Questions: Serial or Parallel on the solar panels & MPPT or PWM charger

ronstoryronstory Posts: 8Registered Users

I've got two old panels off one of the original Arco proof of concepts and yes, they are 20 years old. I wrote done the voltage and amp when I tested them years ago and they kick out 20.98V and 20.10V with 1.98A (same for both) when I measured them long ago. Now I have misplaced my engineering notebook from that era and don't remember the load we cobbled together for testing.

I also tested them a few month back in the summer and the voltage I measure was almost exactly what I wrote on the back of the panels long ago. (Note to self... this is when I realized that I don't remember the test config that got me that data. :cry:)

Now my question, I'm looking at mounting them on an airstream trailer that I'm restoring and was wondering whether I should connect them in series or parallel. The storage on the airstream is a single 115 AH (at 20A) flooded 12V battery.

With the difference in voltage but the same amperage I'm tempted to just wire them in series and then look at a controller that can take the 41-42Vish as an input and then translate it into a 3 stage charging process for the battery.

Ideas? Recommendations? Crazy ideas?

All help appreciated.



  • ronstoryronstory Posts: 8Registered Users
    Question 2of2: To go MPPT or just PWM?

    From the previous post, I'm trying to determine whether I wire two old but capable panels in serial or parallel config. Now my 2nd question... should I go for a MPPT controller of the more simple PWM controller types?

    I been cruise these forums and folks seem to agree at MPPT is better... but their are scenarios where they seem to work better than others. Where I'm going to use them is with two 21ish volt panels and to just help keep a single 115AH 12V flooded battery in the might Airstream topped off or from losing too much power while boondocking.

    If things go completely south, I could plug in the tow vehicle and then charge the battery in the airstream... but what a waste of gas. :blush:

    So, in this type of application is MPPT a good fit? I know it may depend on the answer of serial vs. parallel wiring... but that's why I'm the newbie afterall. ;)

  • BB.BB. Posts: 27,681Super Moderators admin
    Re: Question 1of2: Serial or Parallel on the solar panels?

    Your panels sound fine for charging your 12 volt battery...

    The can be connected in parallel and use a (cheaper) PWM type solar charge controller...

    Or you can connect them in series and use a MPPT type solar charge controller.

    The Vmp (maximum power) sounds like around 15.5 to 16.5 volts

    1.98 amps * 16 volts = 32 watts per panel

    The problem is that MPPT controllers (maximum power point tracking) use more energy internally, and can be less efficient with smaller solar panels. Morning Star makes MPPT controllers that would work--but, unless you get more panels--there is probably not a good reason to use MPPT and the panels in series (lower current in panel leads, longer runs with less voltage drop--the usual electrical stuff).

    wind-sun_2031_1362431Morningstar SunSaver MPPT Solar Charge Controller

    Otherwise, you will want to put them in parallel with a standard PWM charge controller. You do not want to put them in series--it will just waste about 1/2 of the panels power (40 volt output clamped to ~14 volts at the battery anyway).

    Morning Star line
    Steca or Blue Sky

    ~4 amps of current. Recommeded charge current is ~5%-13% of battery capacity:

    4 amp / 0.05 = 80 AH
    4 amp / 0.13= 31 AH

    So, these panels are a bit small to really "charge" and "equalize" your 115 AH (at 20 hour rate) flooded cell battery. They will need help from an alternate charging source (AC or Generator) to really go "off grid"... However, they will help keep your battery charged during storage and light energy usage.

    Also, it would be a good idea to look for a solar charge controller with a Remote Battery Temperature Sensor (RBTS or BTS) bolted to the battery... Storage battery charging voltage change with respect to temperature, and many charge controllers will under or over charge the batteries if a RBTS is not used.


    PS: I merged the threads because the two were pretty closely related...

    MPPT is pretty much standard for the larger charge controllers as they can be somewhat more efficient (0-30% maximum). But, they are also more flexible as the work well with multiple panel configurations and efficiently over wider temperature ranges. Also, you can make choices to run the panels at higher voltages (in series or series/parallel) to reduce the amount of money you spend on longer runs of copper wire from the panels to the charge controller/battery shed...

    Typically MPPT makes sense for 400 watt or larger systems. The Morning Star MPPT for 200-400 watt maixmum smaller systems.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Posts: 27,681Super Moderators admin
    Re: Questions: Serial or Parallel on the solar panels & MPPT or PWM charger

    Lastly, I should add that if you try to charge your trailer using the tow vehicle with the 12 vdc connection through the typical trailer light plug--it is probably a terrible waste of fuel. The voltage drop though the 12 vdc connection will limit your charging current.

    A better bet, fuel wise, is to get the pretty efficient (and quiet) Honda eu2000i or eu1000i inverter/generator and a good quality charger (like the Xantrex TC 20 or TC 40). Run the 20 amp on the smaller 1000 watt peak generator, or the 40 amp on the 2000 watt peak generator.

    By the way, the TC family has gone obsolete and is replaced by the TC2 family (the TC2's have not hit NAWS yet).

    In any case, it is recommended that any charger (including AC) you get have a remote battery temperature sensor.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ronstoryronstory Posts: 8Registered Users
    Re: Questions: Serial or Parallel on the solar panels & MPPT or PWM charger


    Thanks for merging the threads, wasn't sure of the etiquette on this forum.

    Great information and thanks for pointing out that I'll be below the minimum recommended charge rate for the battery. I also have a new Xantrex XADC 60A converter/charger to keep the battery charged when shore power is available and power the Airstream. Yes, I agree that the charging the battery with the truck is a terrible waste of gas. :blush:

    I'd looked at that Morningstar MPPT since it had the higher max input voltage of 75V where the other were spec'd at 36V. Now that could be a bit of specmanship on Morningstar's part but it would allow me to connect the panels in serial with margin.

    So since I'm on the light side of the battery charge amperage with my panels... would a PWM in parrallel be "better" than the the MPPT in series?

    Also, with the slight voltage difference in the panel... would that matter in the parallel mode?

    As for the generators... I've been looking at both the honda EU inverter line and the Yamaha EF line. I've been leaning toward the Yamaha EF2400is due to the fact is would be capable of running the AC if needed. I'm also investigating be doing a LPG conversion on whatever generator I buy since I'd rather just carry one aux fuel source and the airstream has two 30gal tanks I can just tap into so no extra gas can.

  • Solar GuppySolar Guppy Posts: 1,959Solar Expert ✭✭✭
    Re: Questions: Serial or Parallel on the solar panels & MPPT or PWM charger

    The Electronics and caps in the MorningStar Mppt are all 100V rated, the 75V is a very conservative designed controller, not "specmanship" as you suggest

    For your power range the SunSaver will harvest the most power, a PWM controller would work fine as well, but you lose some watts having the panels at the battery voltage and not the vmp of the solar panels ( voltage maximum power )
  • ronstoryronstory Posts: 8Registered Users
    Re: Questions: Serial or Parallel on the solar panels & MPPT or PWM charger


    Cool, glad to hear that they use high quality caps. I work in the computer industry and a couple years back got bit by the tantalum cap feasco. Yuk.

    Another question about the MPPT, if I wire the panels in series... with the MPPT then maximize the power to get the best amperage to the battery?

    Also, Bill makes the observation that with a small panel setup like mine it the MPPT could have a higher 'idle' energy usages, is that a big issue with the Morningstar MPPT unit?
  • BB.BB. Posts: 27,681Super Moderators admin
    Re: Questions: Serial or Parallel on the solar panels & MPPT or PWM charger

    Take a look at the Morning Star Spec Sheet (PDF Download, 2 pages)...

    At 25 watts, it is already 92%+ efficient, by 60 Watts, is 96%+... So, I would not have a concern with your panels.

    Solar Guppy has looked as a "professional" at the Morning Star using his calibrated test equipment and found it to be a very a very solid unit and worth the price...

    The question for you is--" it worth the price" per watt.

    If you are looking for a 30% improvement in power over a well designed and configured PWM--you probably will not see that except in rare conditions.

    I don't think you can go wrong with the Morning Star MPPT... And I, personally, like any good MPPT controller with series panels operating well over Vbatt (the higher the input voltage, the efficiency will drop a bit due to parasitic capacitance--but not much to worry about).

    The other issue is to run your power vs load calculations. 60 watts of solar panel is not alot. Basically, by the time you add all of the loses up--you are lucky to get much more than 150 Watt*Hours a day 6 months out of the year (and less, if the panels are not tilted towards the sun).

    If you can justify the price of a MPPT controller and more panels, you will not be unhappy.

    Regarding the generator--I understand your A/C power requirements. My suggestion is to figure out how much power you use from the battery/during the time you will be running the generator and size to that.

    Needing to run, relatively large generators, vs the average load, just wastes fuel.

    The Hondas have "eco throttle" which allows them to cut way back on fuel consumption, even down to 1/4 load... Most generators reach minimum fuel consumption around 1/2 load (+/-).

    An eu2000i may have problems starting your AC (unless it is pretty small). But the eu2000i will run 400 watts for 15 hours on 1.1 gallons of fuel.

    Your battery bank, assuming around 10% of 115 AH, or 12 amps * 14 volts is 168 watts, take 80% for the battery charger and you are looking at 168/0.80=210 watts maximum needed generator power.

    The smaller the genset, the better off you will be (assuming your RV DC power usage is relatively low because of conservation and energy efficient loads). I don't know as much about the Yamaha's--but I would very much watch your size/fuel consumption and make that an important part of your buying decision.

    Perhaps, the bigger Yamaha and a small Yamaha/Honda 1000 watt as a pair would be a good compromise. You probably would only need to take the bigger Yamaha when you know you will need A/C or more power for tools/coffee maker/friends/etc.

    Regarding the "...etiquette on this forum..."--just nice, friendly, and have a bit of fun.


    PS: MPPT charge controllers are typically buck mode switching power supplies and they have various algorithms to optimize the solar panel's power output via P=I*V... Different brands hunt/search/assume the Vmp/Imp of the solar panel strings in various ways--but the good products (such as those mentioned around this forum) will have similar, positive, results. As long as the solar panel input voltage and current are within the controller's specifications, they will efficiently down convert the high voltage / low current into low(er) voltage / high(er) current required for the battery banks.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ronstoryronstory Posts: 8Registered Users
    Re: Questions: Serial or Parallel on the solar panels & MPPT or PWM charger

    Drat... somehow my post got lost... I swear I hit 'submit reply'. :cry:


    Thanks for taking the time to share all your expertise. As for the generators... I've been comparing the Honda and Yamaha (and Robin and Kipor)... but have settled on the big two. I've checked consumer reports... but I've found I perfer Consumer Search since they coalesce both the reviews and real world user opinions in to recommendations. See and they give both Honda and Yamaha high marks for fuel efficiency with some giving the nod to the smaller Yamaha.

    Based on my panels only provided about 60W, I'd just be happy if I could use them to keep the battery charged on the trailier without needed to plug it to the house. I expect to use the trailers for weekend trips and then plug it to home when we return to fully charge the battery. I'd like to then be able to unplug it and have the solar panels keep the battery charged until the next 2-3 weeks when we take our next trip.

    I like the idea of the MS MPPT and the price is not unreasonable considering that I could upgrade to larger panels later. Are there solar panels that are more rugged for RV use? I understand that the longer panels may need additions support or rails.

    As for my energy usage, it is still TBD from a real measurement standpoint, but with cooktop, furnace, water heater and refrigerator all on LPG... that just leaves the lights, vent fans, water pump and furnance motor. I'm upgrading all the lights to LEDs (1.2W) and with have about 20 lights with a CLF in the bathroom. Add in two fanstastic vents and furnace my peak would be 10A with everthing reasonable on. A more typical would be 2-3 amps for 3-4 hours in the eve and possible 4-5 amps if it was hot enough to need the on vent fan on.

    So a reasonable Ah useage would be 10-12Ah per day for pleasant temps and up to 20AH per day for hotter climates... but not so hot as to need the AC. ;^)

    I'll start looking around for a efficient generator as well.
  • icarusicarus Posts: 5,108Solar Expert ✭✭✭✭
    Re: Questions: Serial or Parallel on the solar panels & MPPT or PWM charger

    10 amps at 120vac= 1.2kw

    10-12 ah/day= 1.2-1.44 kwh/day

    A pretty tall order with a small panel array,

  • ronstoryronstory Posts: 8Registered Users
    Re: Questions: Serial or Parallel on the solar panels & MPPT or PWM charger


    Those were 12V loads, not AC. Most of the Airstream is 12V so I would perfer to pull straight from the battery not incur the loss of inverting / converting.

    So 10A-12A *12V so 144W and with more of the fans running 20A, or 240W. We have the tendency to head to bed a couple of hours after dark or when we tire of the campfire. The LED lights really help. ;)
  • mike95490mike95490 Posts: 7,783Solar Expert ✭✭✭✭
    Re: Questions: Serial or Parallel on the solar panels & MPPT or PWM charger

    Watts consumed are "Watts Consumed" no matter if they are 12V or 115V watts. Just a little is lost in the conversion, and you still will have a flat battery, quickly.

    Granted, you gave 12V amps, and someone may have used that, and overconverted as if they were 120V amps, but you are still pulling out more power than your little 60W panel can replace.

    4A for 4 hours, is 16AH @ 12V
    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: ,

  • ronstoryronstory Posts: 8Registered Users
    Re: Questions: Serial or Parallel on the solar panels & MPPT or PWM charger

    The point I was trying to make and apparently failed was that watt consumed suggested by Icarus was based on P=IE calc with E being off an order of magnatude on the high side.

    I agree, it's unlikely that the small panel will keep the battery completely topped off. With the small panel I'll lose less than I would otherwise and it's close to what I'll typically consume on a summer day. Not a bad thing considering that I have my current panels and it's basically 'free'.

    With the battery being the storage device... the panel will allow me to extend my stay if desired without a running a generator or plugging into the tow vehicle. Not to mention I can always buy larger panels to solve the charge issue.

    Provided that I keep the storage battery above 12.3V, I'll be able to fully charge it upon return home with the Xantrex TC XADC. Still need to shop around for a generator.

    Make sense?
  • BB.BB. Posts: 27,681Super Moderators admin
    Re: Questions: Serial or Parallel on the solar panels & MPPT or PWM charger


    Batteries tend to be the most abused portion of an off-grid system... And, if not properly taken care of, can fail in a matter of months.

    Everyone just wants to make sure that people have realistic expectations so they are happy with their systems and they will last a long time (but just about everybody "lunches" their first set or two of storage batteries the first time the "go off grid").

    By the way, one other suggestion--if you camp and drive, then dry camp again--installing a properly sized inverter in your tow vehicle and running an AC cord back to the trailer to power a appropriately sized AC battery charger will probably do a better job of charging your battery vs trying to use the 12 volt battery connection from the tow vehicle.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • MoeMoe Posts: 60Solar Expert ✭✭✭✭
    Re: Questions: Serial or Parallel on the solar panels & MPPT or PWM charger

    We don't have solar on our Airstream, primarily because we prefer to park in the shade whenever possible and where we are, that's often possible.

    When it isn't, we have awnings on all windows (except the front) and use them, but quite often we wind up running both Fantastic Vents on medium all day (12 hours x 2 amps x 2 fans = 48 amp-hours).

    At night, we close the windows and shades for security, run one vent on medium, with the other open and the motor not turning. (12 hours x 2 amps x 1 fan = 24 amp-hours). So that's up to 70 amp-hours daily just for the fans.

    But not all of that is coming out of the batteries (two Group 27 100 amp-hour 12 volt Lifeline AGMs). We run at least one of our two Honda EU2000i generators for about 4 hours each evening, from about 4PM to 8PM or so, when two of the vents are running, so 16 amp-hours of the fan use goes directly from converter/charger (60 amp Progressive Dynamics Intellipower with Charge Wizard) to the fans, leaving 50-something coming out of the batteries.

    Our furnace fan draws about 10 amps and the duty cycle varies throughout the day. We don't have a "battery monitor" with amp-hour readout, but I'd guess it's about 25% average throughout the day, or about the same consumption as the Fantastic Vents--50-something coming out of the batteries outside generator run time.

    We have 11 or 12 12-volt fluorescent Thin-Lites with dual 8-watt tubes (1.6 amp draw) and use those instead of the halogen spots or incandescent wall lamps. The water pump (6-7 amp) doesn't run long enough to worry about. It and the lights are mostly run when the generator is running, so they don't come out of the batteries.

    There are lots of phantom loads that do--the LP detector, the radio remembering its station settings, the digital thermostat system for the heat pump, the water heater circuit board, the refrigerator circuit board and gas valve, the Winegard antenna if you forget to turn it off, and so on. We also keep a Midland Weather Alert radio on 12 volts continuously.

    The generators do the job for us and where we store the Airstream, we have 15A power to keep the batteries charged. Despite its 34 foot length, there isn't much space up top for solar panels with two Fantastic Vents, two skylights and the heat pump. So solar wasn't very practical for us. YMMV.

    If you haven't found the Airstream Forums, check them out.
  • ronstoryronstory Posts: 8Registered Users
    Re: Questions: Serial or Parallel on the solar panels & MPPT or PWM charger


    Yes, the AC chargers are *way* smarter than the Fred Flintstone primitive 10 gauge wire running from the isolator to the charge line on the trailer. I'm got a large Xantrex MSW converter... but I'd rather get a TS converter to run that new XADC converter charger. Looks like I may be easier to just get the generator.


    Thanks... we plan to do most of our camping in the NW so our temps are pretty reaonable most of the time... I would only turn on the fanastic vents for a couple hours to do the "air change"... and then let the windows do their thing in the cooler eve.
  • BB.BB. Posts: 27,681Super Moderators admin
    Re: Questions: Serial or Parallel on the solar panels & MPPT or PWM charger

    If the XADC (sorry, don't know the brand/model) is a modern charge controller with Power Factor Correction (on the AC input)--it probably will work great with the MSW inverter.

    I would call the manufacturer and ask them how it works with a MSW inverter.

    As long as you plug from the inverter to the charger (and don't ground the MSW inverter's AC white/neutral line like is done in normal house wiring)--you should be just fine.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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