mini split and LIon in RV

[olmz]

Hi,

I am struggling to understand how to configure a 115 volt, 12000 btu mini split (mr cool) with "x" number of Lion 100Ah, 12 volt batteries (battleborn) in parallel....in a 5th wheel RV (so my options are limited)

the max wattage (20A x 115 volts) is 2300 watts.

I have been told that at full blast with the 4 batteries I have at present, I would only get about 2 hours, as among other factors, there would be some loss converting DC to AC (I would run the AC off of an inverter, most likely a magnum 2812 inverter)...

Would some kind and knowlegable soul tell me if you agree with this, and if "yes", what kind of battery bank I would need to run this kind of minisplit?

many thanks!

oliver

 

[olmz]

PS the batteries are to be charged with 3 LG 375 watt panels if we have enough roof space, but two, minimum, if we don't.

on shore power, of course, none of that matters.....but the question is about, obviously, off-grid.

all comments more than welcome. thanks.

 

[MisterSandals]

Some reading...

Search results for query: mr cool

diysolarforum.com

 

[olmz]

pps with apologies for stutter stepping, but in case it helps "you" to help me, the panels will run thru a midnite 150

 

[olmz]

Some reading...

Search results for query: mr cool

diysolarforum.com

thanks so much.

 

[rmaddy]

Here's some rough math to show how you might figure this out.

If the A/C is 20A at 115VAC then as you say, the wattage is 2300W. Since you need this from an inverter and an inverter isn't 100% efficient, let's assume 90%, then 2300W from the inverter is about 2550W from the battery.

A 12V 100Ah LiFePO₄ battery is 12.8V x 100Ah = 1280Wh. Figure you can use 90% so that's about 1150Wh usable.

So 1150Wh / 2550W = 0.45 hours (nearly 27 minutes). That's per battery. That assumes the A/C runs at 20A 115VAC constantly.

For example, 4 hours of constant runtime would require 4h / 0.45h/battery = 9 batteries.

None of this takes into account recharging the batteries via solar or shore power.

 

[olmz]

Here's some rough math to show how you might figure this out.

If the A/C is 20A at 115VAC then as you say, the wattage is 2300W. Since you need this from an inverter and an inverter isn't 100% efficient, let's assume 90%, then 2300W from the inverter is about 2550W from the battery.

A 12V 100Ah LiFePO₄ battery is 12.8V x 100Ah = 1280Wh. Figure you can use 90% so that's about 1150Wh usable.

So 1150Wh / 2550W = 0.45 hours (nearly 27 minutes). That's per battery. That assumes the A/C runs at 20A 115VAC constantly.

For example, 4 hours of constant runtime would require 4h / 0.45h/battery = 9 batteries.

None of this takes into account recharging the batteries via solar or shore power.

thanks very much for demystifying. it is obvious that I have been in dreamland on this. no longer, thanks to you and here.

 

[rmaddy]

Do keep in mind that 2300W for a mini split sounds really high. I have a 5000BTU window A/C that only uses 430W at full blast and mini splits are supposed to be pretty efficient. I'd scan through those "Mr Cool" posts and find out how much wattage it really uses.

 

[MisterSandals]

I have a 5000BTU window A/C that only uses 430W at full blast and mini splits are supposed to be pretty efficient.

Yea, i was thinking 2300W must be surge/startup. I have a 9000BTU Pioneer 22 SEER split mini and it uses around 1000W
when the compressor and fan are running on high. I have not checked surge amps - if it was easier to check i would, sorry.

 

[CamoGreg]

Yea, i was thinking 2300W must be surge/startup. I have a 9000BTU Pioneer 22 SEER split mini and it uses around 1000W
when the compressor and fan are running on high. I have not checked surge amps - if it was easier to check i would, sorry.

My Pioneer 9K BTU 22 SEER is scheduled to arrive soon and from what I've read from others is they stay comfortable averaging around 600W if not running full blast.
2300W seems very high to me as well.

 

[olmz]

Do keep in mind that 2300W for a mini split sounds really high. I have a 5000BTU window A/C that only uses 430W at full blast and mini splits are supposed to be pretty efficient. I'd scan through those "Mr Cool" posts and find out how much wattage it really uses.

i thought so too, but the standard formula w=a X v takes us there. that said, I have been asking around trying to find what the numbers really are..
I did talk to someone from mr. cool who said that 2300 watts could be expected on startup, so even though I would expect the usual amount to be much lower, it seems that I have to expect that number at some time, anyway and therefore have a battery bank, etc., that is able to handle it.

many thanks.

ps the amperage and wattage I have seen on 9k v 12000 units shows about half the maximums vs. the unit I am looking at....but I cannot get by with 9k, unfortunately...although it must be 115 volts. thanks again.

 

[rmaddy]

I did talk to someone from mr. cool who said that 2300 watts could be expected on startup

That would be only for a second or two, not hours. That startup surge would use virtually no battery power at all. The actual runtime wattage is far more important for determining the amount of battery needed to run.

The startup surge is really only relevant when choosing the size of inverter you will need so it can handle that very brief surge.

 

[olmz]

thank you, maddy. that was my guess too, but as I have learned, even at, say, 1200 watts, just to make up a number, running the mini split I am looking at for more than a handful of hours a day, is going to take much more "battery" than I presently have (that being those 4 battleborn 12v 100Ah batteries).

as to the inverter, while I was hoping to use something smaller, I do have that magnum 2812 if I need it (but all these components are inside the rear of an RV, so it is getting kind of crowded, too). thanks again

 

[olmz]

Do keep in mind that 2300W for a mini split sounds really high. I have a 5000BTU window A/C that only uses 430W at full blast and mini splits are supposed to be pretty efficient. I'd scan through those "Mr Cool" posts and find out how much wattage it really uses.

believe it or not, mr. cool's spec sheets omit any wattage number on, anyway, at least the unit I am looking at. I don't know why, but when I called them, they used the standard wattage formula and didn't give me much beyond that. perhpas it is because wattage usage is so much a function of the environment that they don't want to get pinned down, but there was nothing.

 

[RobertGreen]

believe it or not, mr. cool's spec sheets omit any wattage number on, anyway, at least the unit I am looking at. I don't know why, but when I called them, they used the standard wattage formula and didn't give me much beyond that. perhpas it is because wattage usage is so much a function of the environment that they don't want to get pinned down, but there was nothing.

Yeah, the specific wattage is going to depend on environmental factors. For an air conditioner, it's going to operate less efficiently when it's very hot outside and so forth. That's why these units are often rated in SEER, EER, or with other metrics that will hopefully allow a quick understanding of their relative efficiency. IME, expecting around 1000w of power use for each 10K btu of cooling is a reasonable rough estimate for a lot of the midrange mini splits.

IDK what model you're using, but if it's the 12K BTU 120V "Advantage" unit, the EER is 9.5 according to this document (https://mrcool.com/wp-content/dox_repo/mc-adv-3-br-en-01.pdf)
Which would often put the power use somewhere around 1260W when running at High under unfavorable conditions

 

[olmz]

helpful rough estimate. many thanks

do you run your AC on shore power or is it part of a solar and battery set up?

 

[100 Proof]

If you can get "3 LG 375" panels on the air, that will come pretty close to covering your steady state mini-split demand. In that scenario, your battery bank will last MUCH longer. I'm running a 5,000 BTU window air conditioner off of ~750W of solar panels with no draw from my battery bank, other than when a cloud blows over.

 

[olmz]

If you can get "3 LG 375" panels on the air, that will come pretty close to covering your steady state mini-split demand. In that scenario, your battery bank will last MUCH longer. I'm running a 5,000 BTU window air conditioner off of ~750W of solar panels with no draw from my battery bank, other than when a cloud blows over.

i sure hope I can. i am dealing with roof space. these panels and batteries are going to be a separate, redundent system. I already have 3 310 watt astro panels and 4 monster concord lifeline batteries (midnite 150 controller and magnum 2812) as my main system, and I cannot remove any of that, at least for now. so we are in the process of trying to figure thru all this. it may be that the best I can do is get only two of the LGs up there, but even if that is the case, and I am off grid long enough to need the minisplit (I have two colemans and they are rapidly aging and plus, getting replacements for this particular model has been impossible for near two years due to their supply chain issues..and they use way too much power to run them off of any storage system)...i can probably do some charging of these battleborns using a simple charger and the current produced by the main system i just described to you.

if times were different from a variety of perspectives, I would plumb all of this into one main system, but I cannot get that done where I am, so it will be two separate systems. I don't really mind as I believe in redundency anyway.

or the world will increase all this military activity, and I won't have to worry about any of this.

thank you.

 

[olmz]

ps i would use a window unit, but this RV does not have the kind of windows where that is possible. the alternative is to start cutting hole, doing this, that and the other thing, and in an RV, that can bring a whole new set of problems...so the mini is my only option.

 

[rmaddy]

A mini split is much better than a window A/C unit. Quieter and more efficient.

Keep in mind that running the A/C mostly on solar is great but of course it only works when the sun is out. My solar easily runs my 5000BTU A/C during most of the day, even with flat mounted panels. It's when running the A/C late into the afternoon and early evening that the battery has to kick in. If you only need to run the A/C for about an hour after the sun is too low for the solar to keep up, then you just need enough battery for that hour of A/C plus your other loads for the night. Then hopefully you get enough solar in the morning to recharge the batteries before it's hot enough to turn the A/C on again midday.