Using AUX to activate transfer switch for HVAC

Started by jwb, December 11, 2022, 02:04:38 PM

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jwb

Hello again. We are planning to install a mini-split HVAC system to replace 2 window air conditioner units and also provide back-up heat in cold weather. We live in a humid mountainous area with temps ranging 0F - 90F, and our solar exposure is compromised in fall and winter. Our 2500w PV array with Classic 200SL and SW4048 can power both window units for several hours on sunny days, but we have been manually changing them to grid AC outlets at night or when there isn't enough sun. It would be great if the Classic could help us switch HVAC from solar to grid according to solar input or battery voltage. Could we use AUX2 for this? We are already using AUX1 on Low Batt Disc to control a SSR that switches grid input on/off to our SW. We did this because the Chevy Volt lithium batteries we bought have an 38-48v charging range that was not compatible with the SW charging parameters (oops!) To keep the SW from overcharging the batteries we have to use the Classic battery voltage to start/stop AC input. That system has been working well (thanks to this forum!) but I'm not sure if AUX2 could control another relay with different parameters for HVAC switching. Any thoughts? Thanks!

boB

You might be able to use the Whizbang Junior current shunt monitor (Aux 2) and then control your grid use based on SOC %  (State Of Charge)  from Aux 1 output.

OR control grid use based on battery voltage itself. 

Can you tell at what battery voltage that your batteries are low ?

Those are the two methods that come to mind quickly.

boB
K7IQ 🌛  He/She/Me

jwb

Hello - SW charging was always problematic with our batteries, but it has become more restrictive since we converted to using only L1 120v for grid input. Now the SW charger clunks loudly and throws a fault if we set charging voltage to start below 44.0v on Classic aux1, so 44.0 is the lowest we allow the batteries to get on a regular basis. However, when grid input is switched off manually, or during a power failure, the inverter works fine down to about 40.5v. Supposedly the lithium batteries are safe to use down to 38v, but the SW inverter shuts down at about 40.5.
We try to reduce how often there are large loads while the charger is active and there is little/no solar input because otherwise we either get short cycling due to aux1 charging 44.0-45.5, or if we raise the aux1 cut-out voltage to something like 46.5, the batteries often reach full charge earlier in the day while there is potentially still plenty of sun.
Sorry if that is confusing. Basically, in our current situation we are either putting the batteries through increased shorter charging cycles, foregoing solar input, or running up and down stairs, setting timers, changing plugs into different electrical outlets, etc. Thus far my wife has been a really good sport about it.

ralph day

What you're doing sounds like what I do with my Outback VFXR3648, Mate3 and 2 Classic 250's.  On HBX mode the Mate will switch in or out the grid when the Heatpump is running.  If ther's evough sun or wind the loads are handled by the renewables (battery), if not then the utility runs thingsl.

HBX mode is low battery transfer...transfers loads to utility (whole house) at 50vdc, disconnects from utility at 53vdc.

jwb

Thanks to Ralph for the confirmation. Our aux 1 port is already in use, so I think we would need to use aux 2 to control a relay for HVAC grid versus solar input. Day Light and/or Nite Light sound like they could help us. It looks like those modes are intelligent timers that open/close aux 2 according to PV input voltage rather than time of day. Will aux 2 Day Light and/or Nite Light cycle only once per day, or could they also cycle mid-day according to cloudy periods?
Another switching option we found is the MOES NEW Smart Solar Wind Charge Controller Dual Power Automatic Transfer Switch, which uses mechanical relays to switch between grid and solar/wind input according to battery voltage. Reviewers warn that if/when a mechanical relay failed, grid could potentially backfeed and damage the inverter. I'm not sure if the SW4048 has backfeed protection to prevent that.
I would rather rely on the Classic and SSR if possible.
Thanks again!