Two battery banks, one inverter ?

photon_trapphoton_trap Registered Users Posts: 23 ✭✭

I will be acquiring a new off grid home to which I want to expand the battery capacity of. I know it is not advised to mix batteries of varying ages (due to charge rates?) but, as the existing batteries are only 3-4 years old, I was wondering if I could add panels, a second charge controller (both of which I happen to have NIB) and feed the Inverter (an existing Magnum MS 4448 PAE) from two separate battery banks. Can this be done?


Jeff A


  • EstragonEstragon Registered Users Posts: 4,495 ✭✭✭✭✭

    In theory, you could use a make before break switch on the dc input to the inverter to switch between two banks. This should keep the inverter lit while switching, but with the banks temporarily in parallel, the switch has to handle current between banks through the switch if they're at significantly different states of charge. You could use a break before make, but the you'd want to turn the inverter off when switching banks. Neither is a particularly good solution.

    Aside from these issues, there's also the longer term problem of what to do when the existing bank starts to die. Say that's 3 years from now. You then have a dying 6-7 year old bank, and a middle aged 3 year old one. Do you replace half a bank every 3 years indefinitely?

    Personally, I'd probably add the pv and controller capacity to the existing bank, try to use some of the extra capacity on an opportunity basis, and just work the bank harder until it dies. You then get to buy a proper bank to better meet your needs at that time, with actual experience of needs having lived in the new home for some time.

    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • PhotowhitPhotowhit Solar Expert Posts: 5,650 ✭✭✭✭✭

    I agree with @Estragon, Add the panels and charge controller, but wait on the battery bank.

    Depending on your location, you might consider setting the new panels at a slightly more south easterly or southwesterly direction to give you a wider charging cycle, sort of a virtual tracking. This will help you make use of the extra panels without drawing down your existing battery bank and help spread out the loads.

    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former, 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • EstragonEstragon Registered Users Posts: 4,495 ✭✭✭✭✭
    edited April 2019 #4

    One other thought... if the 3-4 year old bank is being acquired as part of buying the new home, chronological age isn't a very good gauge of life expectancy.

    P.O./vendor may have been great at maintaining it. Assuming flooded lead acid, they'll have records of SG checks, water consumption, EQs, etc., the bank and connections will be clean, and proper tools and PPE will be at hand and obviously used regularly. If so, and the records show no issues (eg consistently low SG cell(s)), the bank may well live to a ripe old age.

    Another possibilty is the vendor tired of off-grid living chores (maybe a contibuting factor in selling the place), and has abused or neglected the bank. In that case, you may find yourself in the market for a new bank sooner rather than later.

    Absent info to the contrary, I'd bet on the second possibility.

    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • mcgivormcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    edited April 2019 #5

    What capacity and type of battery is the existing bank? What sort of climatic conditions are they subject to? It may be that they are already on the downward slope of life expectancy. Knowing actual loads, would help in determining if the existing bank is either too small, or has lost capacity. This may be an opportunity to do an energy audit because personal needs may be vastly different from the previous users.

    Adding more array capacity to the existing system (batteries) may create further damage by providing more current than the battery can safely accept, all unknown at this point.

    The use of two battery banks is impractical for various reasons, providing further information in a detailed description of the entire system, will assist in others understanding what may be the best solution, if it was well designed initially, it may have been ballanced for the previous operators needs, which will more than likely differ from your needs.

    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
  • BB.BB. Super Moderators, Administrators Posts: 32,006 admin


    I agree with the folks above... Add the array and controller(s) as you like... And see how much energy you use per day (Watt*Hours per day) and the actual harvest (trees/building/mountain/valley shading, etc.).

    And, you may want to look at "virtual tracking"... Depending on where you are located, running one array facing south west or south east (assuming the first array is southern pointing), you are adding total hours of solar per day, with little lost energy due to the non-south facing panels (some places are known for summer afternoon thunderstorms or morning overcast--And facing off south does not cost much, if anything in total harvest). And having 10% dual facing arrays which supplies 10 hours per day vs 15-20% rate of charge 6 hours per day--Your FLA lead acid batteries would be "very happy".

    Basically, confirm/update your energy needs against the array and battery bank capacities... And beat the bank until it is well and no longer of use to you.

    With the new information, you will have a better idea what to do next (larger battery bank, Li Ion/LiFePO4, back up genset, etc.).


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • photon_trapphoton_trap Registered Users Posts: 23 ✭✭

    Thanks kindly for all the thorough explanations !

    The existing system seems well balanced with 9 275W panels, a FM60, Magnum MS4448 PAE Inverter and 8x 270kWh L-16 Trojans, The home is just one BR so I don't think the existing system will be insufficient until I add a planned garage/shop where I will spend a lot of time building things out of metal.

    I realize now I will likely be better off with a second system and the comfort of redundancy


  • jonrjonr Solar Expert Posts: 1,386 ✭✭✭✭

    You can easily create two separate banks feeding one load with diodes, but they have too much voltage drop. A "battery combiner" is a relay set up to act as a diode - it only allows one way flow. Although it's not the usual setup, I believe it would work in this case - allowing flow from a battery only when its voltage is higher than the load voltage and blocking all reverse flow from one battery to the other.

    I am available for custom hardware/firmware development

  • BB.BB. Super Moderators, Administrators Posts: 32,006 admin

    The new system(s) is a ways off... I like to suggest just one system (larger if needed) if possible. 2x more hardware, usually means 2x more work.

    And you also have the issue of trying to balance loads between the two systems (one system with excess loads vs other other system, that day, that has "extra" energy and would you be able to move the energy from one to the other system)--Rather than all charging/discharging/managing just one system.


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • photon_trapphoton_trap Registered Users Posts: 23 ✭✭

    Thanks. I will plan on selling (?) the existing batteries, adding the second charge controller and panels and buying a new set of batteries. Realistically, my need is 1 year out so it's not so painful I suppose


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