Newby questions, solar for RV

jimindenver
jimindenver Solar Expert Posts: 59 ✭✭
in Solar Beginners Corner #1
Hi, I'm Jim and new to all of this.

I wish to build a system to keep our batteries up while we camp off grid. I have purchased two panels second hand for $50 each. One Schott poly 230 and a Canadiansolar 220. I understand that I will need a MPPT controller for these and have been told a Rogue controller would do well but they are not cheap. ( I however am :blush: )

Are there any less expensive options? Our battery bank will only be 200 ah and our needs small a save running the furnace in freezing temps.
If I could keep the bank up with only one panel and spend less on the controller, I would be happy.
Thank you

JinD
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Newby questions, solar for RV

    Welcome to the forum, Jim.

    Can you give us the specs on those panels (Voc, Vmp, Imp, Isc)? And what system Voltage you want to run?
    I'm too lazy to look them up. :p
    Chances are they have "oddball" Vmp which is too high for 12 Volt and too low for 24. That means an MPPT type controller is essential for minimizing power loss. Unfortunately they are all expensive. The few cheap ones out there are no-name knock-offs and not worth wasting any money on. Some of those aren't even really MPPT!

    System Voltage makes a big difference here, as you're looking for 10 to 20 Amps peak current for your 200 Amp hour battery bank. Multiply by 12 you get up to 240 usable Watts for a 12 Volt system. Double that for a 24 Volt. About the smallest/cheapest MPPT controller there is, the Morningstar 15: http://www.solar-electric.com/mosumpsochco.html

    Beware of keeping the furnace going in cold weather. Depending on what type of furnace that is, it can have some pretty big demands for circulation and some specific ones for maintaining control board function (will not like the MSW output of inexpensive inverters).
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  • jimindenver
    jimindenver Solar Expert Posts: 59 ✭✭
    #3
    Re: Newby questions, solar for RV

    sure thing, I appreciate the help

    panels are
    Canadian solar
    pmax 220w
    Vmp 29.3
    Imp 7.52
    Voc 36.6
    Isc 8.09

    Schott poly 230
    Pmax 230w
    Vmp 30.0
    Imp 7.65
    Voc 36.9
    Isc 8.33

    Simple test on a cloudy day had the Canadian solar putting out 33.7 and the schott 34v

    My plan is to use it to keep two DC totaling around 200 AH charged while boondocking. On a single 95ah DC battery we have gone from 12.84v to 12.24v in 12 hrs with the furnace set at 65 while it was in the 20s out. I will be adding a second battery for the 200 ah and still running at 12v.

    I could use the extra power during the day to run a second fridge, a small inverter or fans for the RV although by what you posted one of the panels may be enough.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: Newby questions, solar for RV

    As suspected, the Vmp's are "oddball".
    Normally for a 12 Volt system you'd want a Vmp around 17.5 (35 for a 24 Volt system). That means either panel would lose a significant amount of power with the Vmp "pulled down" to 17.5 - about 40%. This is why the MPPT is necessary.

    Your testing would have shown the Voc, which is for the most part irrelevant.

    For 200 Amp hours you get a formula like this:
    5% minimum peak charge current; 10 Amps @ 12 Volts (lowest you want the system to go) = 120 Watts, less typical efficiency derating of 77% = 155 Watt panel. For a 10% charge rate (better is your using the batteries while recharging) you'd get 310 Watts of panel.

    Working backwards, the larger panel would yield: 230 Watts @ 77% = 177 Watts, divided by 12 Volts = peak current potential of 14.75 Amps. That would be around 7% peak charge rate, perfectly good for the 200 Amp hour batteries providing everything goes well (good sun, low draw while charging). Remember that the sun isn't always good and the loads are inconsistent and often higher than you expect.

    Which brings us to your "second refrigerator". I don't know what type you have, but they are all hard on battery systems. For those with a compressor, there is always a nasty start-up surge. The Peltier type ("car coolers") are just plain inefficient, but may be what you use in your application. Inverters also use power for themselves any time they are on. Be really sure you need one before adding it to the system. Morningstar makes a very nice 300 Watt unit http://www.solar-electric.com/mosu300wasiw.html , and Samlex has some decent smaller ones http://www.solar-electric.com/sasiwain1.html .

    In short, you can in theory keep a 200 Amp hour 12 Volt battery charged from a 230 Watt panel using a small MPPT controller. Providing all goes well.
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  • jimindenver
    jimindenver Solar Expert Posts: 59 ✭✭
    #5
    Re: Newby questions, solar for RV

    Again thank you, this gets more complicated than I thought it would.

    I think I should just spring for the better controller and not be cheap. I may not mount the second panel right off but would like to keep the option open. I may be able to up the bank up to 3 12v deep cycles for 300ah but weight becomes a limiting factor after that.

    We are generally pretty low on the load usage. The main fridge is propane but has a CP as does the Water heater. (its off most of the time) a bit of fan in the bathroom when needed and the propane and smoke detector. Other than that the furnace fan is the big draw and we wont need that at all here soon. Until now we have kept the 95 ah batt charged once a day with a Champion 3500/4000 that we keep if we need the air conditioning. So being able to keep both batteries up silently would be wonderful. I'll leave the Champ at home if this works well.

    JinD
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: Newby questions, solar for RV

    For a little extra money ($325 vs. $223) the Rogue 3024 would handle both panels (check with them as the listed limit is 400 Watts on 12V) and keep the battery bank very happy. :D

    300 Amp hours would give you roughly 1800 Watt hours DC to work with. That's about half of what I use to run the whole cabin with full-size electric 'frige, water pumps, microwave, et cetera.

    Be careful with 3 batteries in parallel, as current sharing becomes a problem. Check the Smart Gauge wiring info here: http://www.smartgauge.co.uk/batt_con.html

    As for the Champion generator ... it's always a good idea to have back-up power!
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  • jimindenver
    jimindenver Solar Expert Posts: 59 ✭✭
    #7
    Re: Newby questions, solar for RV

    I set the panels out for a SC test.

    The schott 230 was around 34v VOC and 9.03 amps

    the Canadian 220 was at 35.5 v voc and 8.54 amps
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
    Re: Newby questions, solar for RV

    This proves the panels work. :D
    But once you hook them up to an MPPT controller the numbers will be different as it "loads" the panels according to what it feels it needs to get the best possible output for the battery at the moment. It will change a lot according to the insolation and the battery state of charge at any given time.
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  • Shadowcatcher
    Shadowcatcher Solar Expert Posts: 228 ✭✭✭
    #9
    Re: Newby questions, solar for RV

    One possible alternative to very expensive MPPT controllers is the STECA line which will handle the voltage from your panel and some good prices on Ebay.
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #10
    Re: Newby questions, solar for RV

    But unless you use an MPPT controller, with those panel voltages you will waste better than 40% of output. Sort of doesn't make sense to install PV and then toss 40% of it away for the sake of a few dollars,

    Tony
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  • jimindenver
    jimindenver Solar Expert Posts: 59 ✭✭
    #11
    Re: Newby questions, solar for RV

    Thank you for the suggestions, I appreciate the info.

    I just spoke to the guy that sold the panels and he is expecting more in a few weeks. I know what I have is more than enough for the RV but after reading above that you can run a cabin completely off of solar , I started thinking of a system for the house. (kinda hard not to @ $50 a panel)

    JinD
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #12
    Re: Newby questions, solar for RV

    And another one becomes addicted to solar! :p
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #13
    Re: Newby questions, solar for RV

    so tell us why these pvs are being sold so cheaply?
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #14
    Re: Newby questions, solar for RV

    And ask him if he will send me a few!

    Tony
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  • jimindenver
    jimindenver Solar Expert Posts: 59 ✭✭
    #15
    Re: Newby questions, solar for RV
    niel wrote: »
    so tell us why these pvs are being sold so cheaply?

    Ahhh this question was asked on the RV forum too.

    I'm told he involved with the installation some how and that the panels are the extras from the jobs. HIs phone was ringing off the hook when I picked my two up and he said to call him this week to see if he could get more from his distributor. He sells them on craigslist so if they are hot he is a idiot. When I made a joke asking why he didn't have controllers, he said they don't order extra controllers for a job but do panels in case some don't work.

    Aside from the wires had been pulled from the tape holding them to the frames and a few small scuffs on the frames, the pair I bought looked new. The tape was still clear and not brittle like they had been mounted in the sun. I could tell from the dust pattern that they had been in his garage for quite a while.

    I have been very lucky with craigslist. Our first generator looked new and was $50. The 3500/4000 we have now cost us $67, it may not be a nifty quiet inverter type but not spending the money there is making solar possible for us. Silent is better than quiet any day in my book.
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