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Old 01-25-2022, 10:07 AM   #1
Mini-Skoolie
 
Join Date: Aug 2021
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Year: 1998
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Power Setup

I have been reading different threads and trying to make sense of some things. Electrical is not a strength of mine so figured I would throw a line out. We are looking to find a way to efficiently power our bus off engine when we are parked, boondocking if you will. Just to where we wont have the need for shore power if we choose. (Which will probably be more often than not) We plan on having 1-2 Mini split AC units, Charging abilities for phones and laptops, exterior LED Lights, Indoor lights, and plan on having our range, water heater, and fridge/freezer combo run off of propane to alleviate the electricity needs. With all that being said, would be be more efficient? A generator setup, solar? Or a combination of both? We would like to run the front mini split off of solar so we could have AC while driving as the bus does not come with Dashboard AC. Our mini split we have right now for the back is 9000 BTU. Could we get away with just having that one? Our bus is a 9 window TC1000. I know this is a jumble of loosely tied questions, so any help would be greatly appreciated.

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Old 01-25-2022, 12:30 PM   #2
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A 300A alternator is around 3600W of power. Assuming its continuous 100% duty, combining that with a battery bank buffer might get you what you want... in reality those assumptions are likely unsafe.



When you say "engine" though it seems like you don't necessarily mean the bus engine. Its also sort of hard to understand what you mean by "most efficient", so the following are guesses around that.


If you have a middle-man battery bank -> inverter -> AC appliances, you can hook up both solar and a generator to charge the bank. Even modest solar will greatly offset your ongoing costs, so I'd consider adding solar input, as much as you can afford. As for engine power, I'd suggest a transfer switch + generator. When the genny kicks on, the transfer switch kicks everything over, and you have a dedicated circuit for charging the batteries at the same time.


I think our use case is pretty close to yours... We have a large full house inverter, etc and we try to run off solar as much as possible. Our inverter is also a decent charger for when we plugin, and we might add a nice genny to do off grid charge. We also have one working split AC + another split unit we're installing, both 9000BTU, The split is great but circulating the heated/cooled air is a problem, which is why we're adding a second unit.



Electrical I consider to be a strength of mine so ask me anything but disclaimer, no advice here should be taken as professional advice, not from me or anyone... I strongly recommend you at least have your work checked by a paid electrician.
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Old 01-25-2022, 01:13 PM   #3
Mini-Skoolie
 
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Quote:
Originally Posted by kazetsukai View Post
A 300A alternator is around 3600W of power. Assuming its continuous 100% duty, combining that with a battery bank buffer might get you what you want... in reality those assumptions are likely unsafe.



When you say "engine" though it seems like you don't necessarily mean the bus engine. Its also sort of hard to understand what you mean by "most efficient", so the following are guesses around that.


If you have a middle-man battery bank -> inverter -> AC appliances, you can hook up both solar and a generator to charge the bank. Even modest solar will greatly offset your ongoing costs, so I'd consider adding solar input, as much as you can afford. As for engine power, I'd suggest a transfer switch + generator. When the genny kicks on, the transfer switch kicks everything over, and you have a dedicated circuit for charging the batteries at the same time.


I think our use case is pretty close to yours... We have a large full house inverter, etc and we try to run off solar as much as possible. Our inverter is also a decent charger for when we plugin, and we might add a nice genny to do off grid charge. We also have one working split AC + another split unit we're installing, both 9000BTU, The split is great but circulating the heated/cooled air is a problem, which is why we're adding a second unit.



Electrical I consider to be a strength of mine so ask me anything but disclaimer, no advice here should be taken as professional advice, not from me or anyone... I strongly recommend you at least have your work checked by a paid electrician.

By engine I mean not on the bus engine. Im totally fine with a genny, but know that means gas dependent, have thought about adding a 100 fuel tank to the bus and tying in a diesel genny so fuel wont be an issue, by efficient i mean bang for buck, if having a small gas gen coupled with a smaller solar with a few batteries would do it I would like that instead of buying a diesel onan or a massive solar system
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Old 01-25-2022, 01:53 PM   #4
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Well, if you don't want to be running the genny 100% of the time you're unplugged and need electricity, you are going to need batteries. If that includes AC appliances, you will need an inverter. Batteries alone do not have much runtime, solar will greatly extend runtimes during good sun.


Really depends on your budget as for how much to invest where. Bang for buck, panels can be had for very cheap if you know where to look. Generators vary in price and for good reasons (noise/tech/etc) so you'd have to see which ones would meet your needs the best- for instance I'd be looking at ( https://www.harborfreight.com/8750-w...ogy-57480.html ) because we have 50A (240V) hookup and want somewhat clean sine output. 30A (120V) hookup will let you use smaller, quieter, cheaper units.
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Old 01-25-2022, 03:18 PM   #5
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An example plan...

Hi,

I will be roughly at the center latitude of Virginia just barely east of the crest of the Blue Ridge. I expect to have my then full time bus unshaded by trees. The goal is to be able to run off of batteries for 1 whole worst case day before I have to run the generator, and that's cloudy in the dead of winter.

Given the altitude the historical overnight low is 1 degF. There are 4230 heating degree days and 1020 cooling degree days.

My Bosch inverter drive mini-split has 8900 BTU / hours capacity to maintain 70 degF internal from 0degF outside with an input of 1.35 kwh/hour of operation when heating. Given the series inefficiencies involved taken to be 80% total that 1.35kWh/hr becomes a bit less than 1.7kwh/hr from my 48VDC system, for an amp draw of 35.1Ah/h (35.1Ah * 48vdc = 1687whr). For cooling it apparently* looks like this: Worst case day is 110degF. That outside is 40 degrees over desired inside ambient so with 80% efficiency overall should pull about 26.4ah yielding 10.8kBTU/hr of cooling.

So that's what the minisplit can do, what do I need ?

[See attached spreadsheet below.]

3" foam board on the floor, 3" spray foam everywhere else, and about 45 square feet of windows and doors, and 635 sq ft of walls and 289 each of ceiling and floor. R value of nominally 20 in the non-door/window areas, call it an R value of effectively 6 in the window/door areas.

I am counting on my triple layer windows to have an R value of 5, as will the door (door actually should beat that...). To keep a 70 degree difference across the envelope should take 5869 BTU / hr., and in all cases I have far more than that.

So, how well will the surrounding forest hold the wind down in winter? Will using fabric shades and shade from the solar panels help much? Will turning the bus to max out solar gain in the winter do much? How far off are my assumptions ? I do not know yet.

I know that as opposed to my design interior temp. of 70 degrees I'm fine with 75 deg F in summer and 65 deg F in winter, that helps a bit.

So with a worst case of a 35.1Ah/h draw for 24 hours I need 842Ah of storage just to stay warm. I plan on 1200Ah of storage at 48VDC and a wood stove for emergencies (or a direct vent propane heater in some cases).

To run that I hope to have 4~8kW of inverter capacity, and nominally 5120W of solar panels. That is 8 crossways on the roof and 8 in tilt up frames on one side. I have room for 4 more if need be.

*The data sheet is not as clear for cooling as it is for heating.


I should add estimates for lifespan. The batteries will generally cycle from 40% to 60% state of charge, only if I know cloudy cold or hot days are coming will I raise the limits towards 10% to 90% state of charge utilization. The panels are rated to have at least 80% capacity in 20 years. The batteries should give me 10~20 at the intended use model. The inverters I'll likely need to replace twice each in 20 years' time at $6k nominal, and the generator plausibly 4 times for $2400 cost. All values in today's dollars. Makes the all up 20 year system cost about $33.4k. It works out to less than $150 / month which is twice per kWh compared to what I am paying now, and 1/4 the usage compared to my all electric grid house.


But the power company wants $20k~40k (they'll make a firm estimate if I commit to their pricing) to bring power to my preferred build site...
Attached Thumbnails
Bus Heat Cool calcs.jpg  
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Old 01-25-2022, 06:06 PM   #6
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Quote:
Originally Posted by TomDPerkins View Post
The goal is to be able to run off of batteries for 1 whole worst case day before I have to run the generator, and that's cloudy in the dead of winter.

Quote:
Originally Posted by TomDPerkins View Post
...Given the series inefficiencies involved taken to be 80% total that 1.35kWh/hr becomes a bit less than 1.7kwh/hr from my 48VDC system, for an amp draw of 35.1Ah/h (35.1Ah * 48vdc = 1687whr)....


So that's what the minisplit can do, what do I need ?
I'm going to throw in another 250W for anything else you want to run off the system, so 1.75kW + .25kW = 2.0kW in total of continuous consumption. Over a 24h period, 48kWh. This is where you can see running purely off battery is just not realistic.


I'm also going to assume insulation is at least covering the major metal surfaces sufficiently, exposed metal walls / etc will make this all futile.


Framing this from personal experience: I have 43kWh capacity and with 3000W of solar I bet split heat (single 9000BTU unit with similar numbers to yours) would run for 3-5 days before battery drained to nothing with reasonably cooperative weather. Snow, input drops to near zero.

If you fill your roof with panels... 2500W-4000W, you'll be able to offset your costs quite a bit, and claw back some used up storage. So if we combine that with a goal of 12h runtime given zero generation (overnight) you'd need something like 24kWh of battery capacity.

My bottleneck at this point is generation. 3000W of solar, I am fighting a slowly losing battle of consumption vs generation running split AC 24/7. Until I double up on panels to at least 6000W I am on the cheapo-Chinese diesel heater train for heat, as that has been working fantastically well. A single unit gets the interior uncomfortably hot on mid-high settings and its tiny 2 gallon tank lasts 12-16 hours, the unit consuming 120W at full tilt. We're installing both a second split and diesel heater in the rear so that we don't have to have so many fans pushing air around to keep heat/cold evenly distributed.

TL;DR: For your worst case scenario there's far more economical options than trying to run the split heat off batteries, although as someone with the goal to run 100% off solar I can understand the allure.
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Old 01-25-2022, 06:12 PM   #7
Skoolie
 
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1200Ah at 48vdc and 90% to 10% state of charge is 46kWh of storage, and the real runtime required of the HVAC tends to 18 hours not 24 even on the worst case day -- and I desire to run a wood stove or propane heater on such days in any case. It think it is realistic to think it will work.

What will at minimum be 5120w of panels is being delivered tomorrow...


Please recall propane direct vent & woodstove are both planned backup heat.


PS. I am doing 3" of spray foam on all walls and ceiling, and 3" foam board on the floor, and paying careful attention to blocking accidental in/exfiltration of air. I have a humidity managing energy recovery 77cfm air exchanger as well, it will run 1 hour out of 4 per inhabitant present.
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Old 01-25-2022, 06:54 PM   #8
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I like the math exercise, but decided not to try to calculate cooling requirements. I just winged it and bought a 12K BTU mini-split and am hoping for the best. I haven't installed it yet. I will do thermal window coverings, that should help.

Here's my logic: my bus is essentially a tin can. Actually, it's a fiberglass can. Maybe more like a greenhouse. When I go into the bus late in the evening the inside ambient temperature is exactly the same as the outside. It essentially cools down very quickly. I'll see how it does when I get the thermal window coverings on, but I'm guessing it won't do very well, certainly not as well as my garage, which swings at least 20 degrees better- on the good side of whatever the outside air temperature is So I just got the biggest unit available.

edit: tough to calculate realistically. No way solar can power it. I have a generator for cooling.
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Old 01-25-2022, 09:04 PM   #9
Skoolie
 
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I really don't expect...

...to get away with not using the generator ever. I do expect to manage without it say 3 days out of 4, with maybe only 3 weeks a year I need to run it every day.
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Old 01-25-2022, 11:04 PM   #10
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Our design incorporates 2.5 to 4.0 kWh of solar panels feeding a Victron multi-plus of 24VDC with 3000 continuous and 6000 peak watts inverter. The batteries will be TWO 24 volt server rack mount lithium units at 5.12 kWh each.
Cooling will be via dual mini split A/C's for both redundancy and cooling distribution (one front over the windshield and one in the rear bedroom. Draw is just under 800W each when cycled on but they're not constantly cycled on. Calculations however were done at 100% duty cycle.
Heating is multi redundant and will be accomplished with (underway), one engine coolant heater near the middle of the bus and the in dash heat/defrost system. When parked, two 5kW diesel parking heaters and a wood stove will provide primary heat. Backup and redundancy will be provided by a 110-120 volt electric space heater and a Mr Buddy propane heater connected to an interior fitting. While heat can kill, cold is far more deadly and so we have numerous redundancies. We also plan on using a 110-120 electric blanket on our queen bed when sleeping.
Calculations show that winter power electric loads will be the least due to non electric main heating methods. Warm weather, in which we will have high duty cycle on both AC units should also come with much higher solar output.
Any electrical shortfalls will be remedied via an on board generator which will be propane powered when our already owned gasoline unit requires replacement. There will also be a deployable wind generator as some of the places we enjoy have lots of wind....and sometimes lots of overcast like ocean shores.
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Old 01-26-2022, 03:09 PM   #11
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ham skoolie
are you planning on a diy wind generator?
i have looked at videos and books for some but question the reliability of some i have seen?
i have also thought of making some alternators look like mini propellors on the hood or roof that charges while driving down the road?
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Old 01-26-2022, 04:56 PM   #12
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Quote:
Originally Posted by Jolly Roger bus 223 View Post
ham skoolie
are you planning on a diy wind generator?
i have looked at videos and books for some but question the reliability of some i have seen?
i have also thought of making some alternators look like mini propellors on the hood or roof that charges while driving down the road?
NO. We will be purchasing a commercial rig with the vanes in the vertical plane. After talking to some in the industry (small scale consumer industry) the general consensus was that they are far more durable as many moving parts and thus failure modes are eliminated.
I've seen a lot of the DIY stuff. Like the ones that use a ceiling fan and then add magnets inside the housing. I'm sure they work because the theory is rather simple. I even have a flash light in my teaching aids that is clear and demonstrates Faraday's principle. However, they're big, bulky, and not likely to be very efficient.
Using vehicle motion through the air to turn a wind generator can be made to work but it's highly inefficient. Lot's of drag added, especially considering the mass needed in the structure to withstand road speed winds, which decreases fuel use.
It would be far better to install a dedicated aux generator or alternator on the engine with totally separate circuitry from the chassis alternator and thus able to be optimized for the electrical needs of the system.
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