Mppt controller for small RV system?

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Jonesz
Jonesz Registered Users Posts: 18 ✭
in Solar Beginners Corner #1
Hi all, new to this forum but have been trying to get my head around what I need for a simple system for my RV. We are not big power users and really only use lights, water pumo and fridge electronics when we are boondiocking. I have 2 AGM 110 AH batteries and want to be able to get the longest stays I can out of them before going back on grid and fully recharging. I am contemplating using a Kyocera 135 W panel to supplement my batteries (really can't stand generators) I was going to buy a Flexcharge 25Amp controller but after reading the FAQ on this sites sponsor page am considering a Morningstar SS-MPPT-15 (15 amp), thinking that the additional $ spent may make my panel far more efficient, even thinking it may handle 1 more similar panel. So my ? is Does the additional money spent on this controller gain me more charging efficiency from my 1 panel. I am thinking from what I have read that this controller (the mppt) instead of the standard controller increases the effective rate of charge of the panel. Sorry for the convoluted ? but didn't know how else to state the problem.
Jonesz
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #2
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    Re: Mppt controller for small RV system?

    Assuming your current panel Vmp~17.5 volts, wiring runs are short (and heavy enough copper to have low voltage drop)... For a small system, the extra cost of a MPPT controller for 10-15% +/- energy gain from sub freezing temperatures is difficult to justify.

    The MPPT controllers are great when you have long wire runs (say you camp in the shade and want to stake two series connected (to increase Vmp-array voltage) panels 50' away in the sun--MPPT controllers are hard to beat).

    Here is a nice thread with video from Keven in Calgary Canada that shows designing and installing solar PV in a small RV trailer.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #3
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    Re: Mppt controller for small RV system?

    bb is looking at it in a dollars and sense way and you could look at it in a power way as you are getting typically about 10% extra over the pwm. this could go higher when your dod is more about half way rather than hardly any drawn out of the battery with dods around 70-80%. the choice is yours to make, but i don't think you'd regret buying an mppt. just be sure that the controller can handle any future planned expansions for if you run 14.8a of pv for instance then you only have room on the controller to give a boost of about .2a and that defeats the potential of an mppt controller. staying at or lower than nec standards of about 80% of the controller current rating will give good results for mppt too.
    15a x 80% = 12a in pv
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  • Jonesz
    Jonesz Registered Users Posts: 18 ✭
    #4
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    Re: Mppt controller for small RV system?

    Thanx so far guys. Bill that is a lot of info. I will digest what I can tonight and do the rest tomorrow. Neil I think you are sort of on the same track I am on. I was thinking powerwise and that the Flexcharge 25 A was $95 and the Morningstar $240. So for $145. I will be getting a more efficient controller which may stop my having to buy any more PVs. And Bill I don't mean to insinuate that cost is no object but am thinking if I buy the right stuff upfront it sometimes is the cheapest way to go. The 135W panel is 7.6A at max so that doesn't allow much room to ad additional panels later should that be necessary. I really hope that it isn't but you never know. So the real ? I guess I have is how much of a difference would a MPPT controller be over a standard controller on a given day in the January Arizona sunshine if they were wired the same way. If I could expect 10-15% more efficiency I would consider that as reasonable. If not you are probably right in saying it would be too costly.
    We used to boondock in our previous RV with an 85 W panel and one 12V AGM so I was hoping that the two batteries, and one 135 W panel hooked up the most efficient way would be that much better. However I might mention that I was more battery illiterate then and used to draw my battery down farther than I should have. I hope to monitor my current batteries better. I also will have a (2010 year) flooded cell marine battery which was in the 5er when I bought it. That will be on a seperate selectable circuit as backup with a Blue Seas switch. If I have to switch over it will be time to find some shore power. Sorry for the ramble
    Jonesz
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #5
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    Re: Mppt controller for small RV system?

    Welcome,

    If you haven't read these I suggest you do so:

    http://www.batteryfaq.org/

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

    Your 135 will yield some where in the range of ~ 250 watt hours of power on a ideal day out the inverter. (Name plate rating/2 to represent total system loses multiplied by 4 to represent the average number of hours of good sun over the calendar year. An RV application might no do quite as well, but depending on location and season might to a bit more. FYI, we live full time with 400 watts of PV, we burn ~6-800 wh a day. On an ideal day, we can generate 1-1.4 kwh per day.

    As for a MPPT, I would look at a good one like the morningstar. The Blue sky 2512 is a pretty good, reasonably price unit, although rather dated nowdays. I personally like the Rogue 3024, but since it will carry 30 amps it is probably bigger than what you need. That said, more people out grow controllers than nearly any other component. (I'm on my third!

    Tony
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #6
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    Re: Mppt controller for small RV system?

    actually tony i think the rogue would be a good fit for him. the rogue isn't certified, but it doesn't need to be in an rv. this would give him room to expand without going big time on the mppt controllers. i'm not sure how much it costs though.
    tony is right about the bluesky stuff being somewhat outdated and if one is going to spend big $ either way then get the latest.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #7
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    Re: Mppt controller for small RV system?

    Regarding the increase in harvest (with proper Vmp=~17.5 volts)--the hotter the weather, the less increase in power from an MPPT controller.

    Solar panels basically increase Vmp as their temperature falls. The MPPT controllers (in a Vmp=17.5 volt setup) use that little bit of extra voltage of cold/clear days to "down convert" to charging the battery bank at a higher current (via Power=V*I -- Lower voltage and higher current for a fixed value of power).

    The MS 15 amp MPPT controller is a very nice controller. But it is limited into how much power it can extra power it can collect. Now, that extra power is mostly available during the winter when you really need it... But you are still left with the difference of 5+ hours of sun in summer vs ~2 hours or less sun in winter (depending on location and when system is used)... 10-20% more of 2 hours of sun is still not that much extra power.

    But--the ability to place two (or more) panels out in optimum sunlight 50+ feet away (by connecting panels in series to boost Vmp-array) allows you to to use much smaller cables and/or longer distances.

    The whole MPPT/PWM issue is complex and with the different specifications between different vendors, the picture becomes even more muddy.

    In the end, define your needs (winter/summer/location/amount of room for solar panels/backup generator if needed, and your loads) then do some paper designs and see which works out best for you.

    Perhaps you will only ever carry 2x135 watt panels and a MS 15 amp MPPT controller will be just fine.

    Or, if you think you may use more (make a solar PV porch for example), then a Rogue 30 amp may be the ticket...

    Or, if you need a fair amount of power, a 24 volt battery bank is a better fit--then the MS 15 amp MPPT at 24 volts will work really nice with a higher supported Vmp-array than the Rogue.

    One of the big issues with solar is they are quite difficult to start small and to grow into a larger system without having to toss major components (larger charge controllers, new inverters/AC chargers for different battery bank voltage, etc.).

    If you can nail down your requirements--It may make it much easier for you to do a couple paper designs for your needs and see what works best instead of all the hand waving (which gets confusing to write and read).

    In the end, most people need more solar panels+batteries than the can fit to power loads that are too large for the available space solar PV system. It does become a game of trade-offs.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #8
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    Re: Mppt controller for small RV system?

    One issue with RV is shun/shade. In most cases RV people like to park in the shade, to avoid excess A/C and or heat build up in the RV. This makes PV harder to deal with. The idea of a portable panel, one that you can move as the need arise will pay a better dividend than a MPPT controller. (Not that I am against that either).

    I have a couple of panels that I move over the course of the day or the course of the year. It allows me to significantly increase my harvest, especially early morning and late afternnoon.

    Tony

    PS I love my Rogue controller and would highly recommend one to anyone. (It is physically large however.
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  • Jonesz
    Jonesz Registered Users Posts: 18 ✭
    #9
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    Re: Mppt controller for small RV system?

    Thankyou everyone for your thoughts. I am going to go for the Morningstar MPPT 15A setup. I will likely never buy another panel to add to the RV other than the one 135 W Kyocera. But as you say Bill if I go to 2 of these panels I am still good to go with the Morningstar. My thinking is that the 10-20% gain in charging efficiency over the standard controller is worth the extra $ spent. "Icarus" you mentioned portability of the panel and I had given quite a bit of thought to a portable stand and am likely going to go that way. I am thinking that I can achieve a better harvest most days, the only drawback is the chance it may sprout feet while I am away so will have to have it disconnected while absent from the RV. I will be able to tilt and track the sunlight to a certain extent while I am onsite as well as keep the panel clean.An added benefit is that I will not have holes in the roof to fill if we decide on a different RV before we are done. We are not summer campers at home during the summer as we live on a lakefront lot so 95% of our camping is snowbirding in winter and only roughly 50% of that time is boondocking. Again thanx for all the input
    Jonesz
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  • bstory
    bstory Registered Users Posts: 9 ✭
    #10
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    Re: Mppt controller for small RV system?

    Hi, newbie here so forgive obvious questions, but I am a bit confused by some of the posts in this thread. I thought the Morningstar 15 amp MPPT controller was limited to 200 watts which would mean that 2 Kyocera 135 watt panels would exceed the maximum wattage. Are you assuming that the inevitable losses will mean that isn't a practical problem or am I missing some key concepts? (Probably the latter, right?)
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #11
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    Re: Mppt controller for small RV system?
    bstory wrote: »
    Hi, newbie here so forgive obvious questions, but I am a bit confused by some of the posts in this thread. I thought the Morningstar 15 amp MPPT controller was limited to 200 watts which would mean that 2 Kyocera 135 watt panels would exceed the maximum wattage. Are you assuming that the inevitable losses will mean that isn't a practical problem or am I missing some key concepts? (Probably the latter, right?)

    You've pretty much got it: two 135W = 270 Watts. Usually @ 77% efficiency = 207 watts / 14.2 Volts charging = 14.6 Amps. In other words the pair of Kyocer's will keep the controller running near peak much of the time. It is an inexpensive way to get the most power possible, as RV installs tend to have less than ideal insolation. Any extra power potential simply won't be harvested.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #12
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    Re: Mppt controller for small RV system?

    There is difference in the ratings between a PWM and an MPPT charge controller...

    PWM controllers are limited to the amount of solar panels that can be placed on them... They have no ability to manage the excess current through them.

    MPPT type charge controllers can be attached to very large arrays/energy sources (like a 24 or 48 volt battery bank to down convert to a 12 volt bank/loads). MPPT controllers are "Buck Mode" switching power supplies and can safely control their output current.

    So, with a MPPT charge controller, I typically recommend the maximum soalr array sizing of 1/0.77 derating factor (1.3x rated output power) as a good cost effective compromise... Solar panels are "over rated" based on how they operate in the real world (panels operate at higher temperatures than the STC rating assumes and output loss voltage/power except for very cold weather).

    So, if we take the ratings for a 12 volt 15 amp controller:
    • 15 amps * 14.5 volts charging * 1/0.77 derating = 282 watts
    You can use even larger arrays--but then in cooler weather/bright/clear days, the controller will hit the 15 amp current limit more often.

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