Options for powering DC fridge with Midnite Classic

ghostofthenorthghostofthenorth Registered Users Posts: 9 ✭✭
Hello folks, 

I've tried scouring the forum for this topic butbhavent really found the information I need.  My setup is 4 x 235W panels, Mindite Classic, 24V battery bank (plus a Samlex inverter).  We are going to replace our propane fridge as it's broken... and I've wanted to go DC anyways.  The fridge is 24V, and the obvious answer to the question of how to hook it up is to run lines from the battery bank.

The further question I have is, for midday when the sun is shining bright and the batteries are fully charged, should I (and can i?) Wire in a sort of transfer switch to allow for the Classic to power the fridge when the batteries are full?  I am assuming that constantly drawing from the 'full' bank will put additional strain on the batteries.  So I was thinking that I could wire it to run off the batteries while they are not fully charged up, and at night, but to use the Classic to power them when the batteries are full.

Perhaps I'm mislead on this, and looking for some guidance.  Thanks in advance for the advice!!



  • littleharbor2littleharbor2 Solar Expert Posts: 1,628 ✭✭✭✭
    edited October 2020 #2
    Once the batteries are full the Classic will basically bypass the batteries and power all needed loads up to the available power from the panels. No transfer switch needed.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 540 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • mike95490mike95490 Solar Expert Posts: 9,362 ✭✭✭✭✭
    First, does your fridge use a compressor when running on DC or a resistive heater element to emulate the propane flame ?

    The Classic has a "Waste Not" mode, where it can trigger a relay to power a load. You must use a relay rated for the DC volts and amps you will be interrupting.   Also, FLOAT can trigger the same relay. 
      You cannot rely on  this to not let a relay "chatter" if you are running a compressor.  A compressor needs at least 5 maybe 10 minutes to let pressure equalize before restarting or you risk overloading it.

    If your solar and batteries are sized properly for your load, you should just let the fridge run on it's own.  Otherwise, if it's shut off, and then gets warm, the next cooling cycle is going to take 3x as long!

    Powerfab top of pole PV mount | 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

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • 706jim706jim Solar Expert Posts: 437 ✭✭✭✭
    Have you purchased the fridge yet? Here in Canada a 24 volt fridge sells for $1800 while a standard similarly sized 120 volt fridge sells for $400.

    And self defrosts too.

    You could buy four conventional ac fridges for the cost of a single DC fridge at least where I reside.

    Island cottage solar system with 2400 watts of panels, 1kw facing southeast 1kw facing southwest 400watt ancient Arco's facing south.Trace DR1524 MSW inverter, Outback Flexmax 80 MPPT charge controller 8 Trojan L16's. Insignia 11.5 cubic foot electric fridge. My 28th year.
  • PhotowhitPhotowhit Solar Expert Posts: 5,657 ✭✭✭✭✭
    The way a charge controller it works...

    Hook the fridge directly to the battery bank!

    Once the charge controller reaches float and the voltage is held at a very high level, actually minimally charging around 26.6-27.4 normally on a lead acid battery. Once a load is introduced the voltage will drop and the charge controller will increase the amount of energy it allows from the array to maintain that float voltage.

    Adding a fridge to an existing system will be a substantial change. Watch for chronic under charging. You might consider adding to your array.

    If you have a lithium battery bank, things work a bit differently. Usually they will be set for a single charge each day. 
    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 backup...lol), 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.
  • ghostofthenorthghostofthenorth Registered Users Posts: 9 ✭✭
    Thanks for the quick feedback.  The waste not mode is likely what I had been thinking if.  Its been a while since I did the original installation and setup.  I dont want to use an ac fridge, as it's less efficient, and when we are not at the cabin, I would have to leave on the inverter.  Seems like an added risk, and at least less efficient.

    Sounds like just connecting it to the batteries is all I need to do.

  • ghostofthenorthghostofthenorth Registered Users Posts: 9 ✭✭
    Also, as a note, this is the fridge we bought: http://www.solartrader.ca/Unique_9.0_cu.ft_Solar_Powered_DC_Fridge
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