Mini Split AC - hacking

aernan

Observer
I just had a very nice chat with a rep from a major AC vendor. He is also into offroading and was nice enough to answer all my questions. Here is some food for thought.

AC/DC Problem:
All inverter based modern split AC systems take 120/220 V AC and turn it into 48 V DC or other voltage internally. They then use that DC to generate the signal to run the compressor. In this vendor's products the AC and DC components can be on separate boards. If you are willing to void the warranty you can direct connect the unit to DC.

Laying the unit on it's back:
The compressor itself has a sump where oil collects and recirculates. The unit will work on off angles but can not be placed on it's back. Placing the unit on it's side or back will starve the pump for oil and cause premature death. If you plan on operating the unit in an odd orientation you would be best to take it apart and only re-orient the radiator fan portion.

Airflow and recommended clearance:
the outside radiator works just like a car radiator. It needs constant airflow. In an AC it's all generated by an electric fan. As long as the air flow through that unit is unrestricted you don't need a giant space around it.

Oversized AC unit:
From my early reading oversizing the AC unit would waste power. This is because the AC compressors was modulated on/off and draws max power when on. Modern inverter based units modulate the compressor speed and don't waste the excess power. So oversizing is actually good to some point. I have also been told that the AC load is most efficient at 60% of capacity. Running at 100% of capacity is actually inefficient.

I just wanted to share all of my finding in case you were wondering. :)
 

aernan

Observer
For me the next important research is to figure out the losses running 48 V DC -> Inverter 120 V AC -> 48 V DC.

I'm assuming the losses are high enough to justify companies making 48 V DC Air Conditioning units.
 

aernan

Observer
Fwiw,
On inverter type minisplit A/C, the compressor maybe speed controlled via inverter/VFD, And potentially hackable to run directly from DC.
Most likely its fanmotors are singlephase AC only.

From what I'm told that inverter (internal to the AC unit) is powered by DC. It may be as simple as detaching the AC -> DC board and connecting to the DC input of the inverter board. No hacking required unless the board does not take 48 V. Then you would need a DC-DC circuit to drive the right DC voltage.

I work with Electrical Engineers and one of them say almost all inverters use 48 V DC internally as the common rail supply voltage. That includes inverters used in cars to make 120 V AC. It also applies to audio amplifiers used in automotive applications.
 

S2DM

Adventurer
From what I'm told that inverter (internal to the AC unit) is powered by DC. It may be as simple as detaching the AC -> DC board and connecting to the DC input of the inverter board. No hacking required unless the board does not take 48 V. Then you would need a DC-DC circuit to drive the right DC voltage.

I work with Electrical Engineers and one of them say almost all inverters use 48 V DC internally as the common rail supply voltage. That includes inverters used in cars to make 120 V AC. It also applies to audio amplifiers used in automotive applications.

This unit has been discussed in other threads

http://www.hotspotenergy.com/DC-air-conditioner/

Its fairly efficient, really efficient if you remove the inverter losses. I considered it heavily, but at this point, the downsides of converting to a 48V system outweighed the upsides of the efficiency gains. Finding MPPT controllers, inverters etc that are mobile specific and made for 48V just really kills it IMO. In my case, it would also require stacking 4 batteries in series which meant that if I lost any of my batteries, I'd have a non functional system.

To do this right, I think you'd need atleast 4 24V batteries or 2 48V batteries, a 48V MPPT and a 48V Inverter/Charger. The other option would be to get a 12-48V converter. I found a few, but the efficiency wasn't much better than my victron psw 12-230 inverter.
 

aernan

Observer
This unit has been discussed in other threads

http://www.hotspotenergy.com/DC-air-conditioner/

Its fairly efficient, really efficient if you remove the inverter losses. I considered it heavily, but at this point, the downsides of converting to a 48V system outweighed the upsides of the efficiency gains. Finding MPPT controllers, inverters etc that are mobile specific and made for 48V just really kills it IMO. In my case, it would also require stacking 4 batteries in series which meant that if I lost any of my batteries, I'd have a non functional system.

To do this right, I think you'd need atleast 4 24V batteries or 2 48V batteries, a 48V MPPT and a 48V Inverter/Charger. The other option would be to get a 12-48V converter. I found a few, but the efficiency wasn't much better than my victron psw 12-230 inverter.

Can you tell me more about your battery and inverter system. Are you using AGM?

Based on the advice from an electrical engineer who is a co-worker he said most inverters use 48 V DC internally no matter what DC power is supplied. So starting with a 48 V DC battery system could remove a lot of the switching losses in the system. I will be building a large 48 V DC lithium pack. Possibly LiPo but more likely LiFePo4 or LTO. I still have piles of research to do to find manufactures and BMS and size the house battery. I'm still at the beginning of figuring out how much insulation and cooling I will need. I believe the AC will be the biggest load.
 

S2DM

Adventurer
Can you tell me more about your battery and inverter system. Are you using AGM?

Based on the advice from an electrical engineer who is a co-worker he said most inverters use 48 V DC internally no matter what DC power is supplied. So starting with a 48 V DC battery system could remove a lot of the switching losses in the system. I will be building a large 48 V DC lithium pack. Possibly LiPo but more likely LiFePo4 or LTO. I still have piles of research to do to find manufactures and BMS and size the house battery. I'm still at the beginning of figuring out how much insulation and cooling I will need. I believe the AC will be the biggest load.

I have 4 victron LiFePo4 12v 200ah batteries and then their host of control system elements (BMS, MPPT etc). Also have 1300 watts of LG Mono solar panels. It's sized such that running the AC all night really doesn't result in appreciable load on my array (my camper is very well insulated, so I'm generally only seeing ~20% duty cycle on my AC). My batteries are generally full by 9-10am (I often see >100amp of solar output in full sun).

My cautionary tale is to be careful how close you fly to the sun with critical componentry. I bought a very fancy rear coil spring suspension system which ultimately failed and required a major rebuild of my outdoor kitchen to accommodate a more traditional suspension set up. I had Overland Explorer fabricate my camper mounting system which also failed in spectacular fashion, also requiring a major rebuild. Both were critical systems which very nearly left me stranded in Baja.

Point being, for major systems, as much as I like to take things apart and rebuild them my version of better, my current thinking is its best to be a little conservative on the critical sysems. My concerns with the 48V native system is it requires nearly every component to be hacked, which worries me. As it is, getting all of these components to play nice together, Inverters, Chargers, MPPTs, solar panels etc, requires some work, so I'd be hesitant to have to do it all on my own. You'd have to bypass the inverters input to the 48V stage, you'd have to find or hack a good MPPT, the list goes on. All to save a 7% inverter loss. I think the better part of valor would be to oversize a 'traditional' lithium battery array to account for these losses, but stick with a more mainstream energy component set up. I say traditional in quotes because as it is, very few campers out there have large solar arrays or lithium batteries, so even putting these in your camper is somewhat bleeding edge, although I think the jury is no longer out, they are a superior choice.

Just one guys opinion whose spent the better part of the last year rebuilding the first iteration of his camper :)
 
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Darwin

Explorer
I think 24 volt is worth considering, you get half the amps running vs 12 volts and many appliences especially marine appliences can be run at 24 volts.
 

S2DM

Adventurer
I think 24 volt is worth considering, you get half the amps running vs 12 volts and many appliences especially marine appliences can be run at 24 volts.

Completely agree. I was torn. Ultimately, keeping my entire rig 12V native including the truck won out just for redundancies sake. But there are some real advantages to 24V systems.

My concern with 48V is the componentry just isnt there. I am doing a full electric conversion on my haflinger farm truck and adding ~900 watts of solar. The motor and battery arrays are 48V, and I can't find a decent 48V charge controller, and definitely none that are made for mobile applications. The systems just arent there yet, and its not something I'd want to hack together, even if I could.
 

aernan

Observer
I'm feeling the same way. My truck has a 24V system in it from the factory so that would be a preferable target voltage. When I did a little bit of digging I was able to find 48 V inverters. I haven't looked at BMS or other components. I know for certain I will not be using straight 12 V. I also feel strongly that I will go Lithium due to it being possible/advisable to cycle the system to 100 discharge and it's reduction in mass/volume compared with AGM.

Designing this rig sure involves a whole lot of web shopping. :)
 

aernan

Observer
I think the controller brings the charge voltage high enough to charge a 24 V battery. It then monitors the Voltage and Amp draw. If you had two units they would probably compete and the charge cycles would never work.
 

dwh

Tail-End Charlie
I think the controller brings the charge voltage high enough to charge a 24 V battery.

No, it lowers the voltage to charge the battery. The solar array has to start out at a voltage high enough to get the job done. So for a "24v nominal" battery bank, the solar Vmp has to be at least 30v.


If you had two units they would probably compete and the charge cycles would never work.

No, they don't compete. They each operate independently. But that's if the units are paralleled to the battery.

He's talking about hooking them up in series.
 

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