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Small Cabin Forum / Off-Grid Living / Solar Size Up
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ahunter
Member
# Posted: 8 Nov 2011 19:11
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I have a 20x20 off grid cabin with loft that was just recently built and I have a question on how much solar panels I need watts wise battery wise. I live in Nashville, TN...sunlight hour is
High-5.20
Low-3.14
Avg.-4.45

I want to be able to run lights and other basic power needs

MtnDon
Member
# Posted: 8 Nov 2011 21:55
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What I view as basic needs is likely different from what you consider to be basic needs, just as what I consider a small cabin is larger/smaller than what others may think.

Start with adding up all the things you want to use, the number of hours and the number of watts they consume. There are many off grid calculators to be found on the internet. One of them may make the task easier. Sometimes it may be easier to estimate hours of use spread over a time of a week, then divide by 7 to get the 'daily'.

I have a calculator in spreadsheet form you may find useful.

Find it here

ahunter
Member
# Posted: 9 Nov 2011 18:48
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thank you

ahunter
Member
# Posted: 9 Nov 2011 18:49
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Thank You MTNDON!! That help a lot

brandywine
Member
# Posted: 10 Nov 2011 20:51
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Hi! We have a cabin outside Standing Stone State Park in Hilham, TN. We just got our solar panels but haven't installed them yet.

We started out by getting a very good understanding of what power we use.

This is what we did on our journey. Start looking at any and all devices you have plugged into outlets. Start noticing how long these items are on, drawing power. Buy a Kill-a-wat device that goes between the plug and wall and measures how many watts flows to the device.

Document all your findings. I am attaching the excel spreadsheet (powercalc.xls) I made. I used formulas from different websites. I hope it helps.

We bought a generator first and wired that into our power box. We ran the generator all the time when we were home ($$$!!!).
Next, we got batteries and an inverter-charger. We hooked up the batteries and inverter and wired that into our power box in our newly built cabin. We have never been connected to utility. We put circuits that we were sure would be ok and put them on the inverter. After a while of everything working, we started moving more circuits over until everything was running off the inverter.

Our 1.5hp water pump that brings us water from the spring box is a 220 pump so we had to plug that directly into the generator. So we still had to run the generator when we needed water. a Pain.

We just bought a FlowJet DC water pump and water tanks (used 250gal ones) and rerouted the water. Now we only run the generator to charge the batteries and to use the 220v spring pump to refill the water tanks. The DC water pump takes water from the tanks to the cabin.

Now, we are ready to install our solar panels. We actually bought laminates (no frame or junction box) for $.59/watt. I bought some junction boxes off eBay and we will be building the whole array framing out of wood. I can't wait. We won't have to run the generator much at all after we get that done.

Lessons learned: don't do a 12v system. You need HUGE wire sizes and you will have to get a large or multiple solar charge controllers to charge 12v . Go ahead and start out with 24v or 48v battery bank. We have a 12v system, but will be re-wiring everything to 48v. I have to buy a different inverter-charger since they are based on the battery bank voltage. That was a big$$$ mistake. Hopefully I can sell my 12v inverter-charger on eBay.

Start out with ONE DAY of battery bank. These calculators want you to have 3 days worth of battery storage in case it is cloudy. Well, we have a generator and batteries are expensive, so we have a one + day of batteries.

You don't have to go all solar at once. Start small and just switch over a few circuits to start.

If you want to stay connected to the grid, you will have to comply with all the codes. If not, you can do it however you like...safely and practically without the overbuild that codes often require for tiny homes.

Ignore the professional installers that criticize all DIYers on the solar forums. Many are A-holes.

We are far from experts but are happy to share any info we have collected on our journey so far.

We are also open to ideas and suggestions from any others!

Mickie & Travis

brandywine
Member
# Posted: 10 Nov 2011 20:52
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It didn't accept the file. Don't know if I am supposed to zip it or what....I can email it.

MtnDon
Member
# Posted: 10 Nov 2011 22:41
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Reading the attachment upload rules I see that XLS files are not allowed. Need to ZIP it.

brandywine
Member
# Posted: 11 Nov 2011 22:20 - Edited by: brandywine
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here is the file zipped
powercalc.zipAttached file: powercalc.zip
 


Nirky
Member
# Posted: 12 Nov 2011 20:44
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Quoting: brandywine
Go ahead and start out with 24v or 48v battery bank.


For my future cabin I was planning on a 12v battery system, because here in the NW we don't get much sun during the winter, and so was going to use a small gas-engine/car alternator setup putting out13.8v to charge the 12v system.

If I went to 24v or 48v, how would I charge the system during the winter season?

TomChum
Member
# Posted: 14 Nov 2011 12:38 - Edited by: TomChum
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Nirky,

Re: 13.8v to charge batteries. That seems just barely adequate.

Regarding 24,48v: I can imagine some mechanical ways to disconnect the battery bank and change it to 12v just for the charging process.

One good reason to consider 48v is if you have long wires from your panel to the battery. Higher voltage costs less than the heavy copper wires that you will need to deliver 12v over long distance. 100 feet (=200 feet), for example is a LONG way to push 12v, you need big wires for that.

With 48v the wires can be a lot smaller. You'll be surprised how much wire costs, for 100 feet (=200feet) it can cost almost as much as another panel. It's better to spend any extra money on more panels gathering more sun. It's non-productive to spend money on bigger wires. 48v is better in that regard. Especially at higher latitudes where there is less sunlight per day.

MtnDon
Member
# Posted: 14 Nov 2011 20:07
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Quoting: TomChum
Re: 13.8v to charge batteries. That seems just barely adequate.


Right! Motor vehicle charging systems do put out that 13.8 v as a rule and it works for them as most of the time a motor vehicle battery is not discharged very much in normal use.

A PV / alternative energy system, on the other hand, has its batteries normally discharged 25% to 75%, with 50% being the maximum point one should aim for. For a typical 12 volt RE (renewable energy) system an absorb charge voltage of 14.1 to 14.7 volts is used most often.


I seriously recommend 24 volts as the system voltage just as brandeywine and TomChum have suggested. If there is virtually no chance that you will ever want more than just a couple of low watt lights then maybe a simple one or two battery 12 volt system would work. I have seen a few people start with a 12 volt system and within a short time find themselves in a bad situation. A situation where the more expensive electronic hardware needs to be replaced to get into a 24/48 volt system. 24 VDC systems should do most small cabins quite well.

I do have an operating off grid system, good for two to three cloudy days and no need to run the generator until the fourth day... maybe the third day in winter. $8K.

MtnDon
Member
# Posted: 14 Nov 2011 20:20
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I meant to say that I'd go for a good battery charger like those made from Iota. They make 12, 24 and 48 volt equipment. Their only fault, IMO, is they do not have an equalize charge setting. Match it properly to the size of the battery bank, which reinforces the best practice of correctly sizing the system the first time.

Correct sizing means the charger should be capable of supplying C/10 to C/8 the amps of the battery capacity. If the battery bank has a amp-hour rating of 220 (C=220), the the charger should be able to supply 220/10 to 220/8, 22 to 27.5 amps.

Power the charger with a suitably sized generator. An inverter generator that throttles up and down as the load changes is ideal.

A top quality inverter/charger may be the best bet. Inverter and charger are in one unit and work together seamlessly.

Xantrex does make some stand alone chargers that do have equalization ability.

Rob_O
# Posted: 14 Nov 2011 21:22
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Quoting: Nirky
If I went to 24v or 48v, how would I charge the system during the winter season?


Get a 24V alternator.

Nirky
Member
# Posted: 15 Nov 2011 17:08
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Quoting: Rob_O
Get a 24V alternator

Thanks much Rob-O for answering my question, I will look into those.

As for the higher cost of thicker copper wire required in a 12v system, this can be mostly mitigated by the use of copper pipe instead of wire and have it shielded by either by garden hose or pvc piping. :) This allows for longer distance (if necessary) between pv panel and battery box.

MtnDon
Member
# Posted: 15 Nov 2011 19:20 - Edited by: MtnDon
Reply 


Using a 24 v automotive type alternator for a 24 v system will encounter the same issues as a 12 v alternator in a 12 v system. That is, the output voltage never reaches the optimum for recharging the batteries. For a 24 v system that is 28.2 to 29.4 typically. Our 24 VDC system has the absorption charge rate set at 29.6 volts.

Copper pipe will work as a conductor. How you equate pipe size / wall thickness to wire gauge would need some calculating.

Bzzzzzt
Member
# Posted: 15 Nov 2011 20:50
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Just to chime in, I'm completely ignorant about solar applications, but I am an electrician. Using copper tubing as a conductor might work, but it will be highly inefficient and might prove difficult to find parts for and you might have to invent a lot of stuff to make it work.

Figure what the total watts of the load is that will be 12V then divide that by 12 which will get you your amperage. For instance, 240 watts 12 volts is 20 amps which requires #12 copper wire.

Here is an ampacity table you can look at.
http://www.armstrongssupply.com/wire_chart.htm
Use the 60 table for most of your applications unless ALL the devices you're using say they are 75 or 90. If you're uncertain stick with 60. Better to oversize than undersize. Find your amps in the correct column and then look to the table on the left and see the wire size required for that ampactiy. You will notice that #12 wire is good for 25 amps. The NEC says it's good for that ampacity but then says you have to protect the wire at 20amps.

DC Power works just a little differently than the AC that I'm used to working with, but wire sizing does not. Use the right stuff for your circuits and you won't run into any strange "What the heck is that?" problems down the road. I prefer stranded wire but solid is a little cheaper. I think I read somewhere that DC power likes stranded a little better but I dunno if that is in fact true. Places like Home Depot or Lowes and a lot of hardware stores sell wire by the foot which is more expensive, but you might not need a whole spool.

As to the solar power, I've been thinking about this system from Northern Tool: http://www.northerntool.com/shop/tools/product_200466740_200466740

Nirky
Member
# Posted: 15 Nov 2011 21:06
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Copper plumbing pipe is not an alloy of anything, it is copper. As with all metals, impurities do exist, but ordinary copper plumbing pipe is not intentionally blended with any other metal to make an alloy, it is simply, copper.

If one had nothing better to do, they certainly could argue theoretically that 99.8% copper wire conducts better than 99.4% copper plumbing pipe of the same mass, but the real-world non-theoretical difference is negligible.

As for 13.8v vs 14.1v for charging 12v batteries, this is what I've found--the difference is just one thing: time. It just takes a little longer to charge the battery with 13.8.

What you term "optimum" and "absorbtion chrage" isn't a function of your battery chemistry, but is perceived customer need-therefore mfgrs set their equipment to please their customers in regards to charging time for their battery banks. Faster = better for customers as it gives them confidence in the viability of their system, but faster also isn't as good for your batteries; they prefer a slower, more complete charge, which means increased discharge capacity. But it also takes longer, so mfgrs balance these factors. With automotive, there is just no need hurry up.

brandywine
Member
# Posted: 17 Nov 2011 21:18
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Invert close to the batteries and run AC wire to the house...cheaper.

MtnDon
Member
# Posted: 17 Nov 2011 22:24
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Quoting: brandywine
Invert close to the batteries and run AC wire to the house...cheaper.


Yes and no. It really depends on the maximum AC amps you want at the house and the distance between batteries and house. In our case because I wanted 30 amps at 120 VAC available it worked either way. Run the numbers and check out each situation on its own. For us it made more sense and convenience to run 325 feet of #2 AWG for a 90+ volt DC (modules in series) run from the PV modules to the cabin where the batteries are. If I would have been satisfied with maybe 10 - 15 amps of 120 VAC it might have worked out differently.

shovelshort
Member
# Posted: 5 Dec 2011 16:25
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I have been on here researching and I am more confused than ever.

I have one of those 10'x20' cabins with a loft that you see on the side of the road for sale. I use it as a hunting cabin out in the middle of nowhere. I am working on hooking up a wood burner for heat but would like some electricity via solar.

I have looked at the calculators and don't feel anymore confident in figuring out what I need or best way to piece a system together. Cabin is along central Ohio. I would like to have 4 lights with no more than 2 running at a time for about 4 hours a day I guess. I would also like to run a 19" TV/DVD for no more than 3 hours at a time for the lonely nights.

I looked at the Extreme 350 Adventure Kit that is on sale for $570 right now.

www.goalzero.com/shop/p/64/Extreme-350-Adventure-Kit/1:3/

I don't know if this will do what I need or is a good price. I would be open to piecing something together for this price if it was better quality, could put out more useful electricity, and easy to expand upon.

Any advice would be appreciated.

MtnDon
Member
# Posted: 5 Dec 2011 16:48
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Your opinion may differ but I think that is too small panel and too small a battery to be of much use. Specs state 12 - 16 hour sun recharge time. What good is that if the useful hours of sunshine is 5 or 6 hours a day?

I could be wrong but this also looks like spam. If it is not spam then I do sincerely apologize.

shovelshort
Member
# Posted: 5 Dec 2011 16:59
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Quoting: MtnDon
Your opinion may differ but I think that is too small panel and too small a battery to be of much use. Specs state 12 - 16 hour sun recharge time. What good is that if the useful hours of sunshine is 5 or 6 hours a day?


I don't have an opinion since I am uneducated to this topic.

Quoting: MtnDon
I could be wrong but this also looks like spam. If it is not spam then I do sincerely apologize.


This wasn't spam. I found it in a search.

Since I have not purchased anything, what would you do if you were starting from scratch? What would be a quality set up if you pieced things together?

MtnDon
Member
# Posted: 5 Dec 2011 17:12 - Edited by: MtnDon
Reply 


Sorry about the spam comment. I administer another site and the way you posted your first message is a classic example of how a clever spammer works. I do apologize.

Start with how many lights and of what wattage and an honest estimate of number of hours per day for each. Do you want to use 120 VAC and an inverter or if you are going to run nothing but a few lights maybe DC only will do?

I suppose with the TV you'd need 120 VAC for that so using 120 VAC for lights simplifies the wiring the way I see it. Look at the back of the TV and DVD to see the rated power use (watts or amps).

Also how many days of autonomy would you like? That is how many cloudy days of use before the batteries are discharged as much as you want; usually discharging to no more than 50% of the rated capacity is best. Less is better.

Do you know how many good sun hours you have in your area? There are charts available. Also figure use for the worst case scenario, usually winter, unless you don't plan on using the place in winter.

Come back here with that.

shovelshort
Member
# Posted: 5 Dec 2011 18:02 - Edited by: shovelshort
Reply 


I have not bought anything yet.

I want to have 4 lights with only 2 on at a time for 4 hours a day. I do not know the wattage because I do not have any yet. What do you suggest? I am only at the cabin for about 2 days at a time with about a week in between. The most I will be there at a time ever is about 5 consecutive days.

I would say about 55 watts for the small TV and DVD player and I do believe I would have to use the converter. What size converter? I would guess according to charts I have seen that I would never exceed 200 watts draw at any given time and no more than 700 watt hrs/day.

As far as autonomy, like I said I will only be there about 2 days at a time with a week in between so I think I would be safe no matter what.

The charts say 4.2 hours of sun for Ohio.

One thing I have decided on is to go with a deep cycle battery from Crown since they are located here in Ohio.

Bzzzzzt
Member
# Posted: 5 Dec 2011 18:12
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Quoting: shovelshort
I looked at the Extreme 350 Adventure Kit that is on sale for $570 right now.

www.goalzero.com/shop/p/64/Extreme-350-Adventure-Kit/1:3/


I'm no expert in the field and I don't have a working system up and running, but I have priced some stuff and THAT setup looks really expensive for what you get. There's only a 30W solar panel and a 350 watt-hour battery (that's like 29 Amp hours) and the max load at 120V is only 10amps with a 350 Watt inverter.

Just my uneducated opinion, but that sounds like crap.

It is my intention to to purchase this 60 Watt system from northern tool:
http://www.northerntool.com/shop/tools/product_200466740_200466740
I already have this 750Watt inverter from Harbor Freight tools:
http://www.harborfreight.com/750-watt-continuous-1500-watt-peak-power-inverter-66817. html
And I've been pricing Marine/Deep Cycle 100-125Amp Hour batteries. I found a 115 Amp hour 12V battery at Wal Mart for $68.

My main intention as far as load goes is a few 120V lights with LED lamps in them, a small pump for my water supply (rainwater catchment,) a box fan in the summer, and perhaps a radio occasionally. I don't ever plan on my load being more than 20amps at 120V. I have a small generator if I need more power.

All of that is going to cost significantly less than $570.

MtnDon
Member
# Posted: 5 Dec 2011 21:01
Reply 


Quoting: Bzzzzzt
I found a 115 Amp hour 12V battery at WalMart for $68.


In my years of experience with RV's, we've had one since 1983, the Marine / RV batteries are barely better than a standard cranking battery when used as a source of power when parked. They simply don't last as long as golf cart batteries. Of course golf cart batteries cost more, probably about twice as much per equivalent power capacity. Their plates are thicker than any Marine/RV battery. Just my opinion.

MtnDon
Member
# Posted: 5 Dec 2011 21:34 - Edited by: MtnDon
Reply 


shovelshort

I don't like dim lighting, so we use 23 watt CFL lamps in our off grid cabin. So I'm using those in the workup; 2 on at a time for 4 hours a day. TV/DVD, 55 watts, 3 hours a day. I allow a 10% loss within the inverter; most will do better than that. With that data and assuming a 12 VDC system I make the daily amp-hour use to be about 33 amp-hours at 12 VDC.

If I figure on 2 days of autonomy and a maximum depth of discharge of 50% and the coldest temperature to be zero I come up with needing a battery capacity of 185 amp hours at 12 volts. (Batteries are less efficient at lower temperatures.)

You can achieve that capacity with 2 golf cart batteries. (Two 6 volt in series)

With 4.2 hours of sun you'd need PV modules that would supply a minimum of 9.25 amps at peak power. That would recharge the batteries in a day of good sun. Peak voltage should be something in the neighborhood of 17 to 18 volts when used with a standard Non-MPPT charge controller. So 17.5 x 9.25 = 160 watts of PV modules, approximately.

If you wanted half that wattage would work given the use cycle of 2 days use and 5 days in between. I wouldn't want to go less than that as it would keep the batteries at less than full charge for longer periods.

If the TV/DVD was on at the same time as two of the 23 watt lamps that would be about 100 watts. A 200 watt inverter could do the job; I'd probably pick one in the 400 watt range myself.


(note an inverter changes DC to AC and a converter usually changes AC to DC, although some devices that change a DC voltage up or down to another DC voltage are also named converters.)

For batteries select ones that are rated for golf cart or renewable energy systems. Crown does make them

Crown Battery Deep Cycle

shovelshort
Member
# Posted: 19 Jan 2012 22:30 - Edited by: shovelshort
Reply 


For starters I want to apologize for keeping this topic going two months after my last inquiry but I am no closer to making a purchase.

I know you have heard this many times, but I first got the idea to put some electricity in my hunting cabin after seeing the Harbor Freight set up on sale for $149. Then I saw the Northern Tool set up that seemed to be better quality. I am a tight wad, but I am not against paying the money for a quality product.

So, after saying this and reading your advice MtnDon, do you believe that the 200 W SV-T-200-LV off-grid kit from Sun Electronics http://www.sunelec.com/index.php?main_page=pv_systems&id=1696&type=OFFGto to be a safe and wise choice for what I want to do? Again staying with the Crown golf cart syle 2 battery set-up.

MtnDon
Member
# Posted: 20 Jan 2012 01:04
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No problem on the time between posts.

On the surface it looks like an adequate kit. The PV module uses poly crystalline PV cells instead of the amorphous cells. That's a plus. Many of the HF and NT PV kits use amorphous cell modules.

This module puts out 200 watts, at 17.7 volts and 11.3 amps. The charge controller can accept up to 16 amps and a peak voltage of 69 volts. That means you can NOT place 2 PV modules in parallel if you wanted to "grow" the system. But you could add a module in series safely.

Because of the manner that these simple charge controllers work you won't get use of the full 200 watts from the PV module. Here's why...First remember that a fully charged 12 volt battery will have an actual voltage of 12.7 volts. To reach the full charge the voltage applied to the battery should be in the 13.2 to 14.4 volts range.

Let's assume the battery is partially discharged with a voltage of 12.0 volts. Let's also say the sun is shining brightly and directly on the PV module. We multiply the 11.3 amps by the 12.0 volts; 11.3 x 12.0 = 135.6 watts. That's all the power that will go into the battery. 200 135 = 65 watts lost.

Note that this apparent waste of module power is common to all PV systems that use simple charge controllers. By simple I mean charge controllers that are NOT MPPT types. So this issue would be common to all the inexpensive kits, including the HF and NT.

So, IMO, that's not a bad kit, but I'm not sure it is the best either. Their price on the 1/0 inverter cable seems rather high to me. So does the MC4 cable.



FYI, an MPPT (Multi Point Power Tracker) charge controller will look at the power output (voltage and amps) of the PV module. It also looks at the battery voltage. Then it computes the best voltage to apply to the battery to get the highest amperage going into the batteries. It is the amperage going into the battery that gets the job done. The circuitry in the MPPT charge controller does the calculations and adjustments continually. A modern MPPT controller is up to 97% efficient in this module to battery power conversion. That's the simple non technical description. I can enlarge upon that if anyone wants.

MPPT controllers are especially beneficial in cold weather and poor sunlight conditions. They are better for charging batteries with deeper discharges and very good if the wire run run from the PV modules to the batteries is long.

The "catch" to an MPPT controller? They cost more, and for that reason Sunelec and others don't often package one in their simple kits. Blue Star Energy and Morningstar make some of the less expensive MPPT controllers, but still probably $200 more than the one in the kit. Outback, high end Xantrex, Apollo make some of the better high end, higher capacity MPPT controllers.

...Not sure that helps you but I hope so.

MtnDon
Member
# Posted: 20 Jan 2012 10:03
Reply 


I need to add something. I ststated above that a second module could be added in series to the system. That is technically correct, but actually would be a dumb thing to do. Why? Well, technically possible because the controller does have capacity to handle the doubled up voltage. Dumb to do because none of the extra power from the second module would get through the controller to the battery.

The paragraph quoted below is the reason why...

Quoting: MtnDon
Let's assume the battery is partially discharged with a voltage of 12.0 volts. Let's also say the sun is shining brightly and directly on the PV module. We multiply the 11.3 amps by the 12.0 volts; 11.3 x 12.0 = 135.6 watts. That's all the power that will go into the battery. 200 135 = 65 watts lost.



So this package/kit can not grow without the charge controller being replaced. Or if growth was needed then a new module and a new controller could both be used and connected to the same batteries.

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