Question about Midnite Kid charge controller capability

blueskiesblueskies Registered Users Posts: 54 ✭✭

Hi, all.

Charge controller question if I may. (Pardon me if I'm not sensible - I have been researching for days)

When I used the sizing calculator for the Midnite Kid and entered four 355 watt panels (2 in series and 2 in parallel for 24 volt battery bank), it indicated I was in good operating parameters. (It said 0.3 units needed)

Can this really work well with a 24v battery bank to a 24v/1500watt inverter for a 520 watt window air unit or will it max it out? (I'm assuming 1120 watts needed for the 520 watt a/c at startup.


  • BB.BB. Super Moderators, Administrators Posts: 31,464 admin
    edited February 2019 #2

    Nominally for a good quality MPPT charge controller, they regulate their output based on rated current (i.e., 30 amps) and will derate as they get "too hot".

    For the Midnite Kid, we can "over panel" an MPPT charge charge controller (as solar panels get hot, their output voltage falls--Typically hot solar panels--Warm day, direct noon sun, no wind), Vmp-std can derate by upwards of 81% (of nameplate rating)... I use 77% as the panel+~5% controller losses to "upsize" the array (generally, you will only see this level of current on cool/clear days around noon--And even then not often--So the "losses" as the controller hits its limit is not very much over the year):

    • 30 amps controller max amps * 29 volts charging * 1/0.77 panel+controller derating = 1,130 Watt array "cost effective" maximum array

    Your "numbers" do not look right for me... 4x355=1,420 is definitely over a Midnite Kid's rating on a 24 volt battery bank. Not 0.3x (30%) rating. (kid tool)

    I am guessing that you used the Midnite Classic sizing tool (80-92 amp rated) and a 48 volt battery bank (I get 0.3 Classic 150 with this configuration). (classic tool)

    The solar charge controller does not really "drive" your loads directly. Yes, if you have 20 amps from the charge controller supplying a 50 amp load, the controller is supplying the 20 amps, but the battery bank is supplying the other 30 amps (i.e., you are discharging the battery bank over time).

    Whether or not "this" is a problem, depends on your energy usage. If you use the A/C for 2 hours, and the charge controller is charging for 5 hours (below is an example, not real life):

    • 50 amp load (A/C) * 2 hours = 100 AH
    • 30 amp charge current * 5 horus = 150 Amps

    So, in the above (totally imaginary) case, you are OK, because, overall, the battery is recharged with more energy (AH) that was taken out by the load.

    Similar case, the charge controller can output only as much power (amps*volts) as the solar array has available--It cannot supply any surge current at all.

    The battery bank is the "buffer" in your system... For a 24 volt battery bank, if you are drawing current, the battery voltage is around 25.4 volts (charged) down to 21.0 volts (that is a dead or near dead battery).

    When charging, you will generally see around 27.0 volts to 29.5 volts charging.

    The "answer" is to design the battery bank to support your loads... Both running Watts and Peak Watts (starting surge). And total amount of energy needed to be stored (Watt*Hours or Amp*Hours * battery voltage).

    Say your numbers are correct for a small A/C unit. 520 Watts running and 1,120 Watts surge. And let's say you want to run the A/C system for 10 hours per day, and it runs 50% duty cycle. And we assume 2 days of storage and 50% maximum discharge.

    Note: This an example, and it probably close enough that it will give you a reasonable answer to your question--But details do matter (you only run with the sun up, run A/C 2 hours 100% in evening, etc.) and you will get different answers. This answer below is "conservative" -- And will give you a larger battery bank and solar array than you may wish--And we can talk details, if needed.

    • 520 Watts * 10 hours per day * 0.50 duty cycle = 2,600 WH per day
    • 2,600 WH per day * 1/0.85 AC inverter eff * 2 days storage * 1/0.50 max discharge * 1/24 volt battery bank = 510 AH @ 24 volt battery bank

    Now a 510 AH @ 24 volt battery bank will supply reliably:

    • 510 AH * 500 WH per 100 AH @ 24 volt battery bank = 2,550 Watt AC inverter nicely

    So, your 1,500 Watt AC inverter should be fine (it may be a bit small for your 520/1,120 Watt inverter--But spec. wise it is fine).

    Now, to charge that battery bank. 5% to 13% rate of charge for solar is suggested. 5% can work for summer weekend usage. 10%+ rate of charge if full time off grid usage:

    • 510 AH battery bank * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 960 Watt array minimum
    • 510 AH battery bank * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,921 Watt array nominal
    • 510 AH battery bank * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 2,497 Watt array "cost effective" maximum

    And there is sizing the array for your hours of sun and loads. Here is a solar irradiation calculator (pick your location):

    I am not sure where you are at (ip address locators say Illinois and West Virginia for you...).

    Let's say you have 5 hours per day (relatively good amount of sun for a summer location, and where you would want to use A/C):

    • 2,600 WH per day * 1/0.52 average AC system eff * 1/5.0 hours of summer sun = 1,000 Watt array minimum

    So, based on solar array and 5% rate of charge (minimum charge for summer weekend system) would probably work OK.

    For a full time/off grid reliable system, I would be suggesting a 1,921 Watt array (based on all of the above guesses).

    This is a starting point for discussion (me spending your money, is easy). What your needs are--Only thing that matters.


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • blueskiesblueskies Registered Users Posts: 54 ✭✭


    Thank you for so many details!!! I will have to digest them when I am not working. You are truly an intuitive, altruistic moderator and clearly very knowledgeable.


  • BB.BB. Super Moderators, Administrators Posts: 31,464 admin

    Thank you for your kind words Dan.

    We all try very hard to be open and helpful here.


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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