Schematic review, please

Beekss
Beekss Registered Users Posts: 4 ✭
in Caravan, Recreational Vehicle, and Marine Power Systems #1
We have a 22' travel trailer, primary use is recreation though my wife has an occasional need to work while on the road. I've already installed a 12v/206 Ah Lithium Ion battery and converted the onboard converter/charger/power distribution center to be compatible with the battery, the next step is to install the solar side. I would like to have flexibility to add more solar and perhaps another battery down the road. Most camping will be spring/summer/fall in areas that are above freezing during the day and only occasionally below freezing at night. Inverter use will be occasional to brew coffee in the morning, run the microwave infrequently, and to charge a laptop computer. Diagram attached, I'm interested in comments on the overall design, and whether the wire sizes and fuse/breaker/shutoff locations and sizes are reasonable. Thanks in advance!
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,626 admin
    #2
    Welcome to the forum Beekss,

    Your photo of the drawing is pretty difficult to read your text... Can you post another high resolution version?

    A few suggestions otherwise... Using circuit breakers instead of switches+fuses may be a nicer solution. One device does both. And you don't have to carry around fuses for backup.

    For your array--Fine as is. You really do not need the array fuse in this configuration. If you add another string in parallel (2x more panels), you should have a "combiner box". Usually, with 3 or more parallel strings, you need to have a series fuse/breaker per string to prevent a short circuit in once string from being fed "too much current' (exceeds series fuse rating in panel specifications). You could add a box now to make additional panels easier.

    https://www.solar-electric.com/learning-center/solar-combiner-boxes.html/

    You probably want to add a disconnect switch/circuit breaker for your DC input to the inverter... It is handy to be able to turn off (disconnect) your AC inverter (and other DC loads) when shutting down for the winter (leave the solar panels+charger connected to battery bank--assuming trailer is stored "in sun").

    You might also look at "large format" solar panels (typically >200 watts). They are usually 1/2 the cost of the smaller (<150 Watt) "12 volt" solar panels. Watch their Voc/Vmp voltage ratings... They are typically Vmp~30-36 volts and placing two in series for a 100 VDC solar input MPPT solar charge controller may be too high of input voltage.

    And how series are you about 2,000 Watt PSW inverter (or larger)? At 12 volts, 2,000 Watts is serious current. For larger systems (typically over 1,800 Watts), you may want to look at a 24 VDC system.

    On the other side, you may want to look at a smaller AC inverter, or even a second "small" AC inverter (200-300 Watts) for lighting and charging battery power devices/laptop computers... Larger AC inverters can take significant amounts of power (Tare Losses) when running smaller AC loads... A large inverter may take 20+ Watts "just turned on". A smaller 200--300 Watt AC inverter will draw around 6-8 Watts. This adds up if running your loads 8+ hours a day for work. Many folks do OK with a large 12 VDC inverter (even up to 3,000 Watts)--But if you can avoid the heavy copper cables, large fuses/breakers, and the Tare Losses of the large inverter--It is something to think about.

    For loads--There are both 12 and 24 volt RV water pumps and even bulbs. You may be able to use "native" 24 VDC power for your RV house loads, and stick with 120 VAC for the rest of the system (especially if you use a smaller AC Inverter) for the "low power" loads.

    Depending on where and when you will be RVing... Are your panels mounted so they can tilt for better harvest (if more power needed)?

    Knowing your loads (Watts*Hours per day) can help you design a system that meets your needs... For example, Denver Colorado with a 600 Watt solar array mounted flat:
    http://www.solarelectricityhandbook.com/solar-irradiance.html

    Denver
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a horizontal surface:

    JanFebMarAprMayJun
    2.41
         		3.27
         		4.49
         		5.42
         		6.28
         		6.70
     
    JulAugSepOctNovDec
    6.35
         		5.68
         		5.03
         		3.90
         		2.67
         		2.18
     

    Looking at:
    • 600 Watt array * 0.61 off grid Lithium battery AC system eff * 3.27 hours of sun Feb Average) = 1,197 WH per day Feb "break even" harvest
    • 1,197 WH per day (Feb) / 12 volt loads = 100 AH per day harvest (Feb)
    For off grid living--Watch those loads that run 12 or 24 hours per day... They can take a whole lot more energy (WH) per day than the larger loads... For example:
    • 1,200 Watt microwave * 0.3 hours per day (20 minutes) = 360 WH per day
    • 900 Watt water kettle * 0.3 hours per day = 180 WH per day
    • 30 Watt laptop computer * 10 hours per day = 300 WH per day
    • DC Fridge/cooler may take upwards of 500-800+ WH per day
    • Typical RV heater 8 amps * 12 volts * 12 hours per night * 0.50 duty cycle = 576 WH per night
    All these loads add up pretty quickly. If you plan on longer stays dry camping (no vehicle charging for week+ at a time, little to no genset usage), then you need to watch your power usage, as well has harvest (no shading during day--Solar panels have little output even if partially shaded).

    You can use a Kill-a-Watt type meter for AC power usage (measure your loads at home for computer, cell phone charging, microwave usage, water boiling, etc.)... For DC usage there are lots of DC AH/WH meters out there too:

    https://www.amazon.com/s?k=kill+a+watt+meter
    https://www.amazon.com/s?k=dc+watt+meter

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Beekss
    Beekss Registered Users Posts: 4 ✭
    #3
    Thank you Sir - See if this one looks any better, some of your suggestions are incorporated. In response to some of your questions...
    • Inverter - looking at the 2000 so that it can run the microwave when needed, which is likely seldom. Otherwise, a coffee maker once a day and charging up the laptop. I do have a generator, but would prefer to limit it's use or even leave it at home.
    • Panels were selected somewhat on size, as the real estate on the roof is somewhat limited. I would be interested in looking at the large format panels if you have any links or suggestions for ones that won't be too large for the controller. I'm a bit gray on selecting a controller, so I would welcome any education along those lines, and how I would size one to a higher voltage array.
    • Another question on panels - can I use 24 volt, or am I limited to 12? I think that I understand that input voltage to the charge controller can be higher than 12 volt, and that it limits discharge to 12 (or 24 if set up that way).
    • Not planning on tilting the panels, at least for now.
    Thanks again!
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #4

    Sorry, New computer, I couldn't do full res, but did a clip so others might reference it without opening PDF.

    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,626 admin
    #5
    If the 150 Watt panels fit your roof better than large format panels--That is fine. Either type of panel will pretty much have the same Watts per sq foot harvest for your system. "Small" footprint panels are certainly much cheaper to ship vs large format panels (packaging, insurance, truck shipping for a single large panel can double the purchase price). Looking for what is available locally to you--A good place to start.

    The choices you have made for panels, controller, etc. are all fine. The bit of fuzzy always revolves around designing a system for future expansion. It is sort of like buying a VW Bug with an idea towards future expansion--The original design is optimized for what you have today. To expand a lot--Means, potentially, lots of changes to the existing system.

    So, what are those possible expansions? A battery bank with one or two batteries? That is certainly something you can do without too much issue. Li Ion batteries are very forgiving about rates of charge. Just stay within in the voltage and current specifications for charging (and discharging), and you are fine. Lead Acid batteries tend to "want" around 10% to 13% rate of charge--So a larger bank really wants more solar panels/alternate charging source(s) for the best life.

    Solar panel Vmp and Voc change with temperature... The panels are rated at 25C/77F... Panels get hot (full sun mounted on roof on a hot day), the cells get hot and Vmp/Voc fall... Vmp can fall to almost 80% in such conditions.

    On the other had, Voc and Vmp rise as panels get colder... In sub freezing temperatures, it is possible for Voc-array-cold to exceed Vpanel maximum input voltage for the MPPT charge controller. As an example, say your array could fall to zero F/-18C. A typically value for temperature correction with solar panels is around -0.33% per degree C (25C standard temperature):
    • Voc * # of series panels * temperature correction = array voltage
    • 22.7 Volts Voc * 3 panels in series = 68.1 volts Voc-array
    • 68.1 volts Voc-array * (-18C - 25C ambient) * (-0.0033 per C temperature correction) = +9.7 Volt Voc-cold rise
    • 68.1 volts Voc-array-std + 9.7 volts Voc-array-offset = 77.8 Volts Voc-array-cold
    So, with your "12 volt" solar panels and a 100 Volt max input charge controller, you could easily put 3 panels in series down to 0F with lots of margin.

    And there is sizing the MPPT controller for the array... MPPT controllers (at least the good quality such as Victron) can take larger arrays and will safely/by design, limit their output current to the rated value (50 amps in your case). So the (more or less) "optimum" array for that controller on a 12 volt battery bus would be:
    • 50 amp MPPT charge controller * 14.5 volts charging * 1/0.77 panel+controller deratings = 942 Watt "max optimum" array for this controller
    • 942 Watt array / 150 Watt panels = 6.28 = 6x 150 Watt panels
    • 3 series * 2 parallel strings = 6 panels
    • 2 series * 3 parallel strings (with combiner box) = 6 panels
    Either configuration works--3s x 2p -- You don't need the combiner box

    2s x 3p -- You need the combiner box (fuse/breaker) to protect panels against short circuits. Also, for a 12 volt battery bus--The 2s x 3p is "slightly" more efficient for the MPPT charge controller (I would not worry about the extra/loss of efficiency--Maybe 1-2% difference).

    Notice with the MPPT charge controller... It will work with a 12 or 24 volt battery bank. On a 24 volt battery bank, the same contorller will support a 2x large array (power=voltage*current -- Controller has current limit, but 2x voltage is 2x power).

    Regarding the maximum "optimum" array. We use around 75% to 77% derating factor. Hot panels, Vmp falls, Pmp=Vmp*Imp -- So panels typically only produce ~77% of their rated output. You could have even a larger array--The controller will just "clip" its output current (to 50 amps max @ room temperature) and you will lose a bit of harvest at this time... Not a big deal--Lots of sun with solar--Generally you have more power than you need anyway--It is those cloudy/rainy/short winter sun days that "kill" solar harvest.

    Generally we aim at 2-3 days of battery storage for a cabin/home. For RVs with lead acid, sometimes just 1 day of storage because of RV limitations for weight and size. With Lithium, you can go >3 days because the batteries are light weight and small--But $$$.

    For example, if you want to to have 2 days of energy storage (bad weather), your battery would supply (note: I use 90% - 20% Max/min charge for lithium batteries, others may use 95%-10%=85% of capacity--Your choice):
    • 200 AH * 12 volts * 0.85 AC inverter eff (if 120 VAC used) * 0.70 of battery capacity * 1/2 days = 714 WH of AC battery power per day
    • 200 AH * 0.70 battery capacity * 1/2 days = 70 AH per day (for two days)
    Your power needs... A propane refrigerator generally uses very little DC power on propane (running a 3 way fridge on 12 VDC or 120 VAC--That is a power hog--Not normally a good idea--12/24 VDC Compressors refrigerators are much more electrically efficient if trying to get away from propane).

    60 Watt water pump--Generally not a big draw. Note: Besides Watt draw (power or Amps @ xx volts), there is total energy consumption (Watt*Hours or Amp*Hours)... For the RV pump... maybe 5-10 minutes per day:
    • 5 amps * 1/6 hour (10 minutes) per day = 0.83 Amp*Hours per day
    • 0.83 AH * 12 volts = 12 Watt*Hours per day
    Things that run a long time can be a solar "killer"... For example your router at 24 Watts (power)... If you run it 2 hours per day vs 24 hours per day:
    • 24 Watts * 2 hours = 48 WH per day
    • 2 amps * 2 hours = 4 AH per day
    • 24 Watts * 24 hours per day = 576 WH per day
    • 2 amps * 24 hours = 48 AH per day
    And this does not even include the computer/laptop/tablet/etc. power usage. Just leaving the router on 24 hours per day uses > 1/2 of your 2 day battery capacity plan.

    And another solar killer is the RV furance. Assume running overnight and 50% duty cycle:
    • 7 amps * 12 hours * 0.50 duty cycle = 42 Amp*hours
    Out of a 70 AH per day (2 days) battery energy plan...

    During sunny days, your 600 Watt array will do fine... During poor weather/heading into winter/northern latitudes, not so much "extra" energy.

    If you have a good propane supply--Using the propane stove to heat water vs an electric kettle--Can also help save power.

    During dark days (stormy weather), your solar array harvest can be down to 5% to 10% of your typicaly sunny day harvest--Not much help (genset, cut back on optional energy usage, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Beekss
    Beekss Registered Users Posts: 4 ✭
    #6
    Super helpful, thanks! You asked about future expansion, maybe two more panels and an additional 206 Ah battery.

    And thanks to Photowhit for converting my PDF!
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,626 admin
    #7
    You are very welcome...

    Some other random comments.

    First, wiring that "leaves" the battery bus (source of very high levels of surge current) should be protected by a properly sized fuse/circuit breaker. The breakers/fuses should be installed "near" to the current source (less chance of a short circuit along the length of wiring).

    You can use larger than needed wire diameter--Frequently done to lessen voltage drop. For example, suggest a maximum of 0.5 volt drop from battery bus to loads. And a maximum of 0.05 to 0.10 volt drop from "battery charger" to battery bus--To ensure accurate battery voltage feedback to the charger (i.e., you don't want to charge at 14.5 volts at the charger, but only deliver 14.0 volts to the battery bank because  of a 0.5 volt drop).

    You have 4/0 wiring from battery bus to OEM DC panel/AC shore power battery charger.... But no breaker/fuse shown.

    An example of voltage drop calcuation. Say 2,000 Watt AC inverter @ 12 volt input with 5 feet (one way run) of wiring. Using a simple drop calculator:
    • 2,000 Watts AC output * 1/0.85 AC inverter efficiency * 1/10.5 inverter DC battery cutoff = 244 Amps max (2x that for surge current support)
    12 volts nominal @ 244 amps max continuous current @ 5 feet using 2/0 copper cable

    https://www.calculator.net/voltage-drop-calculator.html?necmaterial=copper&necwiresize=10&necconduit=pvc&necpf=1&material=copper&wiresize=0.4066&resistance=1.2&resistanceunit=okm&voltage=12&phase=dc&noofconductor=1&distance=5&distanceunit=feet&amperes=244&x=58&y=23&ctype=nec

    Result

    Voltage drop: 0.25
    Voltage drop percentage: 2.05%
    Voltage at the end: 11.75

    And a handy simplified NEC AWG ampacity chart:

    https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

    2/0 copper is good for ~145 to 195 Amps depending on insulation type used. The NEC chart is pretty conservative, and if the cable is exposed to free air instead of conduit--Less self heating effects.

    https://www.bluesea.com/resources/529/Allowable_Amperage_in_Conductors_-_Wire_Sizing_Chart

    As always, it is a good idea to start with the device's manual (inverter, chargers, etc.) for proper installation instructions.

    And, generally, no need for both a Circuit Breaker and an ANL fuse in series... Just use the appropriate Circuit Breaker should be enough.

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Beekss
    Beekss Registered Users Posts: 4 ✭
    #8
    Revisiting as I get closer to building it out. You said "You have 4/0 wiring from battery bus to OEM DC panel/AC shore power battery charger.... But no breaker/fuse shown." How do I size the fuse/breaker? By estimating my actual load, or the potential load? The OEM converter has about 8 15 amp fuses in it.

    Thanks again
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