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.