Help with panel and charge controller selection/design

Adding solar to travel trailer.  We use it roughly twice a month during summer, so maybe 5-6 outings a year.  Mostly boondocking in Colorado.  Current setup: 12v system, two 6V Trojan t145 in series (260 A-hr bank total).  No inverters yet, but I've done big fat 4/0 wiring to battery, planning for a future one.  I've monitored our DC usage and it's anywhere from 60-90 Amp-hrs per 24 hr period.   Biggest wildcard is the furnace blower, if it's cold, and it often is at 10,000', that runs half the night at about 6 amps.  Have a 3500 watt generator that I drag with us, but I only like to use it an hour or so in the morning, then leave and go about my day.  So I'd like solar to charge us while we're out for the day.

I'm almost out of time before this camping season so my plan is to use two 200w 12v "portable" panels this year.  Portable meaning not attached to roof yet. Just manually laid out in the sun and connected up with MC4 connectors to the cc.  Next year I'll get them permanently wired on the roof and add two more for a total of four 200W panels, 800W.  Initially, I was thinking a Morningstar Pwm 45a cc.  I like the morningstars because they're passively cooled and they have a good rep.  But I see an awful lot of advice to use an MPPT cc. I still like the simplicity of pwm, just wire them all in parallel, shading effects (happens a lot where I camp) minimized, pwm is inexpensive, etc. 

The 12V panels I've found:  Hightec Solar RCL-M200w.  200 Watts.  $185 each.  Vmpp=21.052V, Impp=9.5A, Voc=24.335V, Isc=9.87A.  I've read that Vmpp of nominal 12V panels are usually in the 17-18v range so the Vmpp on this one surprised me. Is it too high?

No panels or charge controllers purchased yet.  Wanted to bounce my design idea off the outstanding knowledge base here and see if I'm heading in a bad direction or if there are alternatives I should consider.  Also, im trying to keep this DIY simple and at a reasonable cost.  

Thanks, in advance, for any guidance.


  • mike95490mike95490 Solar Expert Posts: 9,089 ✭✭✭✭✭
    "test drive" the panels.  Go to a solar store, lift up a panel.  Can you carry it down the RV steps ?  Can you slide it into it's storage space? do you need 2 people to move it ?   100w panels are more portable than a 50# 350W panel.

    For a couple trips a year, is it really worth it for solar power?  Small, silent 1kw portable inverter generators are really handy.
    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

    gen: ,

  • speedbrakespeedbrake Registered Users Posts: 10 ✭✭
    Understand your concerns but I'm not deciding whether to do solar or not, that's been decided.  I would just like some help with design.  And yes, I'm familiar with the height weight bulk you detail.  In my case, a 5' x 2' 2" panel weighing 32 lbs.  I got this.  ;)
  • BB.BB. Super Moderators, Administrators Posts: 31,157 admin
    Ok... A little math here (yes, there will be math).

    Let's make some assumptions. 90 AH per day @ 12 volts. And 6 amp heater 12 hours per night, at 50% duty cycle. With a 12 volt 260 AH battery bank.

    Typically, we use 2 days of storage and 50% maximum discharge from the battery bank for Off Grid Cabins and Homes--That is 4x the daily usage of the storage battery. Gives you long(er) battery cycle life

    For RVs, you may want to design for 1 days storage and 50% discharge... RV batteries tend to "age out" before they "cycle out", and given the limited about of space and weight limits, RVs cannot just "throw more lead" and solar panels at the problem. Generally, if you have poor weather or heavy loads from the day before, you may just choose to fire up the genset when needed to charge/run that day's loads.

    For an RV system (weekend/few weeks a year usage), planning on cycling the FLA (flooded cell lead acid batteries) between 50% and 80% state of charge is a "reasonable" design goal (charging from 80% to 100% state of charge can easily take 4-8 hours by itself--May not be enough hours of sun in a day, and you may not want to run the genset that long lightly loaded (noise, fumes, fuel).

    Your battery bank:
    • 260 AH * (0.80 - 0.50 state of charge cycle) = 78 AH of typical cycling capacity.
    And if you "stress the battery bank" for an 80% to 20% State of charge:
    • 260 AH * (0.80-0.20 SoC cycling) = 156 AH
    So, your 60-90 AH over 24 hours is not a bad expectation. And:
    • 6 amps * 12 hours overnight heating * 0.50 duty cycle = 18 AH overnight cycling
    Again, not bad overall.

    And what would you expect from your 400 Watts of Solar Panels (fixed array optimally tilted by season)--Pick Denver, you need to find major city near your camping site (10,000 feet, probably much better harvest unless you are on the north flank of a mountain):

    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel where the angle is adjusted each month to get optimum sunlight.

    A 400 Watt panel tilted to optimum angle (more vertical in winter, more horizontal in summer)... Pick December as typical winter month:
    • 400 Watts * 0.61 DC battery based solar system eff * 4.56 hours of sun (ave Dec) = 1,113 WH per average Dec Day
    • 1,113 WH per day / 13.6 volts float voltage =~ 82 AH per day harvest
    While I would be suggesting that you would only "plan" on 50% to 65% of daily predicted harvest (to minimize genset use), certainly in good weather you are pretty close to breaking even in good December weather with your planned setup.

    If you where looking for the "typical maximum" solar array for your present battery bank--Assuming ~13% rate of charge max:
    • 260 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = ~636 Watt "cost effective" maximum array
    All in all, not bad for an RV system.

    Your thoughts?


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • speedbrakespeedbrake Registered Users Posts: 10 ✭✭
    I LOVE math! (I actually do, I'm probably a little nerdy that way) 

    Good point about RVbatteries  "aging out" before they cycle out.  I hadn't considered that before.  With my infrequent usage, my batts will definitely age out first.  

    Just to clarify, my usage of 60-90 amp-hr per 24 hr period INCLUDES the furnace/blower fan usage. So that's an extra amp margin in my favor.  

    Thank you you for the "what can I expect from 400w of panels", very useful!  I tweaked the solar insolation numbers a little.  Instead of using worst case Dec, I used Oct.  I know some others do, but we don't camp here in the winter.  So, worst case, October.  Again, there's an extra amp margin in my favor.

    Can you flesh out what "typical maximum" solar array means?  I see that an assumption of 13% max charge rate is used.  In my case that'd be .13*260=33.8A.  Does that mean I should not design for more than 33.8 amps of solar going into the battery from my array at any given time?  

    Which brings me to panel selection and charge controller selection:  PWM or MPPT? If I go the PWM cc/200W 12V panel route, as described in op, the Vmpp of 21.052V gets reduced to 14.8 (the Trojan charging voltage), but the max amps I'll get from ea 200w panel won't go above the panel's  Impp of 9.5A.  With three 200W panels in parallel, I'd be charging at a max of 28.5A (10.96% rate of charge).  Four 200W panels, I'd be charging at a max of 38A (14.6% rate of charge). Is this within acceptable norms?

    MPPT I'm less familiar with.  Could someone show a sample MPPT plan, with panel choices and math :smile:?  That'd be appreciated.  
  • BB.BB. Super Moderators, Administrators Posts: 31,157 admin
    We try to "set expectations".... It is very easy to overestimate what a solar power system can supply vs the relatively high power our modern loads consume.

    The 13% rate of charge is a good bookmark for the maximum "cost effective" charging current (i.e., 260 AH (at 20 hour rating) * 0.13 rate of charge = 33.8 Amps) for flooded cell deep cycle lead acid batteries. You can charge faster, but then you need to monitor the battery bank temperature (most middle to high end solar charge controllers do have a remote battery temperature sensor). As the battery get hotter, the charging voltage drops (typically -0.005 volts per C per Cell @ 25C standard temperature). It is possible to get into a runaway battery charging condition (high charging current, batteries get hot, battery charging voltage falls, charge controller sees as battery needing more charging, battery gets hotter, etc.).

    Very roughly, lead acid batteries will accept high charging current with relatively low thermal rise below ~80% state of charge... As the battery bank gets over ~80% SoC, the battery charger should hold say 14.75 volts, and the battery will accept less and less charging current. Above 90% to 100% SoC, the battery acceptance will fall to ~1% (older, FLA batteries) to even 0.01% or less (AGM and newer batteries).

    Regarding PWM and MPPT controllers... Pulse Width Modulated controllers are simply (more or less) just an "on/off" switch that cycles 10's to 100's of times a second (more on, more charging, more off, less charging current).

    MPPT Maximum Power Point Tracking controllers. Are (usually) Buck Mode (down converting) switching power supplies. A switching power supply can take high(er) voltage and low(er) current from the solar array and efficiently down convert to high(er) current and low(er) voltage to charge the battery bank.

    More or less, because of various reasons, MPPT and PWM controllers end up being similar efficiency... PWM controllers are much less expensive (and usually require more expensive "12 volt" panels--for 12 volt battery bank). MPPT controllers are much more expensive but can use a "high voltage" solar array that can use (usually) less expensive "Grid Tied" solar panels.

    More or less, smaller solar array (400 Watts or less) and PWM controller can be built for less money.

    And >800 Watt arrays + MPPT controllers can be cheaper/easier to install (cheap panels + expensive MPPT controller).

    MPPT controllers do have an advantage in cold weather--In subfreezing weather, solar array Vmp-array voltage rises and MPPT controllers can take that 10-15% or so voltage increase and turn it into more charging power for the battery bank (and running your day time loads).

    From my point of view, MPPT controllers excel at running high voltage arrays that can be some distance from the charge controller and battery bank (100's of feet, if needed). PWM controllers need the wire runs from the array to the controller to be pretty short and heavy gauge copper wiring.

    Other MPPT controller advantages include larger/more power microprocessors and networking. More optional charging and control algorithms. And the ability to better charge a battery bank in very hot climates (when Vmp-array falls, it approaches the charging voltage of the battery bank.

    Regarding predictions of system performance and comparisons--Predictions are good to ~10% (i.e., two measurements/predictions that are within 10%, are pretty much dead nuts the same).

    I would not worry about 14.5% rate of charge--Get a charge controller with a remote battery temperature sensor and you should be OK (PWM). Or, with MPPT controllers, many can be reprogrammed to limit their output current (if needed/desired).

    Before you purchase hardware, I highly suggest that you do a few back of the envelope designs and see which works best for you (i.e., a PWM vs MPPT design). Look at the installation issues and your system needs/costs.

    Solar panels need to be "matched" to PWM controllers... I.e., you need Vmp~17.5-19.0 volt rated panels. With MPPT controllers, you can buy other panels (like 2x 200 Watt Vmp~30+ volts) and run them in parallel (or series) to the MPPT controller... MPPT controllers are much more "forgiving" regarding array and panel choices/configuration (note: this is a complex subject, and you need to do the paper design and make sure the panels you are looking at will work for your needs--There are matching and min/max voltages for the array that need to be matched to the controller input limitations).

    What are the 200 Watt panels Imp and Vmp specifications? For "normal" solar panels, Vmp~18 volts is for "12 volt" battery systems. There are portable panels that may not be "standard" match for PWM on 12 volt battery bank. Also, if flexible panels (plastic faced vs glass, or amorphous cells vs crystalline cells) they typically have a shorter life than the normal "glass+aluminum framed crystalline panels.

    There are a lot of choices out there. Use the first pass paper design (as we went through your electrical needs vs hardware needed to generate the power--I.e., sizing battery bank, array, charge controller) and read about some of the controllers and see which ones you like/are worth the money to you.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • speedbrakespeedbrake Registered Users Posts: 10 ✭✭
    The 12V panels I've found:  Hightec Solar RCL-M200w.  200 Watts.  $185 each with free shipping!  From what I've read, that's a very good price for 12V panels.  Vmpp=21.052V, Impp=9.5A, Voc=24.335V, Isc=9.87A.  I've read that Vmpp of nominal 12V panels are usually in the 17-18v range so the Vmpp on this one surprised me. Is it too high?

    Yes. I plan on using remote temp sensing at the battery for the exact reason you laid out.

     Thanks, Bill.  You've been very helpful.  I'll ask you more questions tomorrow.  It's getting late. 
  • littleharbor2littleharbor2 Solar Expert Posts: 1,569 ✭✭✭✭
    edited May 2019 #8
    With that high of Vmp. you are going to lose close to 1/3 of the panel's charging potential with a PWM controller charging your batteries at 12 - 14.8 volts. I think you'd be happier with a good MPPT controller. 

    As to your opening post, regarding the "Big Fat 4.0 Wiring" Bear in mind that large inverters will gobble up lots of power while doing nothing , Like more power than a refrigerator draws in some cases. Your big wiring plan indicates you are going with a big inverter. I would recommend not using an inverter any bigger than absolutely necessary. My experience in my RV's is my first one we used a 750 watt inverter. This easily covered running the satellite receiver, flat screen TV as well as all blender, phone charging and small power tool use I could throw at it. My second RV had a 1000 watt inverter which was IMHO, larger than we needed. If you need to run a microwave off your inverter than size it accordingly but no larger.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • EstragonEstragon Registered Users Posts: 4,478 ✭✭✭✭✭
    Also, if running a microwave off an inverter, you'll want a pure sine wave type.  Before I knew better, I bought a 2kw modified sine wave inverter from a marine dealer who assured me a MSW unit was adequate for my needs.  In fact, the small (~800w?) microwave has a lot less heating power on the 2000w inverter vs shore power.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • speedbrakespeedbrake Registered Users Posts: 10 ✭✭
    I agree with your inverter advice.  I've anticipated that and am planning a low wattage inverter to run tv, DVD player, small watt loads, etc. And a high wattage inverter to occasionally run the microwave.  Gotta have popcorn with the movie ;).  Both with remote switches so I'm not wasting batt pwr to run the inverters (especially the big one) in standby mode.  

    On the panel's:  Yes, you're right, with a PWM cc that 21volts would get stepped down to a charging voltage of 14.8 without any increase in amps.  Charging amps (from one 200W panel) wouldn't ever get any higher than the Imp of 9.5A.  I'd essentially 'lose' the power, or the charging potential, of the higher voltage panel with PWM.  So here's the $64,000 question:  What could I expect, in a charging amp #, from the above 200w panel specs with a good mppt cc?
  • EstragonEstragon Registered Users Posts: 4,478 ✭✭✭✭✭
    In most climates and with decent panel slope to full sun, you can expect roughly 75% of nameplate (STC) rated output, so ~150w from a 200w panel.  For a bank at ~50% SOC, 150w÷12.1v charging = 12.4a.

    At 14.8v, the bank would be in absorb stage, taking less current as it gets closer to 100% SOC, regardless of extra pv capacity.
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • billybob9billybob9 Registered Users Posts: 141 ✭✭
    Since Mike suggested " test drive your panels " I would like to add this. If your putting your panels on the roof each time you camp and are using the vent to do this so you don't have to get on the roof. Then measuring the vent opening is a good Idea. I was lucky with my lance camper as a 100W panel just slipped through the vent in the sleeping space.
  • speedbrakespeedbrake Registered Users Posts: 10 ✭✭
    Not on the roof.  My plan, for this year anyway, is to put the panels in a sunny spot on the ground (pointed S with a latitude tilt angle). Placing panel's up on the roof every camp would be an incredible amount of work!  

    Thanks Estragon, that's useful info.  As much as I wanted to choose a PWM controller (simple, cheap, etc), I keep coming to the conclusion that I'd be "leaving power on the table", as it were.  So I'm probably going to go with an MPPT controller.   Do mppt controller have an amperage draw at all?  My understanding is that pwm cc's were completely passive and draw zero amps while they're operating. Same with mppt, or no?  If so, how much amp draw would that be for say a 45A mppt controller?  

    Could I solicit some recommendations for panel specs for an mppt controller, specifically what range I should look for in both Vmpp and Imp?  Should I be wiring the panels in series or parallel? I'd prefer parallel because shading effects are less severe.  Can I use an odd # of panel's?  Do MPPT controller have an "input voltage range" that I should shoot for?  
  • BB.BB. Super Moderators, Administrators Posts: 31,157 admin
    edited June 2019 #14
    What is the nominal (400 watt array?) and maximum array size you plan on (400, 600,?).
    Staying with 12 volt battery bank?
    A full sized mmpt (80 amp or so) with an ethernet server can take something like 3 to 6 watts at night (on phone, so can't look up specs at the moment).
    A pwm controller (60 amps or less) may take 1/10th that or less. 
    If you have sun every day, and a good sized battery bank,  3 watts load to run the controller is not going to hurt you. 
    If in snow county, 3 months without sun can seriously discharge a battery bank of left connected. 
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • speedbrakespeedbrake Registered Users Posts: 10 ✭✭
    Hello again , Bill! 

    I'd like to start with a 300-400 W array this year and have room to max the array at approx. 800w

    yes. Staying with nominal 12v battery bank

    I don't think my mppt cc would be as big as that 80a you're citing as an example. But a 3w draw would equate to 0.25 Amp-hr and that would be happening 24/7 correct?  
  • BB.BB. Super Moderators, Administrators Posts: 31,157 admin
    Back at my computer:

    As an example, 400 Watt array with 20 Volt Vmp:
    • 400 watt * 1/20 volt Vmp * 1.25 NEC derating (always check manual, not sure 1.25 is needed) = 25 Amp minimum rated PWM controller
    • 400 Watt array * 0.77 panel+controller deratings * 1/14.5 nominal battery charging voltage = 21.2 Amp "minimum" MPPT controller suggested
    So, at a bare minimum you are looking at smaller solar charge controllers if you plan on present 400 Watt array.

    Here is a modern/fairly high end 30 amp MPPT controller that will accept Vmp-array~21-100 VDC (Standard Rating--marketing spec.). (MPPT from Midnite)

    And relatively simple (no bells and whistles) PWM from MorningStar: (PWM from MorningStar)

    This are both good USA companies with phone/online support. Given USA companies, you will be able find much less expensive units if you cast a wider net.

    Whatever you end up picking, both of these are well documented and you can see what the range of options are.

    I do suggest you get controllers that have a remote battery temperature sensor (or controller is mounted in same space as batteries for ambient temperature measurement).

    If you like lots of data, there are Battery Monitor systems. There is an integrated option with the Midnite Kid... Or you can go with completely separate Battery Monitor... A few options: (remote shunt for BMS on Kid Controller)

    Other battery monitor options:

    Trimetric RV 2030 is full monitor (and not easiest to program). (shunt based)
    Xantrex and Victron are very nice units (some with programmable remote alarm contacts). (shunt based)

    And voltage monitoring BMS based:

    MidNite Solar MNBCMS  (easier to wire--May be "good enough" for a simple system)

    Again, these are just suggestions to start your search... There are lots of other good products on our host's website, and other places (cough, cough EBay).

    Your charge controller should be mounted very close to the battery bank (short heavy wiring, around 0.05 to 0.10 volt maximum drop for Charge to Battery Bus wiring drop).

    If you are going to have fairly long distance from the array to the trailer (park RV in shade, stake the panels in full sun, use an "extension" cord from panels/array to charge controller--And MPPT type usually works much better (Vmp-array higher, Imp-array lower, can use much smaller AWG cable from array to controller). Make sure ground mount array is secured against wind/tripping, and cannot easily grow legs and walk from camp.

    Note--Some folks like to use twist lock plugs for this cord to remote array connection. Downside is if somebody drives away and forgets the array, twist lock plugs will drag your array behind the RV (learned the hardway decades ago at local fire station, people kept tripping on AC charging cable for fire engine--Somebody got the great idea to use twist lock--And, sometime later, left the engine with the AC cord still connected...).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • EstragonEstragon Registered Users Posts: 4,478 ✭✭✭✭✭
    Mppt controllers want "headroom" voltage of ~ 20%+, so for a 12v nominal system charging at 14.8v, ~18v minimum Vmp.  Voltage drops on hot panels, so a marginal Vmp panel with parallel wiring should be adjusted for operating temp (sometimes in specs as NOCT), and for voltage drop from pv to CC.  Much over ~ 75-100' will likely get unwieldy in terms of wire and/or panel size needed.

    If series wiring, the Voc should be adjusted up for the coldest local record low temp multiplied by string size to be sure string Voc is well under the controller max.  Odd numbers of panels are ok.  My 48v array is strings of 3, for example, for cold Voc of ~120v max vs CC max of 150v.

    Many CC makers have string calculators on their websites which make these calculations for you.
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • speedbrakespeedbrake Registered Users Posts: 10 ✭✭
    I found a string calculator on Morningstars website.  Have not crunched numbers yet.  Will get to that in next couple of days.  

    I wont have even close to 75.  Max of 20' or so from panel's to cc.  From the cc to battery bank:  1' of 2 awg wire from cc to bus bar, then 2' of 4/0 wire to battery.  
  • tr0ytr0y Solar Expert Posts: 96 ✭✭✭
    @speedbrake ; Solar is fun and all, but at your usage level would not just 1K to 2K quiet little Honda generator be really more cost effective and probably easier ? Throw in an a PSW inverter charger and go with it ? Not trying to stop you but maybe KISS is the best policy here ? 

  • speedbrakespeedbrake Registered Users Posts: 10 ✭✭
    Again, thanks for your concerns that it might not be justified, or cost effective, or easy, or the best plan.  But I'm installing solar.  As stated before, that's been decided already.  

    I'm a big fan of the KISS principle, and generators have a place.  But there are places where generators are not allowed.  Many Glacier Campground in GNP, for instance.  There are also folks that use generators too much (somehow I often have the misfortune to camp next to them) and I don't want to be "that guy".
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