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Old 03-19-2018, 06:03 PM   #1
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THOR #10499
Solar Installation Questions

I have been searching the site and haven't found the answers to my technical solar install questions. I know many of you have installed solar arrays on your coach, so I am hoping that you will be able to help answer a couple of technical questions. I'm an engineer by training, so technical and systems integration is what I do.

When I install the solar array and charger, I assume they are connected directly to the 12V DC bus. How does that impact the coach with the converter/charger also connected? When we are plugged in, both will be providing a charge current to the batteries. Do I need to worry about feedback between the 2 sources as I would with 2 non-synchronized AC sources?

I am looking at a Renogy 400W array, and have a Whirlpool residential fridge. I am assuming that the solar panels will not be sufficient alone to power the fridge, but will be able to at least significantly extend the time between re-charge starting of the generator.

We live in south Florida and most of our travel is within the state, but we do regularly travel up to GA and looking to take a few other, longer trips. It would be nice to stop over night at a rest area to sleep and not have to be woken up by the generator starting or the inverter alarming on a low voltage.

Any experience, recommendations, or general direction to go are definitely appreciated.
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Old 03-19-2018, 06:26 PM   #2
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400 watts would be close if all you were powering was your fridge if: you always got 400 watts out of the solar which we all know won't happen, especially at night.

You don't have to worry about connecting 2 chargers to the system in a DC system; there is no "out of phase" . The only problem you may see is that the converter charger may see the batteries at a higher state of charge than they actually are due to the charging voltage of the solar controller, which may kick the converter out of bulk charging, which will mean it will take longer to fully charge your battery array.
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Old 03-19-2018, 06:47 PM   #3
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Solar panels will generally put out about 18V per panel at full capacity, however there are panels available (generally used for stationary systems) that put out more voltage.

Solar panel outputs go to the input (sometimes called PV for photovoltaic) of the solar charger. There are two types of solar chargers, PWM and MPPT. The PWM chargers are of limited capacity, usually 40 amps and can usually only handle 18-24V. The MPPT chargers can take much higher voltages as input, often up to around 100V. They also can output more amps. You have to have a charge controller. Solar panels don't connect directly to the batteries. A good solar charger is smart enough to regulate the output based upon battery type and current charge state so they won't boil your batteries.


What this means is that if you use a PWM controller, you typically have to run your solar panels in parallel. This means more current down to the charger and bigger wire. MPPT allows you to run your panels in series and provide higher voltage and less current. So you can get by with smaller wire down to the charger.

Both types of charger put out 12-15 volts to charge the batteries. The output wiring is connected directly to the battery (a fuse/breaker in the circuit is recommended) in parallel with other charging sources such as your power converter and engine alternator. Wire to the battery generally needs to be larger (4 or 6 ga or even larger) and as short as possible to keep voltage drop down.

Lots of arguments of the advantages of PWM and MPPT. I have a PWM controller as I will only have 300 watts of power (200 roof top and 100 portable). If you have larger systems, especially over 400W, the MPPT is a requirement. You will find lots of discussions citing the higher efficiency of MPPT, but unless you are going over 400-500 watts, PWM will work well. While the specs say MPPT is better (and more $$), I'm not sure there is a real world advantage in small systems.

I'm in the middle of a project 2 100-watt panels on the roof wired in parallel with 10 ga wire to a pair of 6 gauge wires that go from the roof (refrigerator vent) down to storage compartments and across to the battery charger that is in a compartment near the batteries. This is plenty for me and will support 2 more roof panels in the future. I have a pair of 6V Interstate batteries that give us 225 amp-hours (no residential refrigerator) Eventually I'll add two more 6V batteries along with another 200 watt roof panels.
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Old 03-19-2018, 06:53 PM   #4
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Mr. Transplant,
(Or may I call you "Denver"? 0
Thanks for answering a whole bunch of questions that I was afraid to ask...
The Missus and I are still looking at "kits"*** for this; we just need to screw our courage on a little bit tighter!



*** We've got our sights set on a 400 watt four-panel kit from Renogy...
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Old 03-19-2018, 08:41 PM   #5
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[QUOTE=Thanks for answering a whole bunch of questions that I was afraid to ask...
The Missus and I are still looking at "kits"*** for this; we just need to screw our courage on a little bit tighter!

*** We've got our sights set on a 400 watt four-panel kit from Renogy...[/QUOTE]

Rhenology makes a decent kit. GoPower also makes a kit, but they are very impressed with themselves and cost also double what Rhenology charges. Check wire sizes as sometimes they skimp on wire sizes in kits. Not such a concern with MPPT controllers. I chose to go the pieces and parts route - Flexible panels from Windy Nation and a Bogart SC-2030 controller that goes with my Trimetric battery monitor. The portable panel with the SC-2030 has been keeping my batteries in great shape all winter.

Check out the solar videos at Love Your RV ó http://www.loveyourrv.com/love-your-...-and-upgrades/ if you havenít already. He used a Rhenology kit, but later upgraded to an SC-2030.
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Old 03-19-2018, 08:45 PM   #6
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Old 03-19-2018, 10:29 PM   #7
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if in Florida mostly, you'll have an easy time with 400w, as long as you actually have sunshine most of the day, and enough battery storage for the overnight... other threads discuss how to help 'overnight' with your residential fridge...

as for the solar amps, we just run our array down to our controller, which is in the battery compartment, and then directly to the batteries... it has no contact with the onboard Charger or other systems. The onboard Magnum charger, if we are on Shore Power or Generator, also provides any charging, but both the solar controller and onboard charger monitor the battery levels and adjust accordingly. 'Overlapping' is not an issue, as both can easily be charging the batteries, even at the same time. The solar controller will back off when the batteries don't need it, just like your charger does.
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Old 03-20-2018, 12:08 AM   #8
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There is no problem with the CC output in parallel with converter and alternator, that is the way our system is. When on shore power and the panels are in full sun they will over ride the convertor. I just turn the convertor breaker off most of the time.

400 watts will offset your gen run time but even in FL or GA you can't depend on 6 hours of full sun every day.

I measured a friends system with the ginney running his inverter was putting out 60 amps to the batteries minus the 17 amps needed for the refer. Under those condition they require 5 to 6 hours of ginney run time to completely recharge the batteries. 400 watts of solar under ideal conditions will produce around 22 amps. A WFCO convertor (some not all) may produce up to 20 amps or so for about 30 or 45 minutes when the batteries are at 80% SOC. Other brands will do better but with the reefer you have an inverter/charger is a better option then a convertor and inverter.

None of this takes into account other DC loads.
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Old 03-20-2018, 12:41 AM   #9
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Thanks All. Lots of good information here.

I started checking battery water and connections tonight. All of the cells are full, batteries are all the same type, contrary to my previous belief, and I have 4 of 6 battery terminal connections that were loose. I've been testing my fridge in the batteries and it had been almost an hour now my batteries are still at 12.1V and they haven't been on a charge since Saturday afternoon when we got back from our trip. So it looks like my batteries are in good shape.

Total of 330ah between the 3 wet cell size 27 batteries. Figure at the most right now, I will get 6 hours before the gen need to start and charge the batteries.
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Old 03-20-2018, 04:03 AM   #10
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All righty - I guess I don't understand. A 100 watt panel produces about 6 amps per hour. A residential refrig uses about 8 amps per hour with the compressor running. Limiting the need for the compressor (not opening and closing the door constantly) reduces this 8 amps per hour draw considerably. If you want to recharge the batteries (we have 4 6 volt Trojans), anything beyond the current draw to power the refrig and other 110 VAC goes to that task. I talked with Renogy folks and their best advise was to use a 200 watt system with a 30 AMP PWM controller. Given 12 hours of sunshine in Key West, my system should be backup to 100% (starting from 75% charge) at the end of the 12 hours. Given the temps in KW in the winter and our coffee need, the gennie will still run about 4 hours per day. That is a decrease of about 4 hours per day. At about .6 gal of fuel per hour, saves me about $6 per day in gas. We spend about 2.5 months per year dry camping. Pays for itself in one season.
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Old 03-20-2018, 11:45 AM   #11
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Isn't the fridge about 8 amps of 120V? Almost 1000 watts when the compressor is running?
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Old 03-20-2018, 12:00 PM   #12
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Isn't the fridge about 8 amps of 120V? Almost 1000 watts when the compressor is running?
And it may be light outside for 12 hours in Key West but the solar panels don't operate at 100% for all of those hours, if any. So yeah, Ben's first statement was correct
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Old 03-20-2018, 01:15 PM   #13
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right, it's always easy to 'assume' that daylight hours equates to amp hours, but that's just the solar advertisements kicking in - Solar is only really effective when the sun is directly hitting the panels, and the panels stay tilted toward the sun directly during the full daylight hours. When the sun is first coming up, and toward sunset time, it will not necessarily provide anywhere near the same amps as when it is between 10am and 4pm - 6 to 8 hours is really the 'max' hours for Solar, unless you are in Alaska during the summer where there can be as many as 12-14 hours due to 24hr of daylight.

Looking at Solar as effectively helping to limit the number of hours of generator usage is the best way of considering it's power - the way we use electricity each day will never be the same, and the sun light hours will never be the same due to clouds and shade.
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Old 03-20-2018, 01:20 PM   #14
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VERY good points!
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Old 03-20-2018, 01:39 PM   #15
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I tested the fridge on the inverter yesterday. I found that with only the fridge running from being off and open for 3 days, and all other ancillary loads, that I had a draw of 15A which translates to about 170W. Surprisingly, when I opened the fridge door, it was up to 20A, adding 4A just for the light. Anyway, I think I am going to stay with the 400W kit from Renogy to start off my first solar experience. I am not expecting to replace all battery power, just to supplement and extend the time between the generator starting.
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Old 03-20-2018, 02:10 PM   #16
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Did you purchase your kit yet?

https://www.amazon.com/dp/B00BSZUHRC...a-309877293546

This is the one that we're going to be purchasing; once we're ready to pull the trigger...
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Old 03-20-2018, 02:27 PM   #17
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Originally Posted by Bob Denman View Post
Did you purchase your kit yet?

https://www.amazon.com/dp/B00BSZUHRC...a-309877293546

This is the one that we're going to be purchasing; once we're ready to pull the trigger...
That's te kit I'm looking at also.
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Old 03-20-2018, 02:31 PM   #18
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I'd say that "Great minds think alike.", but I'd never want to insult you like that!
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Old 03-20-2018, 03:05 PM   #19
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I'd say that "Great minds think alike.", but I'd never want to insult you like that!
Haven't purchased it yet, but I think pretty soon. My wife will be heading out on a few road trips this summer and I'd like to have it in place before she goes.

I like to think I have a pretty great mind...but there are probably several others that would disagree. A matter of my personal opinion I guess!
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Old 03-20-2018, 03:30 PM   #20
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That is the same Renogy kit that we installed. Most of the above advice is sound. The one takeaway to really remember is that solar is a supplemental source for charging, on overcast or rainy days you may not get much charging done at all. And in snowy cold weather, if the panels get covered and the batteries are very cold, it becomes much harder to charge them. Solar charging pretty much puts you at the mercy of Mother Nature. So be prepared to use the generator when necessary.
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