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another "two MPPTs one battery bank" question

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  • another "two MPPTs one battery bank" question

    Hi. Often enough I've encountered members saying it's generally okay to use two MPPTs on one bank.

    I've not found clarity on the question I have in mind though.

    The scenario is:

    - same battery bank
    - first charge controller is programmable MPPT
    - second charge controller is UNprogrammable mppt

    I guess ideally I'd set the programmable unit to match the settings of the unprogrammable unit.

    But what if the batteries have specific setting preferences - like one Trojan range lists bulk 14.8 and absorb 13.5 for 12V system.

    The unprogrammable unit bulks (it's LED is labelled 'boost') at 14.8, but absorbs (it's LED is labelled 'float') at default 13.8.

    So - change programmable settings to match the ideal battery specs, or to match the unprogrammable unit settings?

    Considering that Sunking frequently says that there is no setting high enough for us off-gridders, I'm hoping the latter is acceptable.

  • #2
    You have to use common sense, a 5kw array on a dumb controller might well fry your batteries, so put that controller on the 600w array and the adjustable one, on larger array.

    You need to know how your batteries are responding to charging, your nightime loads, and if you are getting enough charge by the end of the day. Not enough charge - increase the voltage to put more amps into them. Too much water use, decrease the time or voltage
    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



    • #3
      Two controllers on a common battery can be done and really not much of a problem. In fact common practice. Ideally you would like for them to be the same model with a comm link so they sync up voltages. When you use two different models still not a real big problem. The unit with the higher voltage setting will be the last one to go into Float. All that means is when the weaker controller reaches its Set Point will switch to Float before the other unit. When it does that all that happens is that controller and its panels essentially turns off until the other unit switches and lowers its voltage. When that happens, then both units will start to load share again. Even then most likely the larger of the two panel wattage will do most of the work. Just how physics works.

      Same for any two power supplies. The supply with a higher state of energy dominates. When the device in a higher state of energy is loaded down enough, the parallel device in a weaker state of energy then will start to contribute energy at a lower rate. One of those things physics takes care of and you do not have a lot of control over. Same applies to say an RV or home system with a genny. When you start the RV (assuming you were smart enough to install a Battery Isolator to allow the alternator to charge house battery) or Generator engine, the Alternator or Charger is a much stiffer source (higher state of energy) of power and takes over from the panels. Essentially the panels and controller shut off because they are fooled by the chargers into thinking the batteries are fully charged with no load so they FLOAT. All that means in this context is no current from controller or in a neutral state of equilibrium.

      Do not worry about back feeding a controller with another charger. All electronic controllers have Pass Transistors and those act like diodes and current can only flow one direction. When you back feed a higher voltage just fools the controller into thinking the battery is charged up, and essentially shuts off because the output is reversed biased and the controller quits trying to push current.

      As for the voltages. Unless you are one of the very few with more panel wattage than batteries, not much to worry about. Pretty darn hard to over charge a battery with solar. I would honestly estimate 95% or greater of solar battery systems are chronically under charged. It takes a lot of planning to design a system that actually is capable of fully charging lead acid batteries year round.
      Last edited by Sunking; 08-10-2018, 06:15 PM.
      MSEE, PE


      • #4
        Thanks Mike and Sunking

        I'll go ahead and try the two different MPPTs on the battery bank in question.

        On this 24V system, max load is 810 Wh. 510 Wh (worst case scenario) will be from a step-down to 12V. Remainder for a 24V fridge straight froms batts.

        Adding another 150 Wh to the load to get me to 960 Wh. Why? Oversizing system, and 960 is the magic Wh number for the two 12V Trojan T200RE batteries I've been able to source.

        On 2.5 minimum insolation, the battery Ah remains 200 on a 24V system, with the off-grid calc spreadsheet showing 25A max charge current for FLA and a required 200Ah for the load of 960 Wh on a 24V system.

        Panel size shown as 576W in the spreadsheet with current at 24A. I have the 130W panel through the 20A LED MPPT already. I will try get as close to the remaining 446 panel wattage as possible, via a new 30A MPPT. Probably via two 220W panels if I can source them.

        I hope this constitutes an adequately sized system. If not, I can go considerably bigger with the two new panels. I can get 320W panels at a good price, and together at 640W and 24V this still works with the 30A MPPT. In this case I'd not need the 130W panel via 20A MPPT already in place, but would use them elsewhere, unless you suggest simply leaving them added for a seriously oversized array for this system.

        Thanks again.
        Last edited by whazzatt; 08-12-2018, 04:40 AM.


        • #5
          I just bought my second 40A MPPT solar charge controller 4215BN to expand my off grid solar system from 800w to 2000w. I read somewhere that f I am using a second controller, I should have an equal amount of watts and volts going through it as the other. Is that true? The charge controllers are identical twins.
          More compelling is my concern at running the second controller through my combiner box, where my system is set up now. Two 400w 48v arrays in parallel go through my combiner box. The negative screws onto the ground bar at the top, while the hot wire goes through a 20a breaker to the insulated (not grounded) hot bar near the bottom by the breakers. The charge controller is attached to the same ground and hot bars of the combiner box.
          The new addition: four 300w, 24v panels will provide 1200w in two 48v arrays in parallel (or I can wire the arrays into the combiner box separately), Open circuit = 40v per panel, short circuit just under 10 amps. If I run the new system through the combiner box, the energy from all panels will be combined on the hot and ground bars and be connected to both charge controllers at the same time. Will this put too much voltage on the controllers, or will they share the load?