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    Looking for suggestions (oldspark time to shine) I want to power my rv-puter from my solar side of things, here is my delema. I would like to also charge the rv house battery from the solar array but not use the rv house battery to power the rv-puter. I would like the existing RV house system to stay the same as it is now. (see primitive stick drawing...grunt...snort) Also I would like the truck alternator charge all batteries as I drive. They are all new batteries and a new 100 amp alternator. starting battery is 24 series 1000cca rv house 27 series deep cycle 105ah and the 2 solar batteries are 27 series deep cycle 105ah. My current draw with rv-puter running is just under 4 amps, solar array is outputting slightly over 10 amps. Most wire lengths that matter are all under 5' and all over rated in size and quality. ( I used to wire up telecom hardware at cell tower sites, they spare'd no expense since we are paying) I should add that being so old the rv has very little draw when running, just a coil,points and condensor. Thanks SNO
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  • #2
    So in essence you have 4 batteries in your system and two charging systems.

    When running you want to charge all 4 batteries but when not running you only want the solar batteries to charge off the solar system.

    What kind of isolator are you using on your truck / rv batteries now?

    I am going to assume you are using a Constant use Solenoid to connect the two together with the truck battery connected all the time and the RV battery only connected during running. I am also going to assume your solar controller won't accept a 12volt in from your truck otherwise you wouldn't be asking.

    Please correct me if any of those assumptions are wrong.

    The easiest way to do this would be to connect your solar batteries to the same connection your RV battery is. While charging they are all sharing the same system anyhow so it shouldn't matter. You will want a 2 way switch that will be able to handle the power running to your solar batteries. This switch would be used similar to how a generator works with three positions. You have connection 1, off, connection 2. This way connection 1 and connection 2 are never connected together regardless of how things fail. You could probably use a Generator switch for this if you can find one that is compatible with DC. Ours at work will switch to the neautral/off position for 10 seconds then go to the other connection but you don't need anything that fancy. We have 480 volts and high amperage running through ours.

    Connection 1 would be from the trucks charging system and connection 2 would be from the solar panels. You don't want the solar panels to be hooked to your vehicle power or you could blow up your solar controller. I strongly suggest one of these generator style switches. You COULD use a 2 way relay or better but they can fail with the contacts arced across which would likely not be a good thing.

    The other thing to be aware of is what happens to the solar panels when the batteries are disconnected? Some units just shut down but others need the battery to operate properly and can get damaged without them.

    You can get fancy and try to use electronics to balance everything out but somehow I don't think you care about that sort of thing.

    Note that also in this system when the truck is off your truck battery is powering the truck, your rv battery would be powering your rc and the solar panels would be charging your batteries running the computer. Which I believe is what you want.

    You might find it makes more sense to move the RV battery to the solar system and just charge them all either off the truck or off the solar panels. Otherwise your going to lose lights in your rv but still have the computer...

    In that case you just use the generator style switch and have it switch automatically between the truck and the rv batteries. Since your rv/solar batteries are the same type and size you should be able to run them together pretty painlessly.



    • #3
      It is just a basic 100amp isolator that was made in 1979 same type you would find today 3 wire unit. Forget the truck battery it is being charge only by alternator when running and only powering vehicle related stuff, it is on 1 side of isolator good and done. The other side of the isolator has the rv (house ) battery connected to it, already factory wired not touching it. but what I want is to chage the house battery when not being charged by the alternator but via the solar after controller. So when engine is running it will be charging all 4 batteries, when engine off the solar will be charging 3 batteries. 1 battery will power existing house system 2 batteries will be powering rv-puter and it's accessories hub/hd/xm/3g router with wifi/speakers/18.5" monitor/phidgets. SNO


      • #4
        Your solar system can probably charge the batteries separately if your controller will allow you to do so. Depends on the controller. If it can then I would just do that for the solar and use generator switch gear to switch sources. Each switch can be pretty expensive so plan well.

        Generators have a neutral "off" position for a variety of reasons. We have seen some of our generators after a lightning hit took out the solenoid and melted contacts together. But the generator switch still kept the piece.

        Otherwise it sounds like you need a diode isolator if you are going to be charging the batteries at the same time but not use them at the same time. This is the only way you can accomplish this. They are available at most RV shops. Problem with a Diode isolator is that you drop your voltage by somewhere in the neighborhood of .7 volts. This might be fine for your batteries but just be aware of it. When charging off your truck they won't be charging nearly as well as the truck battery and the solar will just depend on the voltage they put out.

        For a system where you are going to be putting power from two different sources you still want something like a generator hookup. No way around this. If you cheap out you could pay for it later when you have to replace your solar panels and/or solar charger due to a failure of your switching gear. They have both manual and automatic switches. The automatic ones will likely only work on AC but you could find out at an RV shop.


        • #5
          "Time to shine" eh? Well, being sun-shine I should be qualified - I have enough sun spots!

          Except for the isolation of you rv-putor, your system is no different from any multi-battery alternator system, though in this case you have a 2nd "alternator" - ie, the solar array.

          I assume you have a (Schottky) diode on each panel to protect from damage caused by other power sources (ie, the other panel, or a battery)? Hence the battery(s) and alternator cannot damage the panels. (And obviously(?), OC (open circuit) panels are not a problem.)

          I dare say that you could also parallel the solar controller (regulator) with the alternator - not that you will be doing that - I mention that merely in principle, however that will depend on the solar regulator circuity.

          The problem is the isolation of the relevant batteries. I'd suggest Schottky diodes between the solar controller and the (1) solar batteries and (2) the truck battery.
          I'm assuming the 3-wire isolator is voltage sensing (off the main/truck battery) hence truck battery charging will connect the rv battery.
          If it's a diode isolator, then an additional Schottky to the rv battery.

          Schottkys rated at 5A should suffice, though larger would be nice assuming only one of them might carry the full 4A and maybe more under extreme solar conditions.
          Assuming current sharing 3A or even 2A might be ok, or maybe paralleled... (but carry spares).

          The Schottkys should have a voltage drop around 0.3V. For a set voltage of 14.4V (solar regulator) that should not be a big issue - ie, 14.4 - 0.3 - 14.1V which is still above the "normal battery" charging voltage of 13.8V. Likewise the common 14.2V set point might be ok.
          Of course if your solar regulator has "remote" battery voltage sensing (akin to the S = Sense terminal of 2-wire alternators), then any diode voltage drop will not be a problem other than the variance of the batteries that are NOT being sensed (eg, the rv/truck battery), but that variation with Schottkys will be even less than for diode-type battery isolators that many claim work fine (despite their 0.7V to over 1V drops, and 2-battery variations that can approach 0.5V).

          FYI - even a 4A output of the panels may be less then the float current of the batteries. Though commonly quoted that automotive batteries can have float currents of up to 2A (ie, 8A for your four), a few hundred mA is more likely - especially for new batteries.

          Now, if the Schottky voltage drops are undesirable, or if the Schottkys prove too expensive, then good old relay isolation is required.
          That should be easy enough with a bit of "logic" circuitry, however I don't like the thought of using energised relays for that since they'll probably take a minimum of 3W when energised (3 x 3W relays = 9W IMO is a lot for a 100W solar array).
          A $22 voltage sensing isolator kit that uses a latching relay comes to mind. Its current draw except when flipping its 80A rated relay is a measly 50uA, or 500uA if its indicator LED is included.
          2 or 3 or 4 of those should provide voltage-drop-free interconnection and the logic required, though if the alternator kicks in, they won't disconnect. But I see that as a feature rather than a problem... But if considered a problem, it's simply a matter of disconnecting the solar array, or isolating the rv & truck battery from the solar system.

          Incidentally that same kit provider also has a similar priced solar regulator (with housing) which I think handles 150A but is easily doubled (an extra MOSFET).
          But both circuits are DIY construction with no housing supplied for the isolator (aka "12V-24V Dual Battery Controller"). But that isolator is adjustable and easily reconfigured for other voltages - eg, 24V, or as a "battery protector" (low volts cut out) rather than a battery isolator (high voltage connect).
          I have used both and am totally rapt.

          Of course a good solar system would use MPPTs, and there again, that same company had the cheapest MPPTs I'd seen - fully built for ~$80-$90 (I can't recall if 15A, 30A or 45A).
          But we can talk more about those kits - and if they are still offered - if you are interested. (Australian supplier.)

          There is also battery maintenance - especially if using diodes, or if long (partially) discharged battery conditions arise.
          Usually done on a 6-monthly basis, I'd recommend a good "smart" charger (multi-cycle). But failing that, direct connection to the alternator with a reasonable run at 14.4V or 14.2V (say 10 minutes or more) provided that is acceptable for deep-cycle maintenance (deep-cycles are less tolerant to high charge currents, but that usually does not apply to maintenance). And of course, those "isolator" kits can provide that (maintenance) switching any time.
          Last edited by OldSpark; 09-02-2013, 12:04 AM.


          • #6
            Thanks for the replies. Time to digest and plan the attack. I kinda figuired diodes would be the best route. SNO


            • #7
              Thinking further - based on a 100W solar panel being up to ~10 Amps therefore 2 x 100W being ~20A, the diodes should arguably be 20A.
              And BTW - I merely considered 4A output TOTAL in my reply above - not 4A per panel.

              It does depend how the panels are rated. 100W panels can put out 120W or more, but is that 100W rated equatorially (~1kW/sq.m) or for Canada etc?

              I wonder what the diode ratings are that came with the panels? (Again, assuming they are fitted, but for parallel panels they should/must be fitted.)


              • #8
                I will go up on the roof when it stops raining, specs are on the back of panel. I think actual output is 17.8v 5.8A There is a bunch of numbers and different specs for shorting current, blah blah blah. I will get actual numbers and then we can proceed in the right direction. thanks again SNO


                • #9
                  of course now I am envisioning just using Diodes between the solar batteries and the alternator system and between the solar panel and the solar batteries. I have heard that you could get a different style Diode that didn't drop the .7 volts which is what OS must be talking about.

                  Or just using a solenoid/relay between your solar batteries and your charging system. Either after your isolator for the rc battery or before it that engages when the alternator is charging.

                  I am assuming that the two solar batteries are acting as one battery in your system. And realistically if your solar controller can handle the power applied by the alternator without issue then you probably are just best off with a solenoid/relay to connect your solar batteries to the trucks charging system and not worry about disconnecting the solar controller.