Am I doing this right?

[miss.h01]

I'll try not to be another annoying solar amateur on here but I have to post this question to ease my mind.

Recently I've been trying to gather some quick solar/electric information for my upcoming camper van conversion, so naturally I started with which items I'd like to run on solar!

This is what I have so far, along with the Ah/h pull from the respected items owners manual:

Espar Airtronic D2: 0.7 - 1.9Ah/h (this is only the range of which settings I'd like to use as of now)

MaxxFan Deluxe: 0.2 - 1.2Ah/h (same as above)

Dometic CFX 75DZ: 1.30Ah/h

I was planning on purchasing two Eclipse 100W solar panels which pull ~5.7A.

Figuring the average day draws 5-6 hours of usable sunshine, buying two panels would bring in roughly 56 - 68A per day.

I'd like to run the Dometic CFX 75DZ 24/7, which alone would pull around 31A daily since it pulls 1.30Ah/h, correct? 

Looking to purchase panels soon (as I found some on sale!) so I would just like to see if I'm on the correct path to figuring out the calculations and doing them correctly.

I'm off to grab a nap but I'll hop on here when I wake and reply to y'all! Thanks to all who reply and help me out.

 

[abnorm]

Well you left out the STORAGE of the Solar Energy in your example...…………

How many and what kind of BATTERIES will you use ?

 

[jimbob394]

I don't have a Dometic, but my Snomaster cycles the compressor on and off throughout the day. It says on average it'll be running for 12 hrs out of 24. So while the fridge is on, its not pulling the full 2.5A 24/7. I'd check with Dometic or maybe someone on here knows how many Amps per 24 hrs it might pull.

Sent from my Pixel 4 XL using Tapatalk

 

[Trebor English]

My 100 watt solar panel generates a max of 5.5 amperes.  The panel is mounted flat on the roof so it is never pointed properly.  I typically get less than 5 amperes, four is common.  The duration varies seasonally and 5 to 6 hours doesn't happen for me.  It is usually more like 4 to 5 hours, often less.  Four amperes for four hours is only 16 amp hours wile five amperes for 5 hours is 25 amp hours.  If I were to get another panel then I would get 32 to 50 amp hours on a good day.  Of course, the minimum is zero. 

Energy use by refrigerators varies widely.  If your outside temperature is 100 degrees F and your inside ambient temperature is higher the refrigerator will use twice as much as it does when the temperature is 70.  The thermostat makes the compressor run longer when the ambient temperature is hot.  When the temperature inside the fridge is set to 0 degrees F for freezing the energy use goes up a lot.  Of course, on days when you use the heater it will be cold so your fridge will use much less electricity so you shouldn't need to plan for both being maximum unless the heater blows on the fridge.

I noticed in the Amazon ad that both the CFX 75DZ (70 liter)  and the CFX 95DZ  (85 liter) use exactly the same 1.30 amp hours per hour with the same test conditions, 90 ambient, 5 small side, 37 large side.  I would expect the larger unit to use a bit more electricity.  My point here is that your results may be different, particularly if you go places hotter than 90 degrees. 

Bottom line, many people keep a small efficient 12 volt fridge with 200 watts and 200 amp hours of battery.  You have selected a fridge / freezer larger than most.  It might work fine.

 

[B and C]

As has been pointed out, you don't always get the max out of panels due to overcast or latitude. As you use power, you will find more things to add that take that power. Got a cell phone and a laptop? They aren't on the list. Hotspot and booster?

Batteries fill up during the day from the excess power you are not using at the moment and discharge at night so your bank has to be big enough to get you through the night and maybe a couple of rainy days too.

I recommend buying as many panels as will fit or you budget allows. A marginal system will leave you wanting.

If you decide to add panels at a later date, finding compatible panels to the ones you already have may be problematic. In this case you would need to buy another controller for the new panels and run the systems in parallel.

 

[bass_sears]

Couple of thoughts, although the other comments have already touched on good points.

See if you can find a performance chart for that fridge at the temps you want to keep it, that will give you a more accurate idea than just taking the max current it could draw and multiplying by 24 hours. Not sure if Dometic has them, Engel (close competitor) provides a chart of power draw based on ambient temp and temp you want to keep the cooler:
https://www.engelcoolers.com/pub/media/wysiwyg/pdfs/MR040_performance_curve.pdf
... which I found pretty accurate compared to our actual use. Keep in mind these small fridges don't have a lot of power (that's why they don't draw a lot of power) and if you're putting warm things in there all the time they will take a long time to cool off and it will take a lot of running time (i.e. power) to get things cool... it's not like a home fridge where you can stick a case of warm beer in there and it will be cool in 1/2 an hour...

Another way to get real world power use is to bring that Dometic home, plug it into a Kill-a-watt, and use it for a week the way you would use it in the van and see how much actual power you draw, can't beat empirical evidence.

Before you go any further figure out your storage plan for power too, because it makes a big difference to how you handle rainy days etc. Also are you going to be charging off your batteries when driving (driving much?), or shore power? Factor in other sources of power not just solar. A small generator like an EU1000i can go a long way to making it so you don't have to way over build your system just to cover for that once a year 10 days of rain, just run the gen to charge your batts. Try to build something that will work for most of the year in most of your expected circumstances.

5-6 hours of solar a day is generous at best. You can find online charts that will tell you how much solar energy to expect by month based on location, that will give you some idea of how much less you'll get in winter than summer, but keep in mind most of those calcs are based on angling your panels toward the sun which you're not talking about (if there's a way to get your panels so you can detach and angle them consider that, I only have 100w panel on our van but I have gotten 95w at 4PM in January before just because I could angle the panel just right).

I would agree with the "get as many panels as you can afford" line above, no one, ever, anywhere said "Boo, I have too much free power" ;-)

You're ahead of the understanding curve using the phrase "amp hours" not "amps", so good on yah for that. Going forward one tip I would give is to start thinking of things in watt/hours not amp/hours though. Watt/hours are basically interchangeable regardless of setup you have (12v, 24v, 120v etc), but amp hours are dependent on the voltage you're talking about and so you can get confused if you start moving into bigger systems that are 24v or 48v and you can badly over or under estimate power draws if you don't adjust amps based on volts, easiest just to convert everything to watts right from the start.

 

[miss.h01]

Wow, thanks for the replies everyone.

I'll do some research on the Dometic CFX 75DZ when I get home. I'll shoot Dometic an email right now about it.

To answer an above question; I am planning on charging my batteries when I drive. Not sure if I'll be hitting the road everyday but I do plan on moving around a bit.

I was planning on buying a single 100Ah Lithium battery, which would only be enough for a single day without power, right? I heard not to let batteries fall below 50% to stop them from being harmed, so if I use roughly 50Ah a day I should be good for a single day? If that's the case I may look into two 100Ah's or maybe a single 150Ah battery.

I have a lot to think about, this is only the beginning of my campervan project and it's already stressing me out, ugh!

 

[highdesertranger]

you need to do more research.

for one lithium batteries don't have the 50% cap.
another it takes hours and hours of driving to fully charge a battery.

before you buy anything ask questions, we are here to help.

highdesertranger

 

[Trebor English]

The Battle Born 100 Amp hour lithium LiFePo4 battery will be plenty for several days with that fridge, probably 3.  Battle Born have BMS, battery management system, and they shut off to not self destruct.  Once you go three days using it with no solar input it will not charge up with 20 minutes of driving. 

Lithium batteries are different from lead.  If you account for many charge / discharge cycles for ten years one lithium battery is similar in price to two or three sets of lead acid batteries.  With lithium you buy ten years worth at a time.  You can't return the unused portion. 

Lead batteries are slow to charge and require frequent full recharge.  If you only recharge to 90% full the capacity lowers making that the new full capacity.  Lithium batteries don't suffer from the fading capacity of sulfated lead acid batteries.  They charge quickly.  They will take a lot of current.  A big deeply discharged lithium battery can take enough to cause a vehicle alternator to overheat and fry, particularly at idle speed.

 

[highdesertranger]

Terbor no disrespect but what Lithium batteries charge fast? all my smaller ones take a long time to charge and I am camping with someone with Battle Born batteries and they take a long time if deeply discharged. highdesertranger

 

[Trebor English]

Lead acid batteries, particularly AGM, can take current pretty quickly up to about the 80% full level.  A 100 amp hour AGM can usually take 20 to 30 amps for a long time.  At 0.4C, 40 amps, an AGM will get to the absorption voltage, about 14.4 volts, pretty quickly then the current starts to drop off.  Then, with the voltage limited, the current slows to a crawl.  A Battle Born with a big RV converter set to 14.6 volts can get over 40 amps.  Then they can maintain the speed almost all the way to the end.  My 5 amp solar panel, of course, won't charge a Battle Born any faster than an AGM except for the very last percent.  To see the difference you would need 8 such panels of 5 amps each to get to 40 amps. 

Bottom line, fast is relative.  All battery charging is slow.  I should have said lead is slower than lithium rather than lithium is fast.  Using lead acid batteries as the standard sets the bar rather low.