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Load from 2 seperate batteries

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  • Load from 2 seperate batteries

    I've been reading that paralel 12v batteries is not recommended, also adding a new lead acid is not recommended.

    Suppose, for small DC loads, someone manages to charge two batteries seperately and wants to load, during and after peak hours, as per attached circuit.

    Any problems to be expected?
    Attached Files

  • #2
    Not going to work well because of voltage drop across the diodes, would only be acceptable for very small loads because of the diode amperage issues.

    Then you have the problem of needing 2 chargers.

    Better to just charge both batteries separately, to the same voltage, then parallel them and see how long they last as a pair, being charged and discharged together. Even if not well matched, better than trying the diode.
    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
      Your idea with diodes has advantages:

      If you are constantly connecting and disconnecting batteries, diodes solve the critical problems from connecting batteries together that are at different voltages. Simply connecting two random batteries together with different states of charge is a problem for a few reasons: Initial connection will result in an extremely high surge current which can be a safety and battery-life concern. Also, during that period when the two batteries are at different states of charge and connected together, one will charge the other, possibly resulting in loss of life for that first battery doing unnecessary work.

      If you can get past those issues, you can parallel batteries that are different age.

      As Mike said, there are also disadvantages to using diodes:

      Even Schottky diodes will waste energy because they drop roughly 0.5V, so your system will waste roughly 4% of the battery capacity. Also, you will need two chargers, one per battery.

      Mike's approach solves all of the above issues.
      7kW Roof PV, APsystems QS1 micros, Nissan Leaf EV


      • #4
        in case of load is AC type, after inverter - there are some inverters, which are able to work together in paralel mode. for example 1000W inverter will be connected to battery A and another, same 1000W inverter will be connected to battery B, afterwards they can create one AC 110v/230v network capable of running 2000W AC based load. every battery will be discharged separately and problem of mixing batteries will not occure.

        some of these inverters are hybrid types, or all in one devices, capable to act as inverter and mppt solar regulator and battery charger in one. then you can connect separated PV pannels to each of them and then you will have 2 complete DC circuits and 1 common AC circuit for load.