Trying to design an RV system

halfwrighthalfwright Registered Users Posts: 15
First, a brief description of who I am and what I want to do. I am 66 years old and working and drawing social security, so I have some disposable income to put into a solar system ($4000). My wife and I plan on going full time in July of this year in a 36 ft. Montana firth wheel. We want to be off grid as much as we can.
We will be running two desktop computers, an entertainment center and the normal 12 volt draw—pump, lights and the like. I think that will be 40 amp hours some days and a lot of variance, from 10 t0 60. I am going to get a catalytic heater.
The system I am planning looks like this:

Four Kyocera KD135SX-UPU 135 Watt Solar Module with Junction Box

Morningstar TriStar 45 Amp MPPT Solar Charge Controller

Samlex 2,000 Watt 12 Volt Sine Wave Inverter

Six Diehard 29HM batteries with 200 minutes reserve capacity

This will all be wired with #8 AWG wire.

My questions are many and varied, but to start with the basics.
Is this system enough, too much, or doable? Do the components all “play well together”? Is my estimate of usage in the ball park? I would rather have a slight overkill than to have to rebuild a system that is too small. I have bought nothing so far, so this is your chance to design a system from scratch.
Thanks in advance for your help and I am sure that I will be back with a lot more questions.
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Comments

  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: Trying to design an RV system

    Start with the basics,,, the first basic is to conserve, conserve, conserve! The next is to make all design considerations based on the loads. Any other method is folly.

    For example,, why two desk tops instead of two lap tops,, or better yet why not two tablets? 60 ah/12 vdc is ~~720 wh. The basic off grid rule of thumb is, take the name plate rating of the PV divide by 2 to account for all cumulative system loses, then multiply that number by 4 to account for the average hours of GOOD sun one can reasonably expect, per day, on average over the course of the year. It should also be noted that in any RV application the numbers are actually worse,, and in many cases much worse,, due primarily due to the fact that the PV is seldom ideally oriented to the sun.

    So, your estimated ~720 watt/hours would need ~ 400 watts of PV. 400/2=200*4=800. A couple other rules of off grid thumb. People always over estimate their solar capacity while at the same time they underestimate their loads,, leading to a big deficit. The second rule is that off grid loads always grow with time.

    Finally, batteries need to be sized to meet the charging capacity and the loads. Ideally one would not want to discharge the batteries more than ~ 25% on a daily basis, despite what the battery salesmen tell you. Deep cycle batteries live much longer if they are subject to smaller discharges. Additionally, they should be recharge to near 100% as soon as possible, but never more than a few days. For your 60 ah loads, you would like to have a battery capacity of ~ 400ah to fit in that mold. That same 400 ah of battery would want a charge current minimum of ~20 amps just to maintain itself.

    I suggest that you read and understand the following links before you go much further.

    http://www.batteryfaq.org/

    http://www.windsun.com/Batteries/Battery_FAQ.htm#Lifespan%20of%20Batteries

    Additionally, a 2000 watt inverter is really too big for a 12 vdc source. 2000 watts from 12 volts is ~ 166 amps! Unless you have a huge battery, that battery won't be able to deliver anywhere near that kind of current without going to low voltage shut down. Consider sizing the inverter better to the loads. Larger inverters, used for smaller loads are quite inefficient.

    Welcome to the forum, good luck and keep in touch,

    tony
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Trying to design an RV system

    Welcome to the forum.

    There is one major error in your equipment list, and that is the batteries. Any battery with a "minutes reserve capacity" rating is not likely to be the right kind of battery for this application. That terminology is associated with starting batteries, which are not deep cycle and won't stand up to the repeated heavy discharge/recharge cycling.

    So let's look at it "sideways" from the POV of how much battery you can expect to support with four K135's:
    540 Watts total, @ 77% efficiency (typical) = 415 Watts / 12 Volts nominal (minimum battery Voltage you want to see to keep the discharge below 50% capacity) = 34 Amps potential peak current (the MS 45 is a good choice here). That's 10% of 340 Amp hours. So you could run a battery bank of 12 Volts at anywhere between 260 and 680 Amp hours, with 340 being "comfortably in the middle". This stays within the range of 5%-13% peak charge rate that is usually recommended. You might want to consider using four 'golf cart' 6 Volts for 450 Amp hours @ 12 Volts (inexpensive and easy to get replacements most everywhere).

    With RV systems it's a bit hard to predict the over-all efficiency as the panels are not always at the ideal angle. Fortunately there is always (or should be) a back-up generator in case things go wrong (like it rains through your whole vacation).

    There are several forum members here with actual RV experience who can better tell you what to expect in terms of loads. It's amazing how much bigger they can be than what the preliminary planning indicates! Accurate load numbers are key to any off-grid design, but the extra flexibility of the RV can work in your favour here.
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Trying to design an RV system

    Welcome, here are some quick thoughts.
    1 switch all lights to LED's, lots of savings here, but a bit pricy. Read the strings here on LED's, some sources listed.
    2 use golf cart batteries as they are designed for deep discharge ( don't know if the ones listed are GC type)
    3 use the Wire sizing spreadsheet here to determine wire size http://forum.solar-electric.com/showthread.php?29-voltage-drop-calculator
    4 use laptops as they are power sippers
    5 the entertainment system - get a Kill-a-watt meter and measure what it uses, lookout for phantom loads
    6 choose the inverter with the lowest standby load, some can be very large

    HTH
     
    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
  • VicVic Solar Expert Posts: 3,112 ✭✭✭✭
    Re: Trying to design an RV system

    $0.01,

    Agree that Golf Cart batts are probably a better value, and I like fewer cells per batt (GC=6 V vs the GR-31 = 12 V).

    It does appear that the referenced DieHard is a Deep Cycle, tho:
    http://www.sears.com/diehard-marine-deep-cycle-rv-battery-group-size-29hm/p-02827582000P

    As surprising as it seems, my batts have a reserve rating, 4290 Mins.

    Think that westbranch has it well nailed. Good Luck with the new system. Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Trying to design an RV system
    Vic wrote: »
    It does appear that the referenced DieHard is a Deep Cycle, tho:
    http://www.sears.com/diehard-marine-deep-cycle-rv-battery-group-size-29hm/p-02827582000P

    Marine/RV type aren't really deep cycle, they're a thick-plate automotive battery and don't really work well for RE applications. Better than "starting" batteries, but not in league with the "golf cart" units.
  • halfwrighthalfwright Registered Users Posts: 15
    Re: Trying to design an RV system

    Man and boy!!! Talk about quick and helpful responses, I got them.
    I am going to go through and look and research as recommended.
    It appears that I am in the ball park except for batteries and conservation. The reason I was going to use desktops is because we already have them and laptops are costly.
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Trying to design an RV system

    To add what has already been said... You can use PV Watts to estimate the hour of sun/kWH per month that you will obtain from your array based on how you mount it and where you will be traveling.

    First, for small systems, frequently making them tilt can help. For example, 1kW (1,000 watts--unfortunately, the smallest array program will accept), tilted to latitude with 0.52 end to end efficiency, at Columbia Missouri:
    "Station Identification"
    "City:","Columbia"
    "State:","Missouri"
    "Lat (deg N):", 38.82
    "Long (deg W):", 92.22
    "Elev (m): ", 270
    "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:"," 38.8"
    "Array Azimuth:","180.0"

    "Energy Specifications"
    "Cost of Electricity:"," 7.0 cents/kWh"

    "Results"
    "Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
    1, 3.69, 61, 4.27
    2, 4.73, 69, 4.83
    3, 5.10, 78, 5.46
    4, 5.71, 82, 5.74
    5, 5.58, 79, 5.53
    6, 6.04, 82, 5.74
    7, 5.97, 82, 5.74
    8, 6.03, 83, 5.81
    9, 5.47, 76, 5.32
    10, 5.08, 76, 5.32
    11, 3.56, 53, 3.71
    12, 3.23, 51, 3.57
    "Year", 5.02, 873, 61.11

    Or around 3.23 to 6.04 hours of sun per day (December/August) or 51 to 83 kWH per month. Picking February (9 month of year minimum):
    • 69,000 WH per Feb / 28 days per Feb = 2,464 WH per day per 1kW of array
    • 2,464 WH per day * 1/0.85 inverter eff * 1/12 volts = 242 AH @ 12 per February day average per 1kW of array
    If you use 0.540 kW (540 watt array), just take the above numbers and multiply by 0.540:
    • 69,000 WH per Feb * 0.54kW array / 28 days per Feb = 1,331 WH per day per 0.54 kW of array
    • 2,464 WH per day * 1/0.85 inverter eff * 0.54 kW array * 1/12 volts = 131 AH @ 12 per February day average per 0.54kW of array


    Note, 0.52 is the end to end panel to AC inverter losses. for a DC only system, the 1/0.85 takes out the typical inverter losses.

    But many people will mount the array flat to the roof, as above with 0 degree tilt:
    "Results"
    "Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
    1, 2.21, 32, 2.24
    2, 3.23, 45, 3.15
    3, 4.13, 63, 4.41
    4, 5.42, 78, 5.46
    5, 5.92, 86, 6.02
    6, 6.69, 93, 6.51
    7, 6.42, 89, 6.23
    8, 5.91, 82, 5.74
    9, 4.67, 64, 4.48
    10, 3.64, 53, 3.71
    11, 2.26, 31, 2.17
    12, 1.87, 26, 1.82
    "Year", 4.37, 741, 51.87
    Which can increase summer power, but will reduce winter. For folks that camp in winter, tilting the panel to +10 degrees over latitude will result in (40 degrees from horizontal in this chart):
    "Results"
    "Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
    1, 3.72, 61, 4.27
    2, 4.75, 69, 4.83
    3, 5.11, 78, 5.46
    4, 5.69, 82, 5.74
    5, 5.54, 79, 5.53
    6, 5.99, 81, 5.67
    7, 5.92, 81, 5.67
    8, 5.99, 82, 5.74
    9, 5.47, 76, 5.32
    10, 5.09, 76, 5.32
    11, 3.59, 53, 3.71
    12, 3.26, 52, 3.64
    "Year", 5.01, 871, 60.97

    Several things else to comment on... 8 AWG wire everywhere--Not really. You should calculate the wiring needs for each run. For the solar array, 8 AWG for all of them together may be just fine or even overkill (again, need exact configuration, wire length, series/parallel and pwm/mppt type charge controller, etc.). For wiring in sunlight, you will have to found outdoor UV rated wiring to survive the sunlight (insulation will crack/fail if exposed to Ultra Violet light).

    Also, battery bank... As said before, a 2,000 watt inverter takes a heck of a lot of current at 12 volts... The estimated wiring/fusing required to support 2,000 watts at 12 volts:
    • 2,000 watts * 1/0.85 inverter eff * 1/10.5 volts cut off * 1.25 NEC safety margin = 280 amp fuse/breaker/wiring minimum design
    If your power needs are not near as large... Morning Star makes a very nice 300 watt / 600 watt for 10 minutes TSW 12 volt inverter. It does not use much power when running, but has a couple of standby modes that use very little energy (standby and remote power down). A very nice unit for this type application.

    And to provide a couple other links. The MorningStar TS 45 amp MPPT controller and Rogue 30 amp MPPT 12/24 volt solar charge controllers are both nice units.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Trying to design an RV system
    halfwright wrote: »
    Man and boy!!! Talk about quick and helpful responses, I got them.
    I am going to go through and look and research as recommended.
    It appears that I am in the ball park except for batteries and conservation. The reason I was going to use desktops is because we already have them and laptops are costly.

    Laptops are costly - right up until you have to buy extra PV and battery capacity to make up the 100 Watt per unit difference! Dropping the desktop comp saved us 1 kW hour per day. That's like another 100 Amp hours @ 12 Volts and another K135 panel. (The panel costs as much as the laptop up here.)
  • halfwrighthalfwright Registered Users Posts: 15
    Re: Trying to design an RV system

    After reading and studying and looking some more, this is the system I now have in mind. Please tell me if it is workable for a fifth wheel trailer.

    4 135 watt KD 135SX panels

    Morningstar 45 amp charger w/remote

    Samlex 1500 watt inverter

    2 6V 3600 Exide golf cart batteries for 372 amp/hours

    So far, it adds up to around $2900, so I should be able to get the wire, connectors and all the miscellaneous and still be close to the $4000 budget I had in mind.
    I have 3500 watt generator for standby.

    I am planning on mounting the panels on the white rubber roof. Do I need to raise them 1 to 2 inches for airflow underneath for cooling? Can I do raise them with little problem while traveling? I am going to run the wiring down the refrigerator vent to the small cabinet over the 'fridge. Can I mount the charge controller in the cabinet
    (18X20X12) or does it need more air flow for cooling? I can drill holes in the back into the vent. Can I wire the inverter into the charger's positive pole and mount it in the cabinet also or do I need to run another wire to the batteries?

    And, this is just the beginning of my questions. Wait until I get started and then I will spend as much time asking questions as I do working.

    Thank you for all the help.
    Jim Wright
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Trying to design an RV system

    Looks to me like the equipment will all work electrically. About 1 kW hour per day with good conditions.

    There's some forum members here who have good RV installs. You might want to look at this thread: http://forum.solar-electric.com/showthread.php?14178-Proper-railing-framing-of-panels-on-a-motor-home-roof-Help-please
    bluewickedburner has it done well and successfully.

    The charge controller and inverter both want good air flow. The closer they are run to their maximum capacity the more heat they will generate. The inverter should be wired to the batteries through a fuse/breaker, not to the charge controller. It helps to think of the DC wiring as three separate but interacting circuits: panels to controller, controller to battery, and battery to inverter.
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Trying to design an RV system

    Here is a smaller system: A nice thread with video from Kevin in Calgary Canada that shows designing and installing solar PV in a small RV trailer.

    By the way, is this a MPPT or PWM MorningStar charge controller? And have you thought about a Battery Monitor of some sort?

    Is there anyway you could get away with a smaller inverter (MorningStar 300Watt TSW 12 volt inverter is ideal for small systems; has low power "search" mode and remote on/off power switch) and a smaller inverter/generator genset (Honda eu1000i/eu2000i/Yamaha 1000-2000+ series)?

    A 1,500 watt inverter running near full power can use a day's worth of collected power inside of an hour or so of operation at 700-1,000+ watts. Also, if you can find an inverter with standby mode, it will save you another 10 watts or so without having to manually shutdown the inverter.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Solar Expert Posts: 9,361 ✭✭✭✭✭
    Re: Trying to design an RV system

    possibly use 2 inverters, one for small loads, other for larger ones.

    The MorningStar 300 Watt, is spec'd to deliver 600W for 10 min (depends on how hot it gets, mounted to a large heat sink, you may get more time out of it) so that may serve most of your loads.

    The MorningStar 45 MPPT, does not have the webserver hardware in it that the 60A MPPT does, if bells and whistles mean much to you. Also, they do NOT come as shown, with the digital meter panel on the front - it's an add-on for more $. So maybe seeing the data on a web page, may cost as much as adding a meter to the 45A

    You will need a 12V battery charger for when you run the genset. I don't think the samlex has charger in it. Gensets have lame 12V outputs mostly. Maybe a 40A / 12V smart automotive charger would work for you ?

    Battery Monitor (amps in vs amps out) totalizer may work for you, if configured properly, or a simple DC Voltmeter, if you read it properly, would work too.

    Make sure your 12V appliances, and inverter, can handle the 16VDC applied when charging or Equalizing. Vehicle movement should deal with stratifaction issues for you.
    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 ,

  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: Trying to design an RV system

    Just an observation. With the battery bank you have you are going to be hard pressed to run anywhere near 1500 watt AC from your inverter, which suggests A: that you look closely at your loads, and determine what you expect/can run off your battery, and B: consider an inverter that is more closely loaded for the battery and the loads.


    372 AH of battery will deliver ~ 1800 wh of power, to a 50% DoD. That said, it won't deliver 1500 watts of power because it is too small to produce that amount of power that quickly. You could deliver that 1800 wh over the course of a day,, but not over the course of an hour for example. Even if you could, the inverter is going to go to low voltage shutdown, and if you do it very often you will cook the battery with such large draws.

    As always, all calcs begin with the loads (and their duration(s) Do an realistic load calc, figure your max load (per item like microwave or pump) and size the battery accordingly. Then design a charge regimen to match.

    Tony
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Trying to design an RV system

    To put some numbers to what was said above... Assume around C/8 as maximum sustain power and C/2.5 as maximum surge power:
    • 372 AH * 12 volts * 1/8 discharge rate * 0.85 inverter eff = 474 watts sustained (for roughly 4 hours and 50% maximum discharge)
    • 372 AH * 12 volts * 1/2.5 discharge rate * 0.85 inverter eff = 1,518 watts maximum surge (i.e., starting load for appliance)
    Since many inverters have 2x rated surge current, that would suggest that around 474 to 750 Watts would be the maximum recommended inverter for that bank.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: Trying to design an RV system

    I'm guessing that a 475 watt draw (~35-40 amps) is going to force the inverter to low voltage shut down long before 4 hours.

    Tony
  • halfwrighthalfwright Registered Users Posts: 15
    Re: Trying to design an RV system

    Bill,
    it is the MPPT charger. It does have a remote monitor that I figured into the price. The reason I am thinking of a 1500 watt inverter is because of a 1000 watt microwave that she who must be obeyed insists will be usable. That will be the only really large load on the inverter. We are going to change over to laptops and still take the desktops for use on the rare occasion we are plugged in.

    Mike,
    I had toyed with the ides of two inverters, but I would still need the larger for the microwave. I am hoping that under a low load, it will be efficient enough. Bells and whistles do not mean much to me. I really prefer the old KISS system (keep it simple, stupid.) I am going to get the remote monitor so that I can keep track of the batteries. After some experience, I am hoping that the system will not need much monitoring.
    There is a 12 volt charger built into the 120 system that charges the battery when the generator is running or on shore power. Am I going to have to bypass it? The 12 volt stuff is lights and water pump. The inverter is rated to 16 V, so I should be OK there.

    Tony,

    I am really in a quandary here. Do I need two more batteries? Would the panel output keep two more batteries charged? The largest load, the 1000 watt microwave, will not be used except for reheating. It will run way less than an hour a day.

    Bill,
    You seem to agree with Tony that the inverter is too large. So, my question to you is the same. Would the panel output keep a larger, say, another 372 amp/hr, battery bank up? I have room to add two more panels, but that means a bigger charger. Does anyone know how to convince a woman she can live without a d---- microwave?

    Thanks for all the help
    Jim Wright
  • Blackcherry04Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: Trying to design an RV system

    I'd check your Amp Hr estimate on the batteries you listed . If I am not mistaken they are about 220-235 amp hrs @12 V. Not a big deal if they meet your requirements. If you come off shore power and they are at 100% and you pull them to 50%, then put back in a bulk charge you really only have 35 % ( 77 amp hrs or so ) to play with. If the Solar is sufficient to get you back to 100% then you'd be ok
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Trying to design an RV system

    Microwaves consume large amounts of power (around 1,200 to 1,800 watts; some maybe down to 850 watts). However, they are only operated for ~10-20 minutes per day for many people... So the amount of storage/solar array needed to support them is not necessarily huge:
    • 1,200 watts * 10min/60min per hour = 200 Watt*Hours
    So, 200 WH, while not small, is certainly within the capabilities of your solar panels on a typical sunny day (again, winter/summer seasonal question and where will the camping be done).

    But the typical flooded cell lead acid battery cannot support large surge/current requirements very well.

    If, you go with AGM batteries (Concorde Lifetime for example), they are rated for upwards of C*4 rates of discharge (i.e., the type of use you see in a UPS which will run 15 minutes to dead).

    An AGM battery bank would allow you to reach your peak wattage/current loads without making the physical bank 2x larger... However, AGM batteries are probably 2x the price and may not last quite as long as normal storage batteries.

    Also, large currents for 12 VDC appliances means large diameter wire and heavy fuses/breakers. A 1,500 watt inverter on 12 volts should be sized:
    • 1,200 watts * 1/0.85 inverter eff * 1/10.5 battery cutoff * 1.25 NEC safety factor = 168 amp minimum branch circuit
    1,200 watts is around the maximum, I would suggest, as a 12 volt load... If you could operate at 24 volts, that would drop to a 84 amp circuit instead (less copper, smaller fuses/breakers, etc.). Of course, you have to round up to the next available wiring/breaker/fuse ratings.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: Trying to design an RV system

    A 1000 watt micro, drawn for 1 hour is 1kwh.

    372 ah of batteries at 50% DoD is ~2.2kwh so (even if the battery can deliver that 1kw load) you 1kwh load is nearly 1/2 your batteries safe capacity.

    What you need is more battery, and more PV, and dare I say smaller loads. What you really need is to do a real life daily load calc, and design a system (battery/PV/inverter/charger) that fit well together.

    Tony
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Trying to design an RV system

    Hmm. What exactly do you cook in a microwave for an hour?
    I use mine on 320 Amp hours of 24 Volt batteries. There is no problem.
    Do not expect the system to manage 1500 Watts continuous, nor to supply maximum power even briefly when the batteries are low. Your best bet for high power usage is when the batteries are at or near Float and the sun is still shining on the panels.
    One thing is certain; you ca not run a 1000 Watt microwave for even 1 second on an inverter not capable of over 1 kW, no matter how much battery bank you have.
  • vcallawayvcallaway Solar Expert Posts: 157 ✭✭✭✭✭
    Re: Trying to design an RV system

    My RV setup:

    4 * 6v Golf Cart batteries (Costco)
    2k Xantrex Prowatt SW inverter
    240 Watt Panel (24v)
    Morningstar MPPT 15L charge controller.

    I've never tried to run the microwave off battery. Don't want to waste the battery power.

    Solar panel is flat mounted. 4 ga wire goes from the combiner (mounted under the fridge vent cover) to the charge controller. 8ga wire to the batteries from there. 00 wire runs to the inverter.

    I run a residential refrigerator which consumes 1kw a day max.

    According to the display on the inverter my TV, Sat box, speakers, inkjet printer and mini-computer pull 130w with all switched on. Just checked to make sure. The mini computer and TV pull about the same as a laptop. The 32" screen is nice when surfing the web.

    All of our light fixtures have been replaced with LED. We have a bunch of push on battery powered ones as well.

    When dry camping I turn the refrigerator off at night, depending on ambient temps. When it is cold the furnace is the number one power hog. We have a Mr.Heater rated for indoor use that we run in the really cold temps.

    In my experience the number one mistake in RV solar is under sized wire.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Trying to design an RV system

    fyi, it was asked that we split this off to a new thread, but i do not see the need as you merely ran down an inventory of what you have to show as an example to the op.

    i will comment on the use of that #8 wire from the cc to the batteries as not making any sense and would be too small. did you get the combiner to cc and cc to battery wires mixed up?
  • vcallawayvcallaway Solar Expert Posts: 157 ✭✭✭✭✭
    Re: Trying to design an RV system

    The run from the combiner box to the charge controller is almost 30'. The use of the 4ga wire was to reduce voltage drop. There was not a good location to mount the charge controller other than near the batteries. The wire from the CC to the batteries could have been smaller but I did not want to re-do things if I increased my capacity.

    During warmer weather this system provides all the power we need. Winter not so much. Heat, more time spent inside with computers and TV plus lower solar harvest all add up to needing more power.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Trying to design an RV system

    ok i'll take your word for it that all is fine. just be sure that v drop % from the cc to the batteries is under 1% and preferably under .5% to prevent the cc from being fooled into thinking the batteries are charged when they aren't.
  • halfwrighthalfwright Registered Users Posts: 15
    Re: Trying to design an RV system

    The weather has not been condusive to outside work, but I finally got my 4 140 watt panels (wired in Series) mounted on the roof of my trailer.At the end of the #6 AWG wire, right where the charger will go, I am getting 23.8 amps at 20.9 volts. The specs say the max should be 22.1 volts and 31.6 amps. I know that the specs are an ideal situation. My question is " Are these readings within a normal range?"
  • halfwrighthalfwright Registered Users Posts: 15
    Readings compared to the specs

    I have finally got 4 Kyocera KD140SX-UFBS 140 Watt panels mounted on the roof of my fifth wheel. The readings I am getting at the end of the #6 AWG wire, where the charger will go are
    23.8 amps
    20.9 Volts

    the specs are

    34.7 amps
    22.1 volts.

    I know the specs are an ideal situation. My question is "Are these readings within a normal range? Are they what I should expect to see?"

    Thanks for your help
    Jim Wright
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Readings compared to the specs

    Sounds a bit low--Are the panels pointing at the sun (cosine of angle to sun--reduction in current output). Also, does the controller say it is in MPPT mode right now? And is the sky clear/sun feel hot/bright?

    You are looking at ~65% of rated output right now. In good conditions, you should be at ~77%-82% of rated output. Great conditions >85% of rated output:

    23.8 amps * 20.9 volts / (34.7 amps * 22.1 volts) = 0.65


    Oops... Actually, you are looking at the wrong numbers for the panels, I think:



    Maximum Power Voltage (Vmpp)

    17.7V


    Maximum Power Current (Impp)

    7.91 A





    Open Circuit Voltage (Voc)

    22.1V


    Short Circuit Current (Isc)

    8.68A



    23.8 amps * 20.9 volts / (4*7.91 amps * 17.7 volts) = 0.88

    So, that is just about what I would expect for very good conditions.

    Voc and Isc are open circuit and short circuit numbers--Not the point for Maximum Power.

    -Bill

    Please do not duplicate questions in different threads--It makes it a bit confusing about where we should reply and continue the conversation.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • halfwrighthalfwright Registered Users Posts: 15
    Re: Readings compared to the specs

    I do not have the controllor wired in yet. The way I got the amp reading was by shorting out the wires from the panels and used an amp meter. the voltage I used a multi-meter. The panels are flat mounted on the roof. It was shortly after noon when I took the readings, clear and bright. I can see how that could change the amp reaing, but not the voltage.
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Readings compared to the specs

    If that was shorting out the panels--Then yes, I would expect closer to Isc.

    Voltage is very temperature dependent... So, hot panels with little air circulation, yes, you can see significant Voc reduction (I guess you are checking each panel at a time?--And hopefully all panels are within 5-10% (worst case) of each other?



    Temperature Coefficient of Voc

    -0.36 %/°C


    Temperature Coefficient of Isc

    0.060%/°C



    -0.36 %/°C * 10C rise (18F rise) * 22.1 volts = 0.80 volt depression from STC ratings

    You can see upwards of 30C rise (~54F) with flat panels and no wind/air circulation.

    In general, the Isc of a solar panel should be a very good representation of the amount of energy hitting the solar panel (within ~5% or better).

    23.8 amps / (4*8.68 amps Isc) = .69 = ~69% of standard sun (1,000 W/m2)

    Cos-1 (0.69) = ~47 degrees from direct noon time sun (Cos 0 degrees=1.0)

    So, you may have a combination of off-center sun and less than clear sky (humidity/dust/haze)?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ShadowcatcherShadowcatcher Solar Expert Posts: 228 ✭✭✭
    Re: Readings compared to the specs

    One of the better, and more entertaining solar/RV guru's is Handy Bob, who is just a bit opinionated but has lots of experience making solar work in an RV.
    https://handybobsolar.wordpress.com
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