High Voltage Battery Only with 240v Inverter? What's the highest efficiency possible?

**SunEagle** · 09-23-2021, 11:53 AM

Hello NotGonnaLie and welcome to Solar Panel Talk

I have not heard of a stand alone inverter like that so my guess it would be a very unique piece of equipment. The only ones that will accept a high DC input voltage and produce 240V AC are grid type or hybrid inverters. Although those are usually connected to solar panels and not a battery for the DC input

**Mike90250** · 09-23-2021, 04:59 PM

You may be able to find a used industrial UPS that uses a high voltage battery, and salvage the inverter from it.

**NotGonnaLie** · 09-24-2021, 03:20 PM

Originally posted by SunEagle

Hello NotGonnaLie and welcome to Solar Panel Talk

I have not heard of a stand alone inverter like that so my guess it would be a very unique piece of equipment. The only ones that will accept a high DC input voltage and produce 240V AC are grid type or hybrid inverters. Although those are usually connected to solar panels and not a battery for the DC input

Thank you. Do you suppose something like the SMA SunnyBoy would treat a battery hookup as if it were PV? I don't see anything about the SunnyBoy requiring extra gear to be connected to the solar panels, unlike the SolarEdge stuff.

Originally posted by Mike90250

You may be able to find a used industrial UPS that uses a high voltage battery, and salvage the inverter from it.

True but I doubt they are prioritizing efficiency at all, I'm not going to see anything like the 95-99% range these PV string inverters have. Trying to power off-grid air conditioning from a solar-charged battery array, I want it to be as efficient as possible, even if I have to pay a premium.

**SunEagle** · 09-24-2021, 04:00 PM

Originally posted by NotGonnaLie

Thank you. Do you suppose something like the SMA SunnyBoy would treat a battery hookup as if it were PV? I don't see anything about the SunnyBoy requiring extra gear to be connected to the solar panels, unlike the SolarEdge stuff.

True but I doubt they are prioritizing efficiency at all, I'm not going to see anything like the 95-99% range these PV string inverters have. Trying to power off-grid air conditioning from a solar-charged battery array, I want it to be as efficient as possible, even if I have to pay a premium.

Well solar panels are current sources while a battery or power supply are voltage sources. I don't think a DC battery will work but then again I have been proven wrong in the past.

**bcroe** · 09-24-2021, 04:40 PM

I will point out, that it might be possible to run an air conditioner DIRECTLY from
your 400VDC battery, skip the inverter. I am talking about the latest mini split
units, which can be more than TWICE as efficient as old stuff. Here the input is
250VAC, but that is immediately rectified to around 350VDC to drive the variable
speed motors. Skip the rectifier (and the inverter), go in direct with a big power
savings (and other advantages).

This is how the Conditioning units powered directly from solar panels work, often
with a line backup input. Have not tried it yet, more research needed. Bruce Roe

**NotGonnaLie** · 09-24-2021, 04:59 PM

Originally posted by bcroe

I will point out, that it might be possible to run an air conditioner DIRECTLY from
your 400VDC battery, skip the inverter. I am talking about the latest mini split
units, which can be more than TWICE as efficient as old stuff. Here the input is
250VAC, but that is immediately rectified to around 350VDC to drive the variable
speed motors. Skip the rectifier (and the inverter), go in direct with a big power
savings (and other advantages).

This is how the Conditioning units powered directly from solar panels work, often
with a line backup input. Have not tried it yet, more research needed. Bruce Roe

That's fascinating, my batteries are LiFePO4 so they hold an extremely flat voltage curve as they discharge and are almost permanently near at that voltage. How did you discover that those units are doing that rectification to 350 volts DC rectification? I was looking at the Mitsubishi MSZ-FH06NA https://www.mitsubi****echinfo.ca/si...9_201910_0.pdf

Is there a way that I could confirm that a specific model could be hardwired to the battery, short of purchasing it and opening the unit up? I'm sure the manufacturer isn't keen to expose that info for fear of electrocuting customers.

**Ampster** · 09-24-2021, 07:31 PM

Originally posted by NotGonnaLie

........

Is there a way that I could confirm that a specific model could be hardwired to the battery, short of purchasing it and opening the unit up? I'm sure the manufacturer isn't keen to expose that info for fear of electrocuting customers.

In addition to heat pump AC units that can be run off DC there are also ones that are variable speed that I assume use an internal inverter to modulate the speed. I have not done a deep dive but I assume most of those are variable frequency three phase inverters. The ones I am thinking about are the ones that have high SEER ratings.
My next purchase is probably one of those units that can be run off 240 AC so I can use my existing grid tie system to power it. That way when not using the heat pump AC, I can sell excess solar power to the grid with out any additional hardware. Since efficiency is in the title of this thread, that is going to be the most efficient system for me.

For someone off grid the math is different.

**bcroe** · 09-24-2021, 11:27 PM

Originally posted by NotGonnaLie

That's fascinating, my batteries are LiFePO4 so they hold an extremely flat voltage curve as they discharge and are almost permanently near at that voltage. How did you discover that those units are doing that rectification to 350 volts DC rectification?

Is there a way that I could confirm that a specific model could be hardwired to the battery, short of purchasing it and opening the unit up? I'm sure the manufacturer isn't keen to expose that info for fear of electrocuting customers.

To be sure, find a panel operated AC unit, note the range of panel voltage possible. I
have not explored that, or opened up any of the 6 variable speed motor heat pumps
here YET. But inside each is an AC motor driven by a DC to AC inverter, no doubt 3
phase. My rather high 250VAC line gives 350VDC with a basic rectifier, as the peak
is 1.414 times the rms.

There are NO USER SERVICEABLE PARTS signs on such equipment, so moving
to straight DC input involves good technology and safety knowledge today. Maybe soon
it will be possible under warrantee. My first approach will be to see if DC can just be
applied to the AC input to flow thru the rectifier, such as the ancient AC/DC radios did.
Bruce Roe

**peakbagger** · 09-25-2021, 06:22 AM

Looks like this Plug Out Power makes what you want. They supply inverters that convert various Hybrid cars to home generators. I think they are 240VAC output units.

**Ampster** · 09-25-2021, 11:39 AM

Plug out Power looks to be an improvement over the inverter I hooked up to the 12 volt battery in my Tesla. I did not see a price but to deliver 240 volts AC it has to cost
more than the $200 I spent.

**peakbagger** · 09-27-2021, 03:42 PM

Definitely a lot more. I think ithat are in the $3,000 range.

**NotGonnaLie** · 09-27-2021, 10:54 PM

Originally posted by bcroe

To be sure, find a panel operated AC unit, note the range of panel voltage possible. I
have not explored that, or opened up any of the 6 variable speed motor heat pumps
here YET. But inside each is an AC motor driven by a DC to AC inverter, no doubt 3
phase. My rather high 250VAC line gives 350VDC with a basic rectifier, as the peak
is 1.414 times the rms.

There are NO USER SERVICEABLE PARTS signs on such equipment, so moving
to straight DC input involves good technology and safety knowledge today. Maybe soon
it will be possible under warrantee. My first approach will be to see if DC can just be
applied to the AC input to flow thru the rectifier, such as the ancient AC/DC radios did.
Bruce Roe

I think see what you're talking about now. Because of how RMS works, changing AC to DC with a bridge rectifier will always get 1.414x the AC voltage input as DC output?

Doesn't the rectification introduce some inefficiency? 250 volts AC times 1.414 is 353.5 volts DC. If you were measuring 350 volts DC from a 250 volt AC input like you said, that would suggest the bridge rectifier in your unit is 99% efficient?

It sounds like without knowing the efficiency of the rectifier in whichever unit I try to run on straight battery DC, I can't know what the compressor inverter will be expecting / receiving. Unless I actually buy it first and test with a multimeter?

**bcroe** · 09-27-2021, 11:56 PM

As I said this may be an experiment for those with the experience giving good technology
and safety knowledge. If a unit has any circuit running directly off the AC input (no I have
not taken mine apart YET), it will fail. 99% efficient is about what I would expect from a
bridge rectifier at this voltage.

The safe way to do this is use HVAC designed to be run directly from solar panels. I do not
know if these go up to 390VDC at this time, though my solar system certainly does. Bruce Roe

**Mike90250** · 09-28-2021, 12:21 AM

Originally posted by NotGonnaLie

.........
It sounds like without knowing the efficiency of the rectifier in whichever unit I try to run on straight battery DC, I can't know what the compressor inverter will be expecting / receiving. Unless I actually buy it first and test with a multimeter?

Please folks, this is not science lab. This is high voltage gear and if you are guessing or hoping, you are in trouble already.

No User Serviceable Parts Inside

If you touch the wrong thing, or accidentally short the wrong thing, AC or DC, it's quite possible to kill you....

Bruce knows his stuff, I don't know you and you are asking far too basic questions for me to ignore.

danger do not touch hurt while you die.jpg

High Voltage Battery Only with 240v Inverter? What's the highest efficiency possible?

High Voltage Battery Only with 240v Inverter? What's the highest efficiency possible?

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