Desert Rat ✭✭


Desert Rat
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  • Re: Automatic control of off grid mini split A/C

    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.) :)
  • Automatic control of off grid mini split A/C

    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.
  • Re: Off-Grid Mini-Split, no Batteries?

    OK, I ran the mini split through my Kill A Watt meter today, and here are the results. I turned on the mini split at 10 AM, when the outside temperature was 81 degrees.The compressor came on at around 12:30 PM when the outside temperature was 88 and the inside temperature reached 80. When the compressor first came on the meter showed a draw of ~1000 watts, Each subsequent time the compressor cycled on during the day it showed a draw of ~460 watts. The outside temperature reached 93 at around 1:00 PM, and the afternoon high was 97. At around 6:00 PM the sun was blocked by clouds and my battery voltage dropped to 25 at which point I turned off the mini split. Total usage during the 8 hour run time was 2.59 kWh. The inside temperature in the room with the mini split never exceeded 80. Note that it could have been lower, but that's where I had it set. It may seem odd to some, but after living here for 13 years with no A/C, anything cooler than 80 degrees in the house would have been too cold. So far I'm happy with the new unit. I have no intention of using it after sundown. It cools off drastically here in the desert at that point, and I always open doors and windows at sunset.