I'm current designing a battery for my 40' 5th Wheel. I'm looking at 16 x 272 Ah Lichen cells. My question/options are around the best way to set up the system. 2 x 8s for 24V with 2 x 100Ah Overkill BMS, 4 x 4s for 12V with 4 x 100 Ah Overkill BMS. I will be running a Victron 3000 Multiplus, plus lots of solar, etc. Besides the savings in Charge Controllers and wiring, which are somewhat off-set by the cost of a couple of 70A DC -DC converters, are there any other efficiency reasons for going with the 24V system.
LiFePO4 setup with 16 x 272 Ah Cells
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GXMnow
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What are your power demands in the trailer? If you need more than 2,400 watts or so, I would suggest the 48 volt system with all 16 cells in series. The lower current at the higher voltage is more efficient. If 2,400 watts will cover your needs, you do get the advantage of having dual redundant batteries with two separate 8S units. You will need heavier cables in a few laces, but the charge controllers, fuses, switchgear, etc. is all a bit cheaper. Once you exceed 32 volts, you can't use the cheap ANL fuses anymore. Yes, you can push past 2,400 watts on 24 volts, but my thinking is 100 amps is getting a bit much. I would only go 12 volt if most of the loads are all 12 volt items, and the total power is always under 2,000 watts. 24 to 12 volt buck converters are cheap, efficient, and reliable now. Run the 24 volt around the trailer, and put a small converter where you need 12 volts. That way you can use smaller cheaper wire. Have the inverter close to the batteries, high current short distance again. At 12 volts, even the charge controller starts costing more as you need double the charging current of a 24 volt system.
The issue with 48V is the huge jump in cost for the inverter/charger. My current plan is to eliminate the RV AC system and install a mini-split. Then 2400 watts is lots, if I keep the current AC, then that would be the biggest draw...that said we plan on chasing 70 - 80 degrees so won't need AC very much. If I need more than 2400 then I would get a 5000 watt Quattro...which should still be good with 2 x 100A BMS on a 24V system.
RollingThunder
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Have you considered a 48v DC mini split?
John Frum
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Cool, could you please link to one?
RollingThunder
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RollingThunder
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Here's the my full list of candidate unit's I've found:
48v, 12kbtu, $780, 3yr warr.
48v, 9kbtu, $1166, 2yr warr.
48v, 12kbtu, $1108.
48v, 9kbtu, $530, 5yr warr.
Edit: one more: 48v, 9kbtu $716, 1yr, 5yr warr.
But who knows if the specs are real....
48v, 12kbtu, $780, 3yr warr.
48v, 9kbtu, $1166, 2yr warr.
48v, 12kbtu, $1108.
48v, 9kbtu, $530, 5yr warr.
Edit: one more: 48v, 9kbtu $716, 1yr, 5yr warr.
But who knows if the specs are real....
GXMnow
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The first one says 12K BTU but draws 3,500 watts. OUCH!! The 9K BTU is still 2,600 watts. The MrCool DIY 12K BTU unit runs on a 120 volt 15 amp circuit. Even if it pulls the full 15 amps at 120 volts is just 1,800 watts. Something is not adding up with these DC units.
RollingThunder
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GXMnow
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The fourth one at least has proper numbers.
Can't say if it is actually any good, but at least their spec sheet seems possible. It says 950 watts, and also lists 21.87 amps maximum. At 48 volts, that would be just over 1,000 watts, so it's close. That is okay numbers for a 9,000 BTU unit. It seems odd that the EER rating in their specs is only 5.2 though. I was looking at the 12,000 BTU MrCool DIY install unit for my upstairs room. The central air just can't keep it cool up there when it is 115 degrees F outside. Got to love living a few miles from the Mojave desert. The MrCool unit has an SEER of 22 BTU per watt. Not sure how EER compares to SEER, but going by BTU per watt, the 9,000 BTU at 950 watts would be 9.47 which is a bit more than the 5.2 EER they list, but still less than half of the MrCool. Using this logic, the MrCool should only need 550 watts to run. Just to check my numbers, I also looked up a basic energy star rated window air conditioner. I found an LG 12,000 BTU unit. It needs 990 watts with a CEER of 12.0 Not EER or SEER, come on.. But that does still work out to 12 BTU per watt. Window units are typically less efficient than mini splits, so this looks about right.
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Can't say if it is actually any good, but at least their spec sheet seems possible. It says 950 watts, and also lists 21.87 amps maximum. At 48 volts, that would be just over 1,000 watts, so it's close. That is okay numbers for a 9,000 BTU unit. It seems odd that the EER rating in their specs is only 5.2 though. I was looking at the 12,000 BTU MrCool DIY install unit for my upstairs room. The central air just can't keep it cool up there when it is 115 degrees F outside. Got to love living a few miles from the Mojave desert. The MrCool unit has an SEER of 22 BTU per watt. Not sure how EER compares to SEER, but going by BTU per watt, the 9,000 BTU at 950 watts would be 9.47 which is a bit more than the 5.2 EER they list, but still less than half of the MrCool. Using this logic, the MrCool should only need 550 watts to run. Just to check my numbers, I also looked up a basic energy star rated window air conditioner. I found an LG 12,000 BTU unit. It needs 990 watts with a CEER of 12.0 Not EER or SEER, come on.. But that does still work out to 12 BTU per watt. Window units are typically less efficient than mini splits, so this looks about right.
RollingThunder
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Thanks a lot for taking a look, I was also having trouble interpreting what they had on their spec sheets. They just don't seem to make sense, and I had not come across EER till I started looking at them. I didn't know there was also a CEER lol. It's definitely not confidence inspiring enough to proceed with ordering one just figure out just what the heck the thing is.
With the price of solar, battery cells, and associated components dropping so quickly, I think a quality 24v/48v mini split could sell well, should a reliable dealer sell one at a price comparable to an AC unit. How great would it be to have air conditioning off of the inverter.
With the price of solar, battery cells, and associated components dropping so quickly, I think a quality 24v/48v mini split could sell well, should a reliable dealer sell one at a price comparable to an AC unit. How great would it be to have air conditioning off of the inverter.
There is really no reason the manufacturers couldn't repackage the current mini split technology into a single efficient rooftop unit for the RV market. Most of the current rooftop RV AC's pay no attention to efficiency, they are basically the same technology that has been around for 40 years. I think efficiency has been largely ignored because the only time people have traditionally run AC is with campground shore power or a generator. As you mention, the boom of solar and big lithium banks are bringing AC off grid into the discussion and every watt matters when you are using solar/batteries.
The only reason mini splits are "split" is because you can't typically have all the components directly on opposite sides of a wall (or roof) with a residential install. There are probably some efficiency advantages to the stand alone condenser/compressor unit, but the main efficiency for mini splits comes from the modern variable speed compressors and lack of ducting. You could have both of those things in a traditional RV rooftop form factor. There is a hole in the market and someone could sell a bunch of units if they offered a 10k BTU rooftop AC that pulled under 1k watts. The technology is there, it's really just waiting for one of these companies to package it up for RV use. A rooftop unit just makes a lot more sense for RV's compared to a traditional residential split system with multiple components.
For now, just remove the RV AC system, fill the hole and get a normal Mini-split. If you really want to use a roof mount, there are several options out there that use ceiling mounted "indoor units" but wold need some work to cover them on the roof. New Horizons RV have made a few 5th Wheels with ceiling mounted head units!
John Frum
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Less stuff on the roof to shade the panels ftw.
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I'm installing in a van, so a wall mount evaporator and big condenser isn't a great option. I've seen the roof mount residential units, but you still have the big condensor unit. I've seen some folks put together systems using condensers/compressors components underneath the van and evaporators in cabinetry, but all the examples I've seen look pretty "hacky" at this point. I'd be good with a system like that if someone could package it up nice and make it reliable and repeatable. I also agree on the roof approach taking valuable space room for solar, but it just simplifies the install so much having everything in one unit. There is a probably a place for both approaches, but you wouldn't want to ignore the rooftop unit since there are millions of RV's out there with holes already in their roofs ripe for upgrade.
I have the Mr Cool unit installed in our fifth wheel. What a huge difference. So quiet and effective. Really makes the rev feel similar to a home environment. Pioneer also makes a highly efficient 120 v unit. I decided to switch over to 24 volt when I installed the mini split. Lighter wires and easier on the controller etc. I had been generating a lot of heat and my 12 volt inverter was pushing high amps. I still have it but just using that pack and inverter for light duty.
I am building a system and went with 12v. 16 x 272ah cells configured into 4 12v batteries. 4 Overkill 120a BMS units. Kind of kicking myself now. Wish I would have went 24v now due to MPPT controllers. I am planning on 1,800 to 2,400 watts of solar. Depending on roof space. I will need 3 of the 100/50 Victron MPPT controllers as the 50a is the output of the units. 50a x 14.4v =720 watts. So I can only put 720 watts of solar per controller. this would be double if I had 24v. Hope this helps. I have thread RVs with DIY Lithium if you want to follow.
So I have been doing more and more research...looking at scrapping the Overkill BMS and going with a 2p8s configuration on the cells with a Electrodocus SBMS0 along with 4 or 5 their DSSR20 Charge Controller .(depending if I can fit 8 or 10 solar panels once the AC is removed).
Are you comfortable with getting no more then 80% of the panels rated performance under any circumstances?
