT controlle.r

    - 1200Ah bank minimum for 12v.

    - Fusing for 175A from the bank to my inverter.

    - Xantrex XM Pro 1800 Inverter that I really shouldn't pull more than 1400watts through.

    Look feasible? Also, it looks like I might be able to get a really good price on used tel-co AGM's that would put me at 1500ah@12v, (or 3000ah @12v if I want to get two more units.) Would this setup be able to keep a 3000ah bank healthy if I were in an off grid mode if I moderate draw? I would definitely plan to add an AC based charger to float when the grid is up and to use on the genny as well. (Something larger than my current 10A Trucharger of course.)
  • BB.
    BB. Super Moderators, Administrators Posts: 33,477 admin
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    Re: Help me scale my panels / batteries correctly

    Regarding the battery bank sizing--we really try to recommend 5% minimum rate of charge for the battery banks unless you are just using solar to float them for long term storage/stand by--then you could probably get down to 1% float service--but then use house/genset for serious charging.

    See if you can find the manuals for the batteries you are looking at--they should have information on how to maintain float service.

    3,000 AH at 12 volts:
    • 3,000 AH * 14.4 volts charging * 5% = 2,160 watts
    And, for a bank that large--you should be looking at 24 or 48 volts to keep the DC current, fusing/breakers, and wiring to something reasonable:
    • 2,160 watts / 10.5 volts = 206 amps
    • 2,160 watts / 21 volts = 103 amps
    • 2,160 watts / 42 volts = 52 amps
    Having a lot of nice batteries is a very compelling setup--however, lots of batteries have their own service and support issues.

    For a large bank--I would seriously reconsider your 12 volt decision.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • stereoman405
    stereoman405 Solar Expert Posts: 56 ✭✭✭✭
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    Re: Help me scale my panels / batteries correctly
    BB. wrote: »
    Regarding the battery bank sizing--we really try to recommend 5% minimum rate of charge for the battery banks unless you are just using solar to float them for long term storage/stand by--then you could probably get down to 1% float service--but then use house/genset for serious charging.

    See if you can find the manuals for the batteries you are looking at--they should have information on how to maintain float service.

    3,000 AH at 12 volts:
    • 3,000 AH * 14.4 volts charging * 5% = 2,160 watts
    And, for a bank that large--you should be looking at 24 or 48 volts to keep the DC current, fusing/breakers, and wiring to something reasonable:
    • 2,160 watts / 10.5 volts = 206 amps
    • 2,160 watts / 21 volts = 103 amps
    • 2,160 watts / 42 volts = 52 amps
    Having a lot of nice batteries is a very compelling setup--however, lots of batteries have their own service and support issues.

    For a large bank--I would seriously reconsider your 12 volt decision.

    -Bill


    Ok, sounds like the 3,000AH approach will be a bit much. I'm looking at Absolyte IIP 6v 1500ah arrays for a great price and was thinking of getting 4 of them. Sounds like it would be perfect for a 24v setup, but a bit much for a 12v.

    To help with this 12v vs 24v struggle I'm having, is there a general rule of thumb as to how much extra I'll be paying to go 12v for wiring and fusing overhead vs 24 costs? A general percentage to factor in? (Keep in mind that Absolyte IIP's already have binding plates.)

    Option 1
    Basically my options are to get 2 Absolyte's for a 1500ah 12v bank and use my current inverters and get a beefier 12v 50+ amp AC to DC charger. (Cheapest route.)

    OR

    Option 2
    Get 4 Absolyte's for a 3000ah 24v bank, buy a new 24v inverter, a couple smaller 24v to 12v inverters, and a more expensive 24v 50+ amp AC to DC charger. ($1500 more than option 1)

    OR

    Option 3
    Similar to option 2, but skip the the Absolyte IIP deal, get (2) 12v 255ah Concords to run at 24v, new 24v inverter, smaller 24v to 12v DC converters, and a slightly smaller 25A 24v AC to DC charger. ($700 more than option 1, and 1/3 the storage capacity.)


    Bill - First, in your opinion would my current 840w of panels, a TriStar 60a coupled with an Iota 55a charger on a mixed (95% grid - 5% Honda eu2000) float AC power supply be enough to keep an Absolyte IIP 12v 1500ah array healthy? I can't imagine many scenarios where it would drop more than 10% without the genny near by.

    Second - Looking at those options and seeing the gear I'm considering, is it really worth the extra cost to go 24v? I'd like to go 24v, but from my numbers it would be roughly a $700 - $1500 cost to go from 12v to 24 at this point, and I'm already $1200 over budget.

    My goal here is to get the longest life out of my gear I can. I don't want it unbalanced to the point that items are burning up prematurely.

    The chart for these batts state that a float charge of 2.23 - 2.27v per cell is ideal@ 77F. It doesn't mention anything about a general "1% - 5%" rules of thumb so I don't really know what to do as far as scaling charging capacity with battery bank size...

    Sorry if all the math you've provided should make this easy for me to figure out. There are just so many variables it makes my head spin. If it makes you feel better I honestly spend at least 1-2 hours doing research after your responses, so I am trying to do as much of the leg work as possible.
  • bryanl
    bryanl Solar Expert Posts: 175 ✭✭✭✭✭
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    Re: Help me scale my panels / batteries correctly

    re "I know that "bryanl" does not favor them for battery bank monitoring" - the understanding is incorrect.

    Where I am coming from is from several points. One is that direct measures are better than indirect measures. Another is that proper interpretation of readings is always required. Another is that electron counters require correct programming to provide meaningful results and the precision of that programming is misleading due to the inherent variability in battery performance with many variables. Yet another is cost efficiency. Then there are the human factors issues.
    is there a general rule of thumb as to how much extra I'll be paying to go 12v for wiring and fusing overhead vs 24 costs?
    The key is probably to adjust the wiring resistance inversely to the voltage. Double voltage means you could get near similar performance with half the wire resistance or vice versa. It takes 3 AWG sizes for a factor of 2 change in resistance so, in practical terms, going from 12v to 24v will need one step smaller in available wire sizes. The copper cost is probably the only significant change, I think, for most cases.

    Another factor that could be an issue for inverters powering devices that turn on with large current inrush is impedance factors. I'd think the higher voltage would be less of an issue here.
  • stereoman405
    stereoman405 Solar Expert Posts: 56 ✭✭✭✭
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    Re: Help me scale my panels / batteries correctly
    bryanl wrote: »
    ...

    Another factor that could be an issue for inverters powering devices that turn on with large current inrush is impedance factors. I'd think the higher voltage would be less of an issue here.

    Honestly I really intend to have a 1800w inverter that is maybe utilized 70% - 80% max, and a couple native 12v items. Once in a while I might fire up a smaller power tool (700w), but really I'm pictureing the inverter running a fridge and 5-6 lights 90% of the time.

    Let me know if I'm on the right track here for figure the added wire costs in terms of 12v vs 24v.

    As for the wire expenses, I'm looking at a max of 10' from charge controller to my bank, 10' from bank to inverter, and 10' from AC charger to bank. Assuming an 1800w inverter, I'd be pulling a maximum of 150A correct? (1800w / 12v = 150A)

    So using a voltage drop calculator:

    http://www.nooutage.com/vdrop.htm

    12VDC @ 4 AWG @ 150A @ 10' = 6.5% drop
    24VDC @ 4 AWG @ 150A @ 10' = 3.2% drop

    To get the numbers better:

    12VDC @ 2/0 AWG @ 150A @ 10' = 2% drop
    24VDC @ 2/0 AWG @ 150A @ 10' = 1% drop

    Or:

    12VDC @ 4/0 AWG @ 150A @ 10' = 1.3% drop
    24VDC @ 4/0 AWG @ 150A @ 10' = .06% drop

    That's just the inverter leg. The other two legs would only be at 20A - 40A depending on charge state, so the loss would be much less.

    So I'd be in the realm of 1-2% loss through each 10' leg of the system if I used 2/0AWG throughout. I'd be paying $109 for a 10' pair of 2/0AWG vs $174 for a pair or 4/0, so $195 more for wire IF I needed to get the loss down to 1% using 4/0AWG.

    Am I over simplifying it, or is this the jist of the issue? Because if it is, sticking to 12v doesn't seem so bad considering the simplicity of my setup right? Especially since the batts I'm looking at already have binding plates. Or am I underestimating how bad an addittional 1% of loss is?
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
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    Re: Help me scale my panels / batteries correctly

    Stereoman, I am thinking that the allure of those batteries is confusing the issue. You stated early on that you were going at this somewhat backwards, and not starting with your usage. This might be the time to do a quick accounting of what your actual use in the summer might be and then compare that to the battery bank voltage/amps you need to meet that peak demand. Yes, aim for 5 % PV but if you don't have it yet, use what you have got and add as $$ are available.

    Since you have grid power available you could hookup a timer on the AC charger and let the PVs you presently have recharge during the day, if they don't do the full meal deal your charger ( a good 3 or 4 stage one) will kick in the necessary amps needed to complete the charge, a bit in reverse of what we normally do here ...where a charger is used in the AM to bulk charge the bank and then finished off with PV power... Not the best as you want to bulk the batteries asap... to avoid sulphation.

    Or set the timer to turn on in the night ~ 3 AM and get 3 - 4 hours of grid charge before the sun comes up.

    Option 1 seems to fit your other criteria , keep costs as low as possible, and use what you have.

    HTH
    Eric
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • BB.
    BB. Super Moderators, Administrators Posts: 33,477 admin
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    Re: Help me scale my panels / batteries correctly

    Bryanl,

    Fair enough--I was not trying to put words in your mouth--or trying to start any arguments. I agree that any tool, mis-used or misunderstood, is not helpful. I was just trying to acknowledge the differences in the opinions/approaches between us.

    Stereoman--You need to keep wire runs short. And if you need to send the power any distance--then you need heavy wires and/or higher voltage. 12 volts is useless with a 1.5 volt drop. The same thing at 48 volts would be a 6v drop--and since the voltage is 4x that of a 12 volt system--A 48 volt system would have a:
    • ]6v * 1/(1.5v)] * 4x the voltage = 16x the power transmission of a 12 volt system on the same cabling.
    Also, I would review your 12 volt loads... Many do not "like" to work on Deep Cycle battery banks where the voltage may go between 10.5 to 15.5 volts (near dead/long wire run to high equalization of a flooded cell bank--AGMs are probably closer to 14.5 volt max and not as much as an issue).

    Add issues of sending 12 volts any distance from the battery bank, fusing/breakers/switches, and such--I would look closely at running those 12 volt loads off a small/efficient 120 volt inverter.

    If these are continuous loads, vs part time, standby losses for the inverter, etc. all need to be looked at for your system. But an inverter+extra panel to make up for the inverter losses may be well worth the hassles and extra costs to now be just using 120 VAC and normal house hold sized/type wiring vs 12 volt.

    Then you can up your bank to 24 or 48 volts and not worry about the misc. voltage equipment.

    Otherwise--I am going have to leave to you the question of which battery banks. Large batteries certainly have their attraction... But they are also big, heavy and take extra power to keep happy.

    No absolute right or wrong answer--but, as WestBranch/Eric says--keep your eye on load needs and budget. It is easy to lose track.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • stereoman405
    stereoman405 Solar Expert Posts: 56 ✭✭✭✭
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    Re: Help me scale my panels / batteries correctly

    Thanks guys.

    Well for the sake of science it seems like the ONLY alternative here (insert grin) is to go up to my cabin this weekend with nothing but my Kill-A-Watt, some Doritos, and a case of beer and sit there and wait while I meter the load off my fridge and a couple other items. I know, I know, its torture, but for the sake of science I'm willing to make the sacrifice.

    I'll post up on Tuesday after my torturous ordeal and see if those numbers will help my confusion. ;)