small rv off grid pv advice needed

I'm on a tight budget and I'm living off the grid in a small travel trailer. When it was time to get a PV system, I had no choice but to go with a cheap 100 watt (12 volt) solar panel that came with a 30 amp charge controller for $125US. I also bought a couple of "blemished" supposedly maintenance free deep cycle batteries from NAPA for $100 each.

Nearly 2 years later, I regret the battery purchase as I was never told to peel off the warrantee sticker to check them. So they did run down within a year and not wanting to void a warrantee, I took then back to have them checked, only to later find out that they topped them up with tap water.

Almost a year later they were low again, (12.2V & 11.95V an hour after charging, with no load since) and with no warrantee tags to worry about, (thanks to the Napa guy), I topped them up with distilled water. A few weeks of mostly cloudy days, fog and snow haven't helped to restore them and not having another power source available, I've had to use them for my led lighting and to charge my phone routinely. There is some sun in the forecast that I hope will help, but I'm thinking of getting at least 1 more battery (likely a Trojan T125) and another panel.

Due to the differences in batteries and possibly in panels and controllers, I was thinking of running separate batteries each from separate controllers and panels, then join them together using blocking diodes before the load. Does this make sense?

I don't have big bucks to spend on brand name stuff and the cheap import products I bought off of that big online auction site do seem to be working fine so far. I was thinking that it might be better to get a 24 volt panel this time with the mppt controller instead? Any thoughts on my plans so far?

Keith.

Comments

  • BB.BB. Posts: 27,916Super Moderators, Administrators admin
    Re: small rv off grid pv advice needed

    Welcome to the forum Functional. Sorry about your less than fun time with solar... Off Grid solar is difficult to do inexpensively.

    OK... Starting with what you have, a 100 Watt solar panel charging at a recommended 5% to 13% rate of charge for a lead acid battery:
    • 100 Watts * 1/14.5 volts charging * 0.77 panel+controller derating * 1/0.05 rate of charge = 106 AH 12 volt battery bank maximum
    • 100 Watts * 1/14.5 volts charging * 0.77 panel+controller derating * 1/0.10 rate of charge = 53 AH @ 12 VDC battery nominal
    • 100 Watts * 1/14.5 volts charging * 0.77 panel+controller derating * 1/0.13 rate of charge = 41 AH @ 12 volt battery minimum
    So this gives you a range of batteries that can work with a 100 Watt panel. For full time off grid living, ~41 to 53 AH battery bank is better (quickly charge the next sunny day). The 106 AH battery, good for weekend use, or, if you are very careful with sun/loads/etc. a few days of use, then a few days of charging.

    Lead acid batteries are cheap and relatively rugged. However, they do not like sitting for days/weeks at less than ~75% state of charge--They begin to sulfate and their capacity falls. Also, as you found out, if they have exposed plates (low electrolyte levels), that kills them too.

    Do you have a small backup genset+battery charger you can use during bad weather?

    Assuming you are somewhere near Calgary Canada, using PV Watts with the panel, fixed, tilted to 51 degrees from horizontal:




    Month

    Solar Radiation
    (kWh/m2/day)


    1
    3.72


    2
    4.39


    3
    5.24


    4
    5.38


    5
    5.49


    6
    5.59


    7
    6.24


    8
    5.90


    9
    5.26


    10
    5.09


    11
    3.41


    12
    2.79


    Year
    4.88



    You do not have a lot of sun during the winter... More or less, using 4.39 hours of sun for February:
    • 100 watts * 0.52 AC end to end system eff * 4.39 hours of sun = ~228 WH of 120 VAC power
    • 100 watts * 1/12 volts * 0.61 DC end to end system eff * 4.39 hours = ~22 AH @ 12 volt DC
    Your thoughts/questions/corrections to my assumptions & guesses.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Posts: 27,916Super Moderators, Administrators admin
    Re: small rv off grid pv advice needed

    MPPT charge controller for smaller systems--Don't usually buy you very much vs a well designed PWM system with "12 volt" (really Vmp~18 volt) solar panels.

    However, if you can buy "cheap" Vmp~30 or 35 volt Vmp solar panels (very common for Grid Tied solar systems on homes/businesses), then a MPPT controller can be a good deal overall (cheap panels+"expensive" MPPT charge controller).

    In any case, lets work on the paper side of the design first before you start buying equipment.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • FunctionalFunctional Posts: 7Registered Users
    Re: small rv off grid pv advice needed
    BB. wrote: »
    Welcome to the forum Functional. Sorry about your less than fun time with solar... Off Grid solar is difficult to do inexpensively.

    OK... Starting with what you have, a 100 Watt solar panel charging at a recommended 5% to 13% rate of charge for a lead acid battery:

    So this gives you a range of batteries that can work with a 100 Watt panel. For full time off grid living, ~41 to 53 AH battery bank is better (quickly charge the next sunny day). The 106 AH battery, good for weekend use, or, if you are very careful with sun/loads/etc. a few days of use, then a few days of charging.

    Lead acid batteries are cheap and relatively rugged. However, they do not like sitting for days/weeks at less than ~75% state of charge--They begin to sulfate and their capacity falls. Also, as you found out, if they have exposed plates (low electrolyte levels), that kills them too.

    Do you have a small backup genset+battery charger you can use during bad weather?

    Unfortunately I'm without at the moment. I do have a 3500W propane generator in storage but I have to find someone with a 4X4 to get it here.


    Assuming you are somewhere near Calgary Canada, using PV Watts with the panel, fixed, tilted to 51 degrees from horizontal:



    You do not have a lot of sun during the winter... More or less, using 4.39 hours of sun for February:

    Sun rises over mountain about 8:35 AM, sets behind other mountain about 2:35 PM (when clouds allow). The clouds like to follow the ridge line of the western mountains and can make a sunny day 5 kms/2.3 miles east of here, into a bleak day for solar panels on my trailer.

    • 100 watts * 0.52 AC end to end system eff * 4.39 hours of sun = ~228 WH of 120 VAC power
    • 100 watts * 1/12 volts * 0.61 DC end to end system eff * 4.39 hours = ~22 AH @ 12 volt DC
    Your thoughts/questions/corrections to my assumptions & guesses.

    -Bill


    I'm a actually about 350 kms/210 miles west and a bit south from Calgary.

    When I can dig into my battery compartment later today I'll see if I can find the ah rating on these batteries and recheck the voltage and dig out my hydrometer too.

    I should mention that I also have an old Argo brand solar panel 1'X4' long. I'm guessing 40W? I ran the model number through Google but it choked. I've been using the Argo hooked directly to an small car battery 480cca? And charging my DVD player with that, so I know there's "some" sun getting through.

    Keith.
  • jimindenverjimindenver Posts: 59Solar Expert ✭✭
    Re: small rv off grid pv advice needed

    A inexpensive 24v panel and a equally inexpensive Eco-worthy 20a MPPT controller can make a pretty nice smaller system if it fills your needs, fits where you need it to, etc. I say inexpensive instead of cheap because they do function and are dependable but don't have the features that a more expensive controller might. Most I know of use them with 24v panels, a few with 12v panels in series. I find the conversion from mid 40's voltage down to 12v to be inefficient so I stay with the lower Voc of 60 cell 24v panels.

    Here in the states it's easy to do that for around $400 but I know the prices are higher in Canada. Adding panel to what you have and getting a bigger PWM controller may be better budget wise. Just make sure that the controller you pick can handle the voltages required to get your batteries back up to full.
  • BB.BB. Posts: 27,916Super Moderators, Administrators admin
    Re: small rv off grid pv advice needed

    What size battery do you have?

    A 3.5 kWatt genset is pretty big for charging a small battery bank... Will use a lot of fuel vs the amount of power you will actually need.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • FunctionalFunctional Posts: 7Registered Users
    Re: small rv off grid pv advice needed
    BB. wrote: »
    What size battery do you have?

    A 3.5 kWatt genset is pretty big for charging a small battery bank... Will use a lot of fuel vs the amount of power you will actually need.

    -Bill

    The batteries are Napa Group 31's , "31RVS", 810ca, 650cca. I read elsewhere that the Group 31's are 185 reserve and about 110ah?
    The Napa website was of no use.

    Regarding the generator, I only ended up with it in a trade and wanted it to run my 110V might welder and assorted other shop tools when the weather warns up. Assorted projects in the works, like building an efficient wood stove/heater and modifying a small trailer, etc.

    Keith.
  • BB.BB. Posts: 27,916Super Moderators, Administrators admin
    Re: small rv off grid pv advice needed

    In general, those are probably not really true deep cycle batteries.

    Comes down to basics... Amount of power "you need" per day. That drives the size/type of battery. Both drive the size of the solar array.

    Alternative... You can afford "$X". You have YYY Watts of Solar panels, etc... You get the power you get.

    In general, my suggestion is to spend money on solar array and descent hardware, and less on batteries. In tough conditions, batteries are not usually going to last very long. Over sized/costly battery banks die expen$ively.

    Good hardware will generally last pretty well--Even if there are some mistakes along the way. Batteries usually do not.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • FunctionalFunctional Posts: 7Registered Users
    Re: small rv off grid pv advice needed
    BB. wrote: »
    In general, those are probably not really true deep cycle batteries.

    I think they referred to them as dual purpose on one of the websites I looked at.


    Comes down to basics... Amount of power "you need" per day. That drives the size/type of battery. Both drive the size of the solar array.

    Alternative... You can afford "$X". You have YYY Watts of Solar panels, etc... You get the power you get.

    In general, my suggestion is to spend money on solar array and descent hardware, and less on batteries. In tough conditions, batteries are not usually going to last very long. Over sized/costly battery banks die expen$ively.

    Good hardware will generally last pretty well--Even if there are some mistakes along the way. Batteries usually do not.

    -Bill

    If the batteries I have can recharge properly, then I think I'll try for about 200 more watts worth of panels.

    I don't know if it would be advisable to add my old Argo panel to the 100w on the same charge controller. I'm thinking not.

    I could stick to charging one battery with my current PV setup and try charging the weaker one with the Argo? I'd have to keep an eye on it but its an option that beats doing nothing until I have the funds to spare.

    I think I'll go that route for now and see how it works out.

    Thanks!

    Keith.
  • BB.BB. Posts: 27,916Super Moderators, Administrators admin
    Re: small rv off grid pv advice needed

    Just need to know the Vmp/Imp of the panels, and the rating of the solar charge controller (link to specs would be nice).

    I prefer to put batteries together in a single bank vs breaking things up (unless these are in different locations/completely different applications or needs).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • FunctionalFunctional Posts: 7Registered Users
    Re: small rv off grid pv advice needed
    BB. wrote: »
    Just need to know the Vmp/Imp of the panels, and the rating of the solar charge controller (link to specs would be nice).

    I prefer to put batteries together in a single bank vs breaking things up (unless these are in different locations/completely different applications or needs).

    -Bill

    I typed a detailed reply after logging in an when I tried to submit it, it said I had to log in again then lost all I wrote, so maybe tomorrow I'll try again.

    Keith.
  • BB.BB. Posts: 27,916Super Moderators, Administrators admin
    Re: small rv off grid pv advice needed

    Hmm... If you take more than ~90 minutes to hit submit, the token can timeout.

    It probably will not work now (since you have made the last post), but there is a "restore" function that works about 80% of the time for me.

    At the bottom right of the post, there is a "restore post" button. Hit that.

    if you don't see the button, hit the "Go Advanced" button on the lower right, and again look for the "restore post" button on the lower left of the edit window. If neither works--That is it. :cry:

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • FunctionalFunctional Posts: 7Registered Users
    Re: small rv off grid pv advice needed

    My panel is 100W and the only other numbers I can recall off hand are: 20v, 17v and I think it was 6.5 amps. I'll try looking for a model number when its light enough out.

    As for the PWM Model SL-02A-20a 12v/24v 20 amp with automatic identification function and 12V / 24V Battery Auto Sensing, Auto Switching.

    After posting this afternoon I remembered someone who had a panel about the same physical size as mine for $100 but no controller and I'm not sure of the wattage or if it is mono or poly. I think they still have it and if so I'll try getting it next week.

    Would a small difference in wattage be an issue? I guess that if its 24v I'd need another controller?

    Keith.
    (This time I type offline then copy and paste.)

    BB. wrote: »
    Just need to know the Vmp/Imp of the panels, and the rating of the solar charge controller (link to specs would be nice).

    I prefer to put batteries together in a single bank vs breaking things up (unless these are in different locations/completely different applications or needs).
  • BB.BB. Posts: 27,916Super Moderators, Administrators admin
    Re: small rv off grid pv advice needed
    Functional wrote: »
    My panel is 100W and the only other numbers I can recall off hand are: 20v, 17v and I think it was 6.5 amps. I'll try looking for a model number when its light enough out.

    Probably:

    100 Watts / 17.5 volt Vmp = 5.71 Amps Imp

    It may be ~6.5 amps Isc--But that does not count as power from the panel (short circuit current is "not useful" for charging a battery).

    Voc is around 20-21 volts for your panel (not an issue here).
    As for the PWM Model SL-02A-20a 12v/24v 20 amp with automatic identification function and 12V / 24V Battery Auto Sensing, Auto Switching.

    That should work OK.
    After posting this afternoon I remembered someone who had a panel about the same physical size as mine for $100 but no controller and I'm not sure of the wattage or if it is mono or poly. I think they still have it and if so I'll try getting it next week.

    Mono or Poly--Does not really matter for our discussion (Poly panels are slightly less efficient, and therefor a bit larger than Mono panels--And Mono panels usually cost more $/kWatt vs poly).

    Just need the Vmp and Imp rating of the panels.
    Would a small difference in wattage be an issue? I guess that if its 24v I'd need another controller?

    We are looking for Imp and Vmp ratings of the panels. For your controller, ideally, the panels should have Vmp in the range of 17.5 to 18.6 volts or so.

    If you have other panels available, post their ratings here. You can use higher voltage panels, but it gets a bit more "complex" (and possibly more expensive charge controller to use the higher voltage panels "efficiently").

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • FunctionalFunctional Posts: 7Registered Users
    Re: small rv off grid pv advice needed
    Just need the Vmp and Imp rating of the panels.



    We are looking for Imp and Vmp ratings of the panels. For your controller, ideally, the panels should have Vmp in the range of 17.5 to 18.6 volts or so.

    If you have other panels available, post their ratings here. You can use higher voltage panels, but it gets a bit more "complex" (and possibly more expensive charge controller to use the higher voltage panels "efficiently").

    -Bill


    It was kind of hard to see upside down at an angle with a mirror, but there is a sticker with more info than I cared to get. But I did see 23.33v and 19.08v, not the other numbers from my apparently failing memory.

    I also saw:
    600V
    100W
    short circuit voltage ... 5.52A
    current ... 5.23 and
    voltage ... 19.08

    As for my previous mention of totally separating sets of panel, controller and battery, despite the identical numbers on the Napa batteries, one has a different style handle and attempts to charge them together has only resulted in the weaker one holding the "fair" one back.

    Keith.
  • BB.BB. Posts: 27,916Super Moderators, Administrators admin
    Re: small rv off grid pv advice needed

    Probably:

    Imp=5.23
    Vmp=19.08

    These are pretty much traditional numbers for a "12 volt" solar panel. You can parallel them with other panels on a 12 volt controller just fine.

    And that is the problem with paralleling batteries... If there is a weak one (shorted cell, etc.), it can drag the rest of the battery(ies) down too.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CraziFuzzyCraziFuzzy Posts: 19Registered Users
    Re: small rv off grid pv advice needed
    Functional wrote: »
    ... but I'm thinking of getting at least 1 more battery (likely a Trojan T125) and another panel.
    The Trojan T-125 is a 6V golf cart battery. You will need 2 of them to make a 12 volt bank for your system. If your current batteries are compromised, and honestly, aren't well suited to this application, I'd use the two of them as cores on 2 T-125's (or the cheaper T-105's), and go with that. If you don't want to combine panels on a controller, that's fine, as long as the overall output of the controllers is less than what the battery bank can accept easily (which it likely will be). Of course, the ideal would be matched panels to a well designed mppt controller, but that may not be necessary in your situation, and certainyl may not warrant the price. The batteries are likely more important here, as a bad battery creates as much heat as it does actually store energy, and bad batteries in parallel with good batteries is even worse.
  • ChrisChris Posts: 135Solar Expert ✭✭
    Where abouts are you located? I found here in the Kootenays (Trail) the cheapest place I could find for "true" deep cycle batteries was Lordco. T-105+'s tax+core just shy of $200 ea
  • ShadowcatcherShadowcatcher Posts: 228Solar Expert ✭✭✭
    One way to help bring the battery's with in spitting distance charging them to full, is to jump them from the the TV. I installed leads with 150A Anderson PowerPoles to make it easy to connect to a set of 6 ga jumper cables.
  • scrubjaysnestscrubjaysnest Posts: 174Solar Expert ✭✭✭
    Depending on the type of vehicle with a tow package it may have relay isolation between cranking battery and the camper charge line. This may be an option in the short term.
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