Automatic control of off grid mini split A/C

Desert RatDesert Rat Solar Expert Posts: 125 ✭✭✭
I posted a couple of questions previously about controlling air conditioners via the AUX function of an inverter. I have since installed two mini split units and have set up the automatic controls on both. I thought I'd share my findings here.
My girlfriend and I have separate houses, both off grid. We have been planning to install mini splits in both places for a few months now, and have recently completed both installations. I added  new solar arrays to both houses to accommodate the additional loads of the mini splits. Each system was working fine and made plenty of power for us, but I knew that neither would support air conditioning. As Dave A. has pointed out, it's best to turn on the A/C before it's needed rather than try to cool down an already hot house. So I wanted to be able to start up the A/C early in the day while it was still relatively cool. To this end I installed six new 260 watt panels at each house for a total of 1560 watts. I faced three of the panels due east, and three due west. This lets us start up the A/C at around 9:00 AM without drawing from the batteries. Assuming full sun, we can leave the units running until 7:00 PM, again without a draw on the batteries. We don't run them at night.
Each of the new arrays are connected to a Morningstar TS-MPPT-60 charge controller. My inverter is an old Trace SW4024, and hers is an Outback VFX3524. The AUX relays work differently in the two inverters, so I'll describe each setup separately.

In the Trace inverter, the AUX relays are the dry contact type. The settings are SET RELAY VOLTS DC and HYSTERESIS VOLTS DC.
When the battery voltage exceeds the SET RELAY VOLTS DC setting, the relay energizes and closes the Normally Open and COM terminals of the relay. The relay opens when battery voltage drops to a voltage equal to the SET RELAY VOLTS DC minus the HYSTERESIS VOLTS DC setting. I have the SET RELAY VOLTS DC at 27.6 volts and the  HYSTERESIS VOLTS DC at 3. So the relay energizes at 27.6 volts and opens at 24.6 volts. The AUX relay controls a Functional Devices RIB01BDC dry contact relay. I have the mini split connected to the Normally Open contacts of the RIB relay. When battery voltage reaches 27.6 at mid morning (normally around 9:00 AM on a sunny day), the AUX relay closes and thus closes the Normally Open contacts of the RIB relay, and the A/C can be turned on. If it gets cloudy enough for the batteries to drop to 24.6 volts, the AUX relay and the RIB relay will open and cut power to the A/C. If the clouds go away, and it is still sunny enough to bring the batteries back up to 27.6 volts, the A/C will start again. If it is too late in the evening for this to happen, it will just stay off.

The Outback inverter's relay has a 12VDC output, and several different AUX OUTPUT functions. I used the GenAlert function, and programmed the same voltages that I used for my system (GenAlert on at 24.6V and off at 27.6V). For this setup I used a RIBU1CW relay which can use 10-30VAC/DC (or120VAC) for the control circuit. I connected the mini split to the Normally Closed contacts of the RIB relay. So the inverter thinks it is sending a signal to start a generator, but it is opening the contacts of the RIB relay and cutting power to the mini split if the batteries get low.

We've been having our summer thunderstorms here early this year, so we have had a chance to see how the systems are working. I am very pleased with both setups, and they are doing exactly as I had hoped.


  • BB.BB. Super Moderators, Administrators Posts: 32,606 admin

    To ask a few other questions:
    1. What brand model of mini-split (and 120 or 240 VAC)?
    2. What array Wattage?
    3. What battery bank voltage and Amp*Hour rating (flooded cell lead acid or other)?
    4. Are you mostly running when the sun is up, or can/do your run into the evening/night time?
    5. Any estimates on Watt*Hours being used for A/C (sun up WH and sun down WH if applicable)?
    6. Do your units have Heat Pump and do you plan on H.P. operation in winter?
    Nice to hear of another successful mini-split AC installation.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Desert RatDesert Rat Solar Expert Posts: 125 ✭✭✭
    1, The units are Pioneer brand 12,000 BTU w110V, 15 SEER
    2. My arrray is 3000 watts total; my GFs is 2880 watts
    3. My battery bank 225Ah @ 24V; hers 450Ah @ 24V
    4. We are running the A/C during the day only.
    5. I checked mine one day with my Kill A Watt meter, and the draw was 2.59 kWh over an 8 hour period. The high temp that day was 97 deg. F.
    6. The units do have heat pumps. I don't plan on using the heat, but I'm sure I'll try it  this winter out of curiosity.
  • animattanimatt Solar Expert Posts: 295 ✭✭✭
    Not sure but is the 27.6v a little low? I could be wrong but would think a little higher would be better for your batteries. Would seem like your still in float territory. 
  • BB.BB. Super Moderators, Administrators Posts: 32,606 admin
    Thank you Desert Rat,

    I guess that somewhere around El Paso Tx (lots of sun)?

    El Paso
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 58° angle from vertical:
    (For best year-round performance)
    Jan Feb Mar Apr May Jun
    Jul Aug Sep Oct Nov Dec
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Desert RatDesert Rat Solar Expert Posts: 125 ✭✭✭
    animatt said:
    Not sure but is the 27.6v a little low?
    I picked 27.4 because that is the voltage on a typical day at which I can turn on the A/C and not draw anything from the batteries.
  • Desert RatDesert Rat Solar Expert Posts: 125 ✭✭✭
    Meant to say 27.6.
  • Desert RatDesert Rat Solar Expert Posts: 125 ✭✭✭
    BB. said:
    I guess that somewhere around El Paso Tx (lots of sun)?

    I'm near Terlingua and Big Bend National Park. You're right about the lots of sun. I keep careful records, and during a typical calendar year I can count on one hand the number of days my batteries don't reach Absorption. ( And that was before I doubled the size of my array.) :)
  • Hill_CountryHill_Country Solar Expert Posts: 96 ✭✭
    How has your off-grid mini split system been working after almost two years? 

    Any advice for doing things differently?  And how have your Pioneer mini-splits been doing?  We are considering using a 12,000 BTU Pioneer WYS012GMFI22RL (230 VAC) with an EER of 13 and SEER of 21.5 in the future.  Thanks!
    100% Off-grid with: 8 Solarworld 275 Watt Panels, 8 Concorde SunXtender 405aH 6v AGM Batteries, MS-4448PAE 48v Inverter, MidNite Solar Classic 200 Charge Controller, 10,000 gallon rainwater collection system, etc.
  • Desert RatDesert Rat Solar Expert Posts: 125 ✭✭✭
    Just saw your post today. I'm starting the third season on my (and my girlfriend's) mini split. Absolutely no problems with either. I feel that controlling them via the AUX relays is a must.for an off-grid setup. These are a big load, and would quickly damage a battery bank if left to run for long without sun. No issues with the Pioneer units: I've since installed a couple of others.
  • Dave AngeliniDave Angelini Solar Expert Posts: 6,136 ✭✭✭✭✭
    edited April 2018 #11
    They are built to last I have an old Sanyo that is 16 SEER still running well after 11 years.
    But, the new 30+ SEER units can run without sun with a decent battery bank. Fujitsu, LG and Mitsu make some amazing mini-splits.
    We have both and can run them alone or in tandem. Some very good reasons for having both ;)
    It is all good when you are cool and cooking dinner :)
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
    E-mail [email protected]

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