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  • Dual Charge battery bank

    I'm designing an off grid system for my RV.

    My current battery bank config is two 200ah AGM's

    I intend to wire a 30 amp plug to the inverter hooked to the battery bank and simply plug in the RV's shore power cable to the battery bank when not on shore power. Using the battery bank as though it was a shore power source.

    The RV has a built in charging system (12v, 45a max).

    I wish to charge the battery bank using the built in RV charger as well as by solar panels. This will allow the bank to be charged while the gennie is running or while on shore power as well as when the solar panels are used.

    My question is - I think, from my research, that I should use two charge controllers, one for the 12v from the RV charger and another forthe solar panels.

    What say you all?

  • #2
    RV charger

    I do not trust the built in 12v charger by the way, it has burned several batteries over the years.

    Comment


    • #3
      You don't need anything and you do not want to use anything else if you have 45 amps of charge current as that is about all your batteries can handle. Going any higher will Just burn up your batteries. The most important charge source on a RV is your vehicle alternator using an Electronic Battery Isolator. A $50 Battery Isolator can do more work in 30 minutes than a $1000 worth of panels can do in a week. Panels are just there for Show-N-Tell. They really do not do much of anything.......................................... .................................................. .................................................. ...

      When charging a battery from more than one source, the highest energy source is the one that gets used. The lower energy source does nothing. The source with the lowest impedance gets used, and that would be Shore Power, Genny, Alternator, and last in line is Solar. Soar is only used when there is not other source available. So what I mean is if you have either the Genny, Alternator, or Shore power connected abd charging, the panels get bumped off and do nothing until you turn off the higher order of energy.
      MSEE, PE

      Comment


      • #4
        Originally posted by glyndwer View Post
        I'm designing an off grid system for my RV.

        My current battery bank config is two 200ah AGM's

        I intend to wire a 30 amp plug to the inverter hooked to the battery bank and simply plug in the RV's shore power cable to the battery bank when not on shore power. Using the battery bank as though it was a shore power source.

        The RV has a built in charging system (12v, 45a max).

        I wish to charge the battery bank using the built in RV charger as well as by solar panels. This will allow the bank to be charged while the gennie is running or while on shore power as well as when the solar panels are used.

        My question is - I think, from my research, that I should use two charge controllers, one for the 12v from the RV charger and another forthe solar panels.

        What say you all?
        Charge Controllers are designed to take power from solar panels and charge batteries. They expect the power to be the sort of power that comes from panels. You would not hook up a Charge Controller to a generator or an alternator output and expect it to work.

        What is your RV charger? It sounds like it charges batteries, but perhaps you don't trust it for your two 200AH AGMs? I don't blame you, because you have quite an investment there. Your RV charger puts out 45 amps (I would verify this is getting to the battery) and this is about 10% of your battery bank. You don't say what kind of batteries you have and how they are wired.

        Perhaps you have done this already but the most effective way to design a power system is to start by accurately measuring the amperage draw of all the things you want to run to get a peak amperage and then also multiple each one by how many hours per day they will be used to get your daily Amperage Hour (AH) consumption for one day.

        Then you can size your battery bank. If you are going to be hooked up to shore power every night then it does not matter too much. If you are going to be boon-docking a lot, then you want to size your battery to cover the days when you will not have good solar. In the extreme case you will want 5 days autonomy. Also, remember that you only want to use 50% of your battery capacity.

        Another thing is that you will likely have a very hard time making sure your batteries are charged completely with just the solar you put on the roof of your RV. Look at the manufacturers recommendations for your AGM batteries and see how long they will take to charge. You will probably need a generator to make sure you don't ruin those batteries.

        Here is a scenario that will kill your batteries (over time). Lets say you put 400w of panels on your roof to charge that 400AH battery bank. You get at best 400w / 14.4v = 27 amps from those panels. But it will probably be much less because of inefficiencies for conversion and panel orientation. Lets assume you used 200 AH or 50% of your battery. 200 / 27 = 7 hours to bulk charge and at least 2 more hours for absorption, likely longer. The sun does not shine that long. So your battery is not charged back up to 100%. This is bad for the battery. You start to lose capacity permanently. Lets double the panels to 800w. That is 4.5 hours at best to charge those batteries. But you won't get that many hours every day even in the summertime. Forget it in the wintertime.

        You need a generator to make sure this does not happen. The dynamics change considerably if you are parked at a site with hookups and/or if you drive a lot and have a good generator output.

        Comment


        • #5
          Originally posted by lkruper View Post
          Charge Controllers are designed to take power from solar panels and charge batteries. They expect the power to be the sort of power that comes from panels. You would not hook up a Charge Controller to a generator or an alternator output and expect it to work.

          What is your RV charger? It sounds like it charges batteries, but perhaps you don't trust it for your two 200AH AGMs? I don't blame you, because you have quite an investment there. Your RV charger puts out 45 amps (I would verify this is getting to the battery) and this is about 10% of your battery bank. You don't say what kind of batteries you have and how they are wired.

          Perhaps you have done this already but the most effective way to design a power system is to start by accurately measuring the amperage draw of all the things you want to run to get a peak amperage and then also multiple each one by how many hours per day they will be used to get your daily Amperage Hour (AH) consumption for one day.

          Then you can size your battery bank. If you are going to be hooked up to shore power every night then it does not matter too much. If you are going to be boon-docking a lot, then you want to size your battery to cover the days when you will not have good solar. In the extreme case you will want 5 days autonomy. Also, remember that you only want to use 50% of your battery capacity.

          Another thing is that you will likely have a very hard time making sure your batteries are charged completely with just the solar you put on the roof of your RV. Look at the manufacturers recommendations for your AGM batteries and see how long they will take to charge. You will probably need a generator to make sure you don't ruin those batteries.

          Here is a scenario that will kill your batteries (over time). Lets say you put 400w of panels on your roof to charge that 400AH battery bank. You get at best 400w / 14.4v = 27 amps from those panels. But it will probably be much less because of inefficiencies for conversion and panel orientation. Lets assume you used 200 AH or 50% of your battery. 200 / 27 = 7 hours to bulk charge and at least 2 more hours for absorption, likely longer. The sun does not shine that long. So your battery is not charged back up to 100%. This is bad for the battery. You start to lose capacity permanently. Lets double the panels to 800w. That is 4.5 hours at best to charge those batteries. But you won't get that many hours every day even in the summertime. Forget it in the wintertime.

          You need a generator to make sure this does not happen. The dynamics change considerably if you are parked at a site with hookups and/or if you drive a lot and have a good generator output.
          My rig also has a Gennie available, so I have shore power (on occasion) and the alternator available for power sources. We wish to get set up to boon dock for about 14 days at a time ultimately. I'm building as I go however trying to use parts that allow for the end goal of 14 days-ish.

          Here are the batterys I'm considering - http://www.amazon.com/dp/B010TXTTJK/...I1ZV6LQ7BHUF53

          Their location dictates/requires the AGM type as they have to located in the living area floor cabinets

          The existing on board charger works from the RV's load center - it can deliver up to 45a (the size of the load center) to battery charging. I do not really trust it however and would like to use a better charge controller, MPPT preferred.

          We full time, currently we are in a park, generally, during the work week, then boon dock on the weekends. We're working to extend our ability to boon dock.

          As for amp draw - two laptops (almost always on, we work online) , led lighting, various kitchen appliances with occasion use (blender,toaster,small water heater for coffee), cell booster, various cell phone/tablets recharging/ 24" tv about 4 hours use per night

          Comment


          • #6
            Originally posted by glyndwer View Post
            My rig also has a Gennie available, so I have shore power (on occasion) and the alternator available for power sources. We wish to get set up to boon dock for about 14 days at a time ultimately. I'm building as I go however trying to use parts that allow for the end goal of 14 days-ish.

            Here are the batterys I'm considering - http://www.amazon.com/dp/B010TXTTJK/...I1ZV6LQ7BHUF53

            Their location dictates/requires the AGM type as they have to located in the living area floor cabinets

            The existing on board charger works from the RV's load center - it can deliver up to 45a (the size of the load center) to battery charging. I do not really trust it however and would like to use a better charge controller, MPPT preferred.

            We full time, currently we are in a park, generally, during the work week, then boon dock on the weekends. We're working to extend our ability to boon dock.

            As for amp draw - two laptops (almost always on, we work online) , led lighting, various kitchen appliances with occasion use (blender,toaster,small water heater for coffee), cell booster, various cell phone/tablets recharging/ 24" tv about 4 hours use per night
            If you already have a system in place, it would be well worth the effort to get something like a kill-o-watt meter to get the actual power you are using when boon-docking. It is hard to do this without knowing more specifics .... I would make a spreadsheet (I think there is an excel sheet in a sticky on this site that helps size systems) and get that figure before buying anything.

            I have never heard of the Mighty Max batteries (I have not heard of many things) and their web site did not give me enough information to determine if these are quality batteries. There is only a 1 year warranty. In your environment with solar charging I would not count on more than that and feel fortunate if you replace them every 2 years. That being said, if you want a 12v system, you are much better off with 2 6v batteries in series than with 2 12v in parallel.

            As for AGM batteries, I understand the need for safety and am considering what I would do if I eventually have a conversion van myself. What you need to know is that AGM batteries last 1/2 as long as Flooded Lead Acid and cost 2X as much. Is there any way you could put the batteries in an external compartment on your rig? It might cost something, but this set of batteries will not be your last. Normal Trojan FLA batteries have a 2 year warranty and prorate up to 5 years. They can do this because of you take care of your batteries they expect them to last for 5 years.

            The Trojan Trojan 6v 370 AH batteries are designed for a 10 year life and come with a 2 year full replacement and 5 more (7 total) prorated warranty for about the same price you are paying for the Mighty Max. You will probably replace your AGMs at least three or four times in 7 years using them with solar boon-docking for 14 days at a time.

            So, consider your battery budget for 7 years. There is room in that amount of money to have an external battery compartment customized in your rig so that you can use real Renewable Energy batteries and have money left over, I would guess.

            Without knowing your actual energy usage, it would be foolish of me to suggest a battery bank size. When you do that exercise and decide that you need at least 5 days autonomy, you may find you need 6 2v RE batteries instead for a total of 1100 AH.

            Only after that can you size your solar bank, your charge controller and even backup generator to make sure you can charge your bank in a reasonable amount of time.

            Comment


            • #7
              You are not getting it. There is nothing for you to do. The only way you can force the panels to charge your batteries, is turn-off or disconnect all other Power Sources. The batteries will only take a charge from the Stiffest Source available. You have no control except to Disconnect or Turn-Off all sources but the Solar. When you go boon-docking or Dry-Camping, your primary source of power will be the generator or alternator........................................ .................................................. .................................................. .... Solar is a very Soft Source of Power and extremely limited. It cannot compete with a STIFF SOURCE of Power like the Alternator or Generator. The Solar Charge Controller can only regulate a Current Source of a Solar Panel. It cannot regulate or work with a VOLTAGE SOURCE like a Generator or Alternator. As soon as the genny or alternator come on-line, the voltage shoots up on the batteries. Your Charge Controlleer sees that voltage rise, and shuts off. Your controller thinks the battery is charged because there is no current flowing from the panels. You cannot change that. .................................................. .................................................. .................. You batteries also have a maximum amount of current they can accept without over heating and damaging them. A 4000 AH battery you do not want to go any higher than 50 amps. Your best charge efficiency is 40 to 50 amps. For a Solar Panel system the minimum panel wattage required to maintain a 12 volt 400 AH battery is using 500 to 750 watts with at least a 50 Amp MPPT Controller. So unless you have 500 to 1000 watts of solar panels, you will be using a genny, alternator, or shore power for daily cycling while Dry Camping. Only way around that is if you live like a hermit rarely using power.
              MSEE, PE

              Comment


              • #8
                Originally posted by glyndwer View Post
                My question is - I think, from my research, that I should use two charge controllers, one for the 12v from the RV charger and another forthe solar panels.
                Sounds like there are two different things you are talking about.

                1) Charge controller. This limits voltage from a source like PV, preventing overcharge of the batteries. Better ones do several stages of charging, with each stage voltage optimized for that phase of charging. This minimizes charge time while maximizing battery life.

                2) Charger. This converts AC to DC to charge the batteries. These are also available with multistage charging. If you don't "trust" yours then replace it with a better one, since it is important for the life of your batteries.

                There are no normal cases where you would put a charge controller on the output of a charger.

                If you are using the generator or shore power, the charger does the work. If you are using solar (i.e. boondocking) the charge controller regulates voltage to the battery.

                Comment


                • #9
                  Originally posted by Sunking View Post
                  You are not getting it. There is nothing for you to do. The only way you can force the panels to charge your batteries, is turn-off or disconnect all other Power Sources.
                  When you are out boon docking typically your engine is off and, if you are like me, I prefer the generator to be off too. So the ideal would to be able to have it quiet and be charging the batteries during a sunny day. Whatever panels you can mount may not be able to keep the batteries charged for 2 weeks, but they would prolong the time between having to fire up the generator or engine. Not the cheapest way to go but the peace is worth something to me. If you want to stay least expensive, you'd keep the RV in one spot and plugged into the grid.

                  Comment


                  • #10
                    Originally posted by NorthRick View Post
                    When you are out boon docking typically your engine is off and, if you are like me, I prefer the generator to be off too. So the ideal would to be able to have it quiet and be charging the batteries during a sunny day. Whatever panels you can mount may not be able to keep the batteries charged for 2 weeks, but they would prolong the time between having to fire up the generator or engine. Not the cheapest way to go but the peace is worth something to me. If you want to stay least expensive, you'd keep the RV in one spot and plugged into the grid.
                    What do you spend for RV sites with hookups. The least I have paid is $25 and that was not even 30 amps. I can run my generator for far less than that.

                    Comment


                    • #11
                      Originally posted by lkruper View Post
                      What do you spend for RV sites with hookups. The least I have paid is $25 and that was not even 30 amps. I can run my generator for far less than that.
                      Dang, calling me out. I don't own an RV and the few times I've rented one we never stayed in an RV park so I didn't realize that's what they charge for power. How about parked in your driveway plugged into your house? The point I was trying to make, is that there are more factors than simply money in making decisions on how to provide power.

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                      • #12
                        Originally posted by NorthRick View Post
                        Dang, calling me out. I don't own an RV and the few times I've rented one we never stayed in an RV park so I didn't realize that's what they charge for power. How about parked in your driveway plugged into your house? The point I was trying to make, is that there are more factors than simply money in making decisions on how to provide power.
                        LOL, no worries! I rented a conversion van and paid $75 on the California coast and the cheapest was the camping area of an RV park used for tent camping... And that was off season.

                        To your point, I have a generator and manual transfer switch at my cabin and am considering a small battery system so I dont have to listen to the generator at night... And I am grid tied... This is only for backup.

                        So far I have spent $1300 and gotten 8 hours use of it...

                        Comment


                        • #13
                          Originally posted by NorthRick View Post
                          When you are out boon docking typically your engine is off and, if you are like me, I prefer the generator to be off too. So the ideal would to be able to have it quiet and be charging the batteries during a sunny day. Whatever panels you can mount may not be able to keep the batteries charged for 2 weeks, but they would prolong the time between having to fire up the generator or engine. Not the cheapest way to go but the peace is worth something to me. If you want to stay least expensive, you'd keep the RV in one spot and plugged into the grid.
                          Rick what you are not getting unless your panels can replace what was used that day, you have to use another source. That will be every morning running the generator to get the batteries back up before you even get good sun on the panels. Oncee charged up you ggo about your day and the panels keeep the batteries topped off while you are gone and ready for another night. Morning, repeat process. With 12 volts @ 400 AH gives you the ability to use 1 to 1.5 Kwh per day. It takes a 500 to 1000 watt solar panel from sun-up to sun-down to generate that. It all depends on how much you use in a day. Use more than the panels can generate in a day, it has to come from somewhere or you go dark.............................................. ..................... You can do what you want, it takes nothing, it is automatic. It just just does not work like you think or want it too.
                          MSEE, PE

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