Is it OK to run mini split until battery is low nightly?

turtlepower

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My power bill is mostly from my central HVAC. I want to run a mini split run from solar to offset my power bill. I am in South Florida and have 6 panes, totalling 1.5 KW ( of 33 volt solar panels. There are run as 3 units of 2 (66 volts each) to a 60 amp MPPT and about 5 deep cycle marine batteries. This system is not grid tied. The system seems happy but is only charging batteries which do nothing.

I get the batteries cheap from a friend (about $50 new).

I have a two low voltage cut off switches I could use: The Victron Battery Protect and I also have a battery isolator which will only lets electricity though when battery bank is over 13 volts. I could use these to either only allow electricity from the batteries to the inverter when the solar is charging the batteries or when the batteries are at least half full.

I don't want to assume I have enough batteries to last the whole night running a mini split. I don't mind if the mini split turns off a few hours at night.
- I don't want the mini split do be damaged by the power being shut off at night.
- I don't want to have to mess with turning the inverter or AC back on daily.

I'm looking at buying this mini split:

And this inverter:

Q: Will the mini split be damaged by the nightly power shut off?
Q: Will I this system be automatic, or will I have to reset or push buttons every morning with the inverter is given power through the low voltage switch?

Thank you very much.
 

OffGridInTheCity

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I use several ATSs in my off-grid system - including 120v Go-Power(s) like this - https://www.amazon.com/gp/product/B00153EYTO/ref=ppx_yo_dt_b_search_asin_title

You feed in grid for shore side and inverter for the generator side. What happens is when it senses inverter power it will delay 20secs (for power to spin up) and then auto-switch to inverter power. When the inverter cuts-off, it auto-switches to grid. These switch-overs are quick. The advantage of an ATS is your mini-slit will operate 24/7 even if you have low sun (no inverter) for a day or 2 working on the system or storms.

I've not had issues with daily heat-pump ATS switch-overs - which include 9000 Senville mini-split, 12,000 IBM floor unit heat-pump, the whole house Lennox heat-pump, and the Rheem hot water heater heat pump. All 4 of these retain their settings etc.

Even if you don't use an ATS - my guess is you won't have problems with the mini-split going off for a few hours and coming back on - but you might loose settings such as the 'temp'. You might be able to get this answer from the manufacturer.

In my case, I just tried it and had great luck!
 

Bluedog225

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You may want to look into the issues related to mixing and matching new and old batteries given your situation.
 

Hedges

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My math has said that lead-acid batteries cost $0.50/kWh (AGM) and $0.25/kWh (FLA) by the time they are worn out.
That makes them relatively or completely non-competitive with grid power; you save money simply buying from the grid.
What does your math say, give your source of batteries?
 

turtlepower

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My math has said that lead-acid batteries cost $0.50/kWh (AGM) and $0.25/kWh (FLA) by the time they are worn out.
That makes them relatively or completely non-competitive with grid power; you save money simply buying from the grid.
What does your math say, give your source of batteries?
That's a great point. I don't have an answer to your question. But even if the batteries are dead, I'm hoping the solar should still run the mini split with a battery plugged in and turn off when the battery (or battery bank) dies. Maybe the batteries last all night. Maybe the batteries last 5 minutes. But I'm hoping the solar runs the mini split for most of the day and saves money. I'm also hoping to have AC when the power is out.
 

Hedges

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I do something similar, but turn the AC off in the evening when I don't need it any more (mild weather in San Jose, CA)

The batteries are needed to provide starting surge of A/C. "LRA" if given on motor, or about 5x nameplate running amps. Unless it is an inverter-drive mini-split which could have very low surge.

I would suggest having a control to enable/disable A/C at a relatively high battery SoC, like 85%.
Then the batteries should last for years, letting PV power the A/C.

My system has oversized PV array, undersize AGM bank just large enough for one night. During the day, plenty of power for A/C and all other loads.
 

turtlepower

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I do something similar, but turn the AC off in the evening when I don't need it any more (mild weather in San Jose, CA)

The batteries are needed to provide starting surge of A/C. "LRA" if given on motor, or about 5x nameplate running amps. Unless it is an inverter-drive mini-split which could have very low surge.

I would suggest having a control to enable/disable A/C at a relatively high battery SoC, like 85%.
Then the batteries should last for years, letting PV power the A/C.

My system has oversized PV array, undersize AGM bank just large enough for one night. During the day, plenty of power for A/C and all other loads.
So does the AC just turn back on in the morning when the power turns on?
 

Hedges

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I have electronic clock/thermostat. Manually set to heat/cool. Separate switch for gas/electric heat.
With that, A/C on/off would be based on time of day, but used infrequently I just do that manually for cooling. Heat starts before I get up (hopefully still connected to grid; I haven't wired it to automatically switch off electric heat when on batteries, but I should).

My battery inverter (Sunny Island) has two SoC setpoints available. One is 70% DoD, at which point all loads (the house) are disconnected but AC coupled PV is kept on line to wait for morning. With the 0.2C charge rate I've programmed, would take several hours to recharge. In the mean time, excess PV production would be wasted.

So rather than re-enabling based on SoC, time and/or light sensor would be better if you need morning A/C.

The "load shed" at 70% DoD is to prevent over-discharge, but I would try to keep loads light enough that I make it to sunrise with power still on.
 

Hedges

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You could also look for a battery charger or DC power supply to keep battery voltage up. When PV is producing and charging to a higher voltage, it provides all power. At night when voltage starts to drop, AC picks up the load.
 

turtlepower

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I did it. For anyone curious, here's how it went:

I installed a $700 Ebay no-name "toshiba inverter" "12000 BTU Air Conditioner Mini Split 19 SEER INVERTER AC Ductless Heat Pump 110V" and a $170 "ce rated" It has a heat pump and a wifi phone app.

I also installed an Ebay no-name 24 volt 1500 (3000 surge) Watt inverter.

I have 6 used 275 watt, 33 volt panels. $50 each. I have them in 3 pairs. (Sorry I don't know the notation you guys use). I broke the glass on one, but it seems to still work. So each pair of panels gives 66 volts. Each panel shows about 4 amps on my multi-meter. So 3 of these pairs gives about 12 amps. The charge controller says it's getting 850 watts when the AC is pulling 1000 watts. They are laid flat on a level roof. They get dusty. I don't clean them.

I run these panels though 2-to-1 and 3-to-1 cables directly to a EM2460 MMPT solar charger. This can take 1560 watts when hooked to 24 volt batteries and can charge at 60 amps.
I have 4 12 volt Walmart "deep cycle" batteries. 2 "pairs" so it's 24 volt. They are mismatched. I got them new from a guy for $30 buck each. They seem to work well. I borrowed one to power a boat trolling motor and it worked great.

The HVAC draws between 800 and 1,100 watts when working hard. I think the AC will run run hard in cool or heat mode all day, but the weather is too pleasant to know yet. I set the phone app to schedule it to work less hard at night. On low mode it sometimes makes it all night. Sometimes the inverter starts beeping and wakes me up.

I spent about $1,500 so far. I think this will pay for itself within a year or two.
I'm pretty happy about all this.
Duke energy tell me that on average I use 2,000 watts hours in the summer each day. I'm hoping this lessens my power bill and gives a nice backup in the Florida heat when the power goes out.

I plan to expand this as I learn more. Next I'm going to see if I can get a net-meter power meter (source https://www.duke-energy.com/home/products/renewable-energy/generate-your-own). Then more 12 more panels and a hybrid/all-in-one inverter.
 

Bluedog225

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If you sub in some cheap LiFePo4 you would be set. Though no reason not to fluff the lead as long as it goes.
 

time2roll

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Q: Will the mini split be damaged by the nightly power shut off?
Q: Will I this system be automatic, or will I have to reset or push buttons every morning with the inverter is given power through the low voltage switch?

Thank you very much.
I would tend to switch off the AC load vs cut the DC power. Likewise I would not systematically cycle the inverter based on the low voltage shut down and the automatic reconnect.

The lead-acid may not last long. First upgrade I would make is an LFP battery.
 

turtlepower

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I would tend to switch off the AC load vs cut the DC power. Likewise I would not systematically cycle the inverter based on the low voltage shut down and the automatic reconnect.

The lead-acid may not last long. First upgrade I would make is an LFP battery.
That's what I do if i'm in my right mind. But that 3am BEEEPPPPPPPPP make me just flip the switch. But I have noticed if I just turn off the AC the beep stops.
 

time2roll

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That's what I do if i'm in my right mind. But that 3am BEEEPPPPPPPPP make me just flip the switch. But I have noticed if I just turn off the AC the beep stops.
That beep in the night would be fixed or crushed. A battery voltage switched relay could control a larger relay to cut power before the beep.
Probably extend the life of those desperate batteries too.
 

turtlepower

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That beep in the night would be fixed or crushed. A battery voltage switched relay could control a larger relay to cut power before the beep.
Probably extend the life of those desperate batteries too.
Where do I get a battery voltage switched relay?
 

time2roll

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