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  • Capacity recommendations

    I have a 315w@9a solar panel. My camper has a BW model 6406 converter. My camper will only need electricity for 3 LED lights, the furnace (propane if that matters, I assume they all are), and a laptop. What size (Ah) batteries should I be looking at and what sized charger in order to reliably recharge the batteries each day? I believe my panel is big enough to do the job.
    Last edited by Oscar P; 04-05-2018, 10:19 PM.

  • #2
    I have room for two batteries. Also not sure if 12v or 6v will achieve my goal better.
    Last edited by Oscar P; 04-05-2018, 10:20 PM.

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    • #3
      Amusing a 12 volt battery with a MPPT controller , then the charge controller required is 315 watts / 12 volts = 26 amps. With 26 amps of charge amps can support a battery as small as 200 AH and as large as 300 AH and a perfect match is 250 AH or a pair of 6-volt golf cart batteries.

      If you use a PWM controller you only need a 9 amp controller and can only support a battery as small as 70 AH and as large as 100 AH because you just turned your 315 watt panel into a 100 watt panel using a cheap POS Chi-Com PWM Controller.

      So get yourself a Midnite Solar Kid Controller and a pair of Trojan T-105 batteries.

      Have a good day.



      Last edited by Sunking; 04-05-2018, 11:40 PM.
      MSEE, PE

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      • #4
        Does your propane furnace have a blower, or is it plain Blue Flame ?
        If it has blower, we need a good, solid estimate of how many hours out of 24, it runs, and what wattage or amps it consumes
        Powerfab top of pole PV mount (2) | 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

        solar: http://tinyurl.com/LMR-Solar
        gen: http://tinyurl.com/LMR-Lister

        Comment


        • #5
          Originally posted by Mike90250 View Post
          Does your propane furnace have a blower, or is it plain Blue Flame ?
          If it has blower, we need a good, solid estimate of how many hours out of 24, it runs, and what wattage or amps it consumes
          It does have a blower, and expected run time is roughly 2-4hrs (half in the evening getting ready for bed, half in the morning getting the day started) a day. I do not have the wattage or amps it consumes, this has been one of my stumbling blocks. Is there a way to figure this out, or is it better to error on the side of caution and just go large? The camper is a late '80's model and pop up top, so its not a large furnace by any means.
          Last edited by Oscar P; 04-06-2018, 11:39 AM.

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          • #6
            You need to get the right numbers. So, using a amp meter, see how much it consumes.

            If you guess too low, you flatten the battery and are cold
            Guess high and you spent a lot of extra money
            Powerfab top of pole PV mount (2) | 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

            solar: http://tinyurl.com/LMR-Solar
            gen: http://tinyurl.com/LMR-Lister

            Comment


            • #7
              You can measure your amperage with a meter like this...
              https://www.ebay.com/itm/UNI-T-UT203...QAAOSwHjNV99RG
              This meter can measure either AC or DC current, along with the more common Voltage and resistance. It's inexpensive but as accurate as my much more expensive Fluke meter.

              Note that when you use it, you clamp around either the positive or negative wire, but not both; they cancel each other out. This applies to both AC and DC measurements.

              I'd also suggest that if you go the golf-cart, MPPT route (best choice) and your fan comsumes a lot of power, you'll be better serred with a pair of your 315 watt panels, especially if they're in a flat roof-top position. You might though need a larger CC to handle the higher amperage.
              Last edited by MichaelK!; 04-08-2018, 02:19 PM.

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              • #8
                My RV propane furnace blower is rated at 55 Watts.

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                • #9
                  I've had good results using a Mr. Heater, portable propane heater. Eliminates that 55 watt draw altogether.
                  2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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                  • #10
                    A rule of thumb that I got from Northern Arizona Wind & Sun is that 300 Watts of solar panel is needed to charge 80 Amp-Hours of battery.

                    Two Trojan T-105 batteries in series will give you 225 Amp-hours at 12Volts. But you only want to discharge those batteries 40% to 50%. So 0.4 x 225 = 90 Amp-hours. So your 315 Watt panel with two T-105 batteries is about the right ratio (a little low), as long as you only discharge them 40%.

                    I'm in the process of installing three 305 Watt panels to charge four 235 Amp-hour golf cart batteries. My worst case usage is running the propane furnace through the night set at 55 degrees when it is 20F outside, plus my CPAP machine. I don't stay awake to see how often the furnance kicks on I won't get a good test of my system until next winter.

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                    • #11
                      People like the Wave 6 propane heater (~$268 on Amazon), and it doesn't use electricity. But you do have to leave windows cracked open since you have combustion occurring inside your living space. I was nervous about asphyxiating the family, so I went with this 4-battery, 915 Watt panel setup.

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                      • #12
                        Originally posted by MikeNM View Post
                        A rule of thumb that I got from Northern Arizona Wind & Sun is that 300 Watts of solar panel is needed to charge 80 Amp-Hours of battery..
                        Whoever told you that is an idiot. If you were to use a MPPT Controller, 300 watts of panels, and a 12 volt battery produces 300 watts / 12 volts = 25 amps of charge current. 25 amps on an 80 AH battery is roughly C/3 charge current, and maximum you should run is C/6 to C/8. So whoever that idiot is telling you how to cook your batteries with more than twice the maximum current. Ideally you want to run charge current of no less than C/10 and no greater than about C/8. On Trojans you can go as high as C/6 or C/5 in northern climates with short days. So do some simple math, C/10 on a 225 AH battery is 225 AH / 10 H = 25 amps. Perfect for a pair of Trojan T-105's.

                        Additionally you want to limit discharge to 20 to 30% each day. Wherever you came up with 40 to 50% is smoking dope. Most likely someone you found in Arizona Smoke Dope Forum.
                        MSEE, PE

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                        • #13
                          Originally posted by Sunking View Post
                          Whoever told you that is an idiot. If you were to use a MPPT Controller, 300 watts of panels, and a 12 volt battery produces 300 watts / 12 volts = 25 amps of charge current. 25 amps on an 80 AH battery is roughly C/3 charge current, and maximum you should run is C/6 to C/8. So whoever that idiot is telling you how to cook your batteries with more than twice the maximum current. Ideally you want to run charge current of no less than C/10 and no greater than about C/8. On Trojans you can go as high as C/6 or C/5 in northern climates with short days. So do some simple math, C/10 on a 225 AH battery is 225 AH / 10 H = 25 amps. Perfect for a pair of Trojan T-105's.

                          Additionally you want to limit discharge to 20 to 30% each day. Wherever you came up with 40 to 50% is smoking dope. Most likely someone you found in Arizona Smoke Dope Forum.
                          While not questioning the technical part of your post, it may be possible that MikeNM misinterpreted, or misunderstood, or just plain messed up the information somehow.

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                          • #14
                            Thank you, J.P.M., for your sensitivity.

                            This was my first ever post about my first ever solar system, and I just shared what I knew. While I respect Sunking's apparently greater expertise in solar, I do wonder why he is so unpleasant about it. It certainly wasn't my intent to question his fine recommendation on post #3. Sometimes on forums you run across people with anger management problem. Words like "idiot" and "smoking dope" are not edifying. I prefer not to spend my free time on forums with unpleasant people. There are other solar forums where people are much nicer. We are all just trying to learn from one another.

                            On the positive side, Sunking's C/5 maximum charging information is helpful, although in his angry arrogance he seems to have missed some of the details I shared about the system I am installing on my RV. I have four 6V 235 AH batteries in a 12V series-parallel configuration, so that is 470 AH at 12V.
                            470 AH / 5 = 94 Amps max allowable charging rate
                            915 Watts / 12 Volts = 76 Amps max available charging rate
                            So my three 305 Watt panels could only charge my batteries at C/6. (470 AH / 76 Amps = 6.2)

                            My system is designed to keep the RV batteries charged during family winter ski trips when the sun is lower in the sky than in the summer. Our family had a great time skiing in Colorado this year in our RV, but got tired of running the generator for hours the next day.

                            As for Depth of Discharge (DOD), NAWS has some comments that are particularly relevant for an RV system:
                            https://www.solar-electric.com/learn...es%20vs%20Life
                            "Obviously, there are some practical limitations on this - you don't usually want to have a 5 ton pile of batteries sitting there just to reduce the DOD. The most practical number to use is 50% DOD on a regular basis." In this article they discuss the inverse relationship between DOD and battery lifetime.

                            Are MPPT solar charge controllers designed so they don't "cook the batteries?" I know the Intellipower AC charge controller in my RV automatically adjusts the charging voltage so it doesn't cook the batteries. I bought a Classic150 charge controller based on the recommendation from Northern Arizona Wind & Sun, but I haven't studied it yet.

                            While I am a newbie to solar (my RV is my first solar project), I do have some general technical experience, and will continue to look for forums with pleasant, helpful people.

                            PhD EE, PE, PMP, Supercomputer designer, Retired Professor of Electrical Engineering,etc.

                            Comment


                            • #15
                              The design of a controller is to Not Cook the batteries. Your job is to size the controller & array and sun hours, to match the batteries. Very few systems end up overcharging batteries, usually battery death is from chronic undercharge.
                              In your case, your batteries are somewhat under-paneled. That means they will die much sooner than expected, but you should be able to get a year of service out of them. Since your RV is mobile, the sloshing while driving will help prevent stratification of the battery acid. Do you have a battery isolator to allow the vehicle alternator to charge batteries while you drive ?
                              Does battery replacement balance against running a generator ?
                              Powerfab top of pole PV mount (2) | 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

                              solar: http://tinyurl.com/LMR-Solar
                              gen: http://tinyurl.com/LMR-Lister

                              Comment

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