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Old 02-04-2011, 01:18 PM   #1
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looking for info on solar battery charging

I'd like be able to charge my battery on my Globetrotter using a solar panel, to the point I could run a 12v/120 fridge from the battery- I'm thinking of using the Engel SB70F Engel SB70F Front-Open Fridge/Freezer
which seems to have a pretty low draw.
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Old 02-04-2011, 01:57 PM   #2
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The keywords to Solar DC operated refrigeration is insulation, insulation, insulation. Clicking through to the specifications shows how important the ambient temperature is, ie: making sure the external coils have plenty of clean cool fresh air flow - not a closed compartment airspace... Any additional insulation you can add and keep it looking factory installed would be a good thing.

Also - the units feed wire size is very important when its 24/7 operation, some small internal-inverter compressor control units might reject (short-cycle) as low battery protection on start-up from wire losses and battery not being top-charged. If the unit does not have low battery protection maybe you should look into finding a low battery disconnect minder...
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Old 02-18-2011, 04:47 PM   #3
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Thanks for the response- I was wondering about the wire size. The Engel draws from .5 to 2.7 amps.. should I think about two batteries wired together, series, parallel?

P
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Old 02-18-2011, 05:35 PM   #4
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2x6v in series=12v
2 (or more)12v in parallel = 12v
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Old 02-18-2011, 06:25 PM   #5
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I really hate to be the negative one here, but we spent 8 years full time away from shore power on our boat running refrigeration from solar and wind and while it can be done, it takes way more than just adding a solar panel.

We had a small unit with a cold plate and a 12 volt Danfoss compressor and had 6 inches of insulation around the box. Our energy use averaged 35 amps a day and that was by far our biggest energy draw. To feed it, we had 4 six volt golf cart batteries, 210 watts of solar, and a wind generator. It all worked well and we almost never had to run the engine to charge the batteries.

One solar panel (unless it's a really big one, like 200 watts) and a couple of normal sized batteries just won't keep up and if you have a bunch of gray days with little sun, you'll need to be looking for a way to plug in to keep the fridge cold. Looks like the unit you linked is 12/24 volt only. While their numbers look good, I'd suspect they are not accounting for ever opening up that fridge door. I'd bet your amp draw would end up higher than the 35 or so we ran on the boat.

Solar is wonderful and I've got it on my project trailer, but I'm really happy to be putting in a propane fridge and letting the solar support LED lights and a few other 12 volt draws.

Again, sorry for the negative reply, but I'd hate to see you sink money into something that won't do what you want.

cheers,
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Old 02-18-2011, 07:12 PM   #6
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Another perspective...............

The first 'big' project that I will do on my MoHo is to add 900 watts of solar in 6 X 150 watt AM Solar panels that will charge the 6 Lifeline GPL-6CT 300 amp hour golf cart batteries I have already installed for a total of 900 amp/hours of battery capacity.

I'm doing this to be able to power a 9 cubic foot Danfoss compressor marine fridge 12VDC/120VAC and get rid of the LP ammonia/absorption unit. I work on these units every day and IMHO, their time has come and gone! Almost every decent sized gas/absorption (G/A) fridge that has been manufactured in the last 10 years or so has been under some sort of safety recall that relates to a fire hazard from the hydrogen used in the solution used to create the cold in these units.

The G/A units' legendary inefficiency is horrible. Ever had 'RV softserve' also known as a poor excuse for ice cream? Ever left the refer door open for a minute and had the unit take over an hour to recover that lost temperature within the box? Yes, the little Dometic that I had in my '06 19CCD worked as well as it could, but still left a lot to be desired.

Since the Norcold mega-recall on their large 12 cubic foot units, I have had numerous requests to get rid of these unite once and for all and eliminate any danger of a hydrogen fueled fire (there have been MANY!) and replace them with either residential style units or marine units.

I realize that this task is far easier on large motor homes with their on-board generators and large battery banks, but I would surely consider using a Danfoss powered marine fridge in a trailer. I have an Engel top access refrigerator/freezer unit in my service van and run it with 300 amp/hours of batteries powered by 400 watts of solar as the only charge source. It has been running 24/7 on DC for 3 years now and has never even burped once!

Something to consider seriously...............
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Old 02-18-2011, 08:23 PM   #7
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Another perspective...............

I realize that this task is far easier on large motor homes with their on-board generators and large battery banks, but I would surely consider using a Danfoss powered marine fridge in a trailer. I have an Engel top access refrigerator/freezer unit in my service van and run it with 300 amp/hours of batteries powered by 400 watts of solar as the only charge source. It has been running 24/7 on DC for 3 years now and has never even burped once!

Something to consider seriously...............
I know what you're saying Lew and agree that a Danfoss compressor unit is going to be reliable, but how can anyone run 400 watts of solar on a Globetrotter? If you're going to be plugged in all the time it makes sense, but for boondocking it's really hard to make it work on a small trailer. Isn't it?

cheers,
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Old 02-18-2011, 09:43 PM   #8
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I know what you're saying Steve. I have not experimented with less of an array to see how the batteries react to it. I have seen up to 27.8 amps to the batteries from this 400 watt array, and would think that even 200 watts could be an adequate charging source for 300 amp/hours of batteries at 10-14 amps max.

I'm fairly certain that the 200 watt array would generate more than the required 30-36 amp/hours to run a Danfoss over a 24 hour period.
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Old 02-18-2011, 10:36 PM   #9
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Quote:
Originally Posted by Globie64 View Post
I'd like be able to charge my battery on my Globetrotter using a solar panel, to the point I could run a 12v/120 fridge from the battery- I'm thinking of using the Engel SB70F Engel SB70F Front-Open Fridge/Freezer
which seems to have a pretty low draw.
Fascinating topic!

I probably don't know as much as the others about how large an array you can cram onto your trailer, nor would I feel comfortable with 400 watts grounded to the skin of my trailer, but here's what I do know...

If you've got the spec sheet on fridge power consumption, figure out what you'll need, then size your DC battery bank (I'd go 125+ amp-hr batts in parallel) so that your estimated power consumption falls in the range of 20%-40% of your total battery amp-hours. If unsure, I would err on the side of 20% to accommodate for periods of low sunlight so the batteries don't get drained past 50% charge.

After reading the boat experience and the propane fridge comments, I'm wondering how much I really need a fridge. Seems a good icebox would fare about the same, but I'm not really sure how messy they get with the melted water.

Good luck and thanks for the fridge link. I'll go check it out.

parting shot: for a more expensive option, you can purchase flexible solar film - think the silicon sheet without the typical glass "sandwich" of the solar panel. Maybe you could wallpaper the top of your trailer as your array for the fridge. Just take a look at what these happy folks did with that technology. Home | OSU Solar Vehicle Team
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Old 02-18-2011, 11:10 PM   #10
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As to wire size - AWG tables & calculators show 10, 5 and 2% losses and the 2% is where to aim for for in 24/7 operation circuits. While you're at it try and up the PV down-lines to that too, just a customization that bumping up the wire quality from a 5% usual & customary allowance pays a 2 or 3% return every minute the sun shines.. .
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Old 02-21-2011, 11:59 AM   #11
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Thanks to all of you for all the help- this is what is so great about the forums, the wealth of knowledge within the community.

For our one outing, and for an SF-Boston/Boston-SF road trip, we've used a Coleman electric ice chest, for drinks and snacks, and it was fine. We don't need a very large fridge, hoping that smaller fridge = less power necessary. I had looked at the ice box that Vintage Trailer Supply sells, but wondered about how much insulation it had, and thought that maybe there'd be a better built-in icebox solution out there. I had thought about a 3-way dometic fridge, but the cost and complexity made me want to look at other options before taking that plunge.

The wiring info is a huge help, as I'd like to keep the battery up with a PV setup- I also have to change out the old Univolt and wondered about any additional wiring I might want to do with that... and on and on

Thanks a ton

Peter
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Old 02-21-2011, 12:03 PM   #12
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And thanks SpaceEgg for the OSU link, that is awesome. I'm a bit familiar with the film- I did a case study on a museum in Minnesota that used the film, and hope to use it here on a project, putting the film on the flat part of a standing seam roof.
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