Charging batteries using generator & MPPT charge controller

wayneworkman2012wayneworkman2012 Posts: 15Registered Users ✭✭
edited September 2017 in Solar Beginners Corner #1
Morning all,

I have an MPPT charge controller rated for 40A, and can take an input of 12 or 24v, and output 12 or 24v. My battery bank is 600A, it's a cluster of 6 100A batteries in parallel.

I've got this 1200W generator that has both AC and DC outputs on it, and it came with this thin little cable for the DC outlet. One end is sort of like a standard AC plug you'd put into an AC outlet but the prongs are not parallel, they are a little offset (to keep people from plugging it into the wall I presume). The other end are just spring-loaded battery connectors.

The DC output says 12v 10A on it's label.

What I want to try is cutting off the spring loaded connectors, and soldering on a pair of MC4 connectors so I can directly hookup the generator's DC output to the input of my MPPT charge controller. I figure the charge controller can handle the 10 amps since it's rated for 40, and that the MPPT charge controller would actually do a much better job at charging the batteries since it can deliver exactly the right voltage the batteries need as they charge up.

Is this ok to do?



  • mcgivormcgivor Posts: 2,069Solar Expert ✭✭✭✭
    edited September 2017 #2
    Don't do it.! That is not the intended purpose of the DC output, enough said...... there are reasons, not worth explaining, unless you really want to know why not.
      1500W, 6× Schutten 250W Poly panels , Schneider 150 60 CC, Schneider SW 2524 inverter, 8×T105 GC 24V nominal 

  • EstragonEstragon Posts: 2,635Registered Users ✭✭✭✭
    Aside from the problems I suspect Mcgivor is thinking of, the 12v output is likely too low for an mppt controller to charge a 12v battery. A nominal 12v solar panel will put out ~18v.

    The vast majority of controllers buck a higher voltage source down to a lower battery charging voltage. There are a few that are designed to boost voltage, but they're pretty rare. Even if it could work, 10a isn't going to do much for a 600ah bank anyway.
    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
  • mvasmvas Posts: 298Registered Users ✭✭✭
    Since you took the time to ask, then I will take the time to answer.

    The problem is...
    The Max Power Point (MPPT) logic that is built in to the Charge Controller.
    Let's assume the Charge controller starts at 1 amp x 12 volts = 12 Watts.
    Then it tries   2 amps x 12 volts =   24 watts - is more watts!  So, keep increasing amps ...
    Then it tries   3 amps x 12 volts =   36 watts - is more watts!  So, keep increasing amps ...
    Then it tries   4 amps x 12 volts =   48 watts - is more watts!  So, keep increasing amps ...
    Then it tries   5 amps x 12 volts =   60 watts - is more watts!  So, keep increasing amps ...
    Then it tries   6 amps x 12 volts =   72 watts - is more watts!  So, keep increasing amps ...
    Then it tries   7 amps x 12 volts =   84 watts - is more watts!  So, keep increasing amps ...
    Then it tries   8 amps x 12 volts =   96 watts - is more watts!  So, keep increasing amps ...
    Then it tries   9 amps x 12 volts = 108 watts - is more watts!  So, keep increasing amps ...
    Then it tries 10 amps x 12 volts = 120 watts - is more watts!  So, keep increasing amps ...
    Then it tries 11 amps x 12 volts = 132 watts  and then POP, the 10 Amp breaker blows.
    The Charge Controller was "climbing the hill" looking for the Max Power Point.
    But Max Power always occurs just when the breaker pops.

    PV Panel array - The Voltage will fold-back (decrease) as you increase amps, creating a Max Power Point.
    Generator - The voltage does not fold-back (decrease) to any significant degree, to cause a Peak Power Point.
    So the breaker always blows.

    Solution #1:
    Get an inexpensive 120V AC to 10 Amp 2 or 3 stage charger and plug it in to the generator's AC outlet.
    You can now Bulk / Absorb charge your Battery Bank.

    Solution #2:
    Get a 15 amp DC-to-DC Boost inverter.
    Set it to 14.8 Volts MAX and 9.5 Amps MAX.
    And connect it between the Generator 12 Volts and the Battery Bank.
    Bulk Mode = Charge at constant 9.5 AMPS, until voltage rises to 14.8 Volts
    Absorb Mode = Charge at constant Voltage at 14.8 VOLTS, while Amps drop towards 0 Amps.
    A device something like this one ...

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