Dual solar systems?

[Tony O]

OK, I'm in a bit of a quandary over how to configure my trailer now that I have yet another complete solar system ready to install. Hopefully some of the solar experts here who have so often advised hapless DIY types like me can help.

Here's the deal: When I bought my trailer it came with a 45 watt panel flush-mounted on the roof. This system charges the single battery in the default factory spot under the propane shroud in the front of the trailer. That's OK.

Now, I've got a 200w two-panel array that I've configured for portable use, a 40w Renogy MPPT charge controller, two 210AH AGM batteries, a 2000w Inverter and a Renogy MT-50 charge monitor gizmo.

Questions:
1.) Is it practical and/or advisable to tie the two systems together or keep them separate?
2.) If disparities in equipment size/configuration prevent marrying the two systems I'd still want to be able to have each separate system be connected to and function through the existing, factory installed power center, perhaps with separate terminals to connect the inverter to when needed.

The trailer has a terminal with binding posts hidden inside a cabinet at the front of the trailer. (See picture.) The power center is located in the same cabinet but separated from the storage portion of the cabinet by a plywood wall. My thinking is that I could put a battery disconnect or a 60 amp AC disconnect box somewhere in the circuit to switch off (or disconnect) one system when running the other.

I may be overthinking this but I'd really like to get this right the first time. Many thanks in advance for your sage advice!

[Tony O]

Anybody have any comments, personal experience or ideas to share?

[Raspy]

1. Maybe I'm misunderstanding, but it sounds like you are trying to run the inverter directly off the solar panels? The inverter will be connected the the battery, and the solar helps charge the battery.

2. Since one system is portable, it will be disconnected form the battery unless you physically connect it. And if you park in the shade, the built in system will be off and disconnected by the controller.

[Tony O]

Originally Posted by Raspy

1. Maybe I'm misunderstanding, but it sounds like you are trying to run the inverter directly off the solar panels? The inverter will be connected the the battery, and the solar helps charge the battery.

2. Since one system is portable, it will be disconnected form the battery unless you physically connect it. And if you park in the shade, the built in system will be off and disconnected by the controller.

I understand your question and no, I think connecting a large inverter directly to solar panels is probably not a practical thing to do. As you suggest, it must be connected ahead of a battery (or multiple batteries), serviced by a charge controller.

That said, I believe the 2000w inverter may be too large to reliably connect to the single factory 12v battery - especially if I were to connect a heavy load to the inverter for an extended period. But, possibly OK if both systems (all three batteries and panels) were connected together.

The idea of connecting the inverter to only one system assumes that it is not technically advisable to connect both systems together. If that is the case, my thought is to have the inverter exclusively connected to the two much larger 210AH AGM batteries and use them separately.

[Jim Bennett]

What I have done is to put 240W of fixed on the roof and use an 80W portable for the rare occasions I need an additional boost. It all runs through my 30A MPPT controller. This keeps it simple and efficient.

I also use a 1500W inverter which attaches directly to the batteries.

In this case I would use your solar controller to handle all the charging.

I have attached a schematic of what I have done. I did configure the input at 36V instead of 18V to save on some line loss, but the actual savings really is not all that much. I would likely stick to a full parallel 18V input if configuring again.

[CarlD]

Tony,


1. It is not advisable to connect dissimilar batteries together.
2. Solar panels are a current source type device. You can connect them in parallel if the open circuit voltage is similar. Many solar panels are rated at around 18 volts for a 12 volt system. This is easy to verify by measuring the terminal voltage in the sun with nothing connected.
3. Warning. I have a Renogy 20 amp MPPT controller. The manual states not to connect the solar panel without a battery connected as it could damage the controller. Read your manual, it may be similar. I installed a 2 pole battery disconnect switch. One pole disconnects the battery and the other pole disconnects the solar panels.
4. Purchase a couple fuse holders and fuses to put on the battery terminals. I used these https://www.amazon.com/Blue-Sea-Syst...WJVPSGWZFY9703
5. Make sure you have heavy gauge wire from the battery terminals to the inverter. 2000 watts will draw around 180 amps at 12 VDC.
6. I am not sure why your want two separate system. If it were me I would loose the original battery and just use the new 210 Ahr units and wire things so you can easily connect the portable solar panels to the system in parallel with the 40 watt solar panel.

[Tony O]

Originally Posted by Jim Bennett

What I have done is to put 240W of fixed on the roof and use an 80W portable for the rare occasions I need an additional boost. It all runs through my 30A MPPT controller. This keeps it simple and efficient.

I also use a 1500W inverter which attaches directly to the batteries.

In this case I would use your solar controller to handle all the charging.

I have attached a schematic of what I have done. I did configure the input at 36V instead of 18V to save on some line loss, but the actual savings really is not all that much. I would likely stick to a full parallel 18V input if configuring again.

Attachment 128717

Thanks very much for this, Jim. Your diagram really helps to get an concrete idea how my system might be configured.

CarlD too made some valid observations and recommendations. In particular his advice to lose the original battery and revert everything to the two large batteries makes a lot of sense. While I suspect most folks would want as much power capacity as possible to maximize overall potential usability I see the logic in the simplicity of CarlD's proposal. As well, I might be able to use the original factory battery location to carry an additional smaller LP bottle. Hmm...

Being a devout boondocker that always prepares for worst case scenarios I still haven't accurately measured my typical power use to gauge whether I can "survive" without a generator. Having used one for years I'm finally at the point where I would like to leave that noisemaker at home. In the past I've plugged a 100' heavy-duty drop cord into the gen after hauling the gen as far away from the trailer as possible, hiding it behind big trees or over the edge of embankments. It helped a lot but then I'd have to march out there every time to start or turn it off.

What I'm thinking now is to use both systems separately, the original system for lights (LED's are installed), water pump, fans, etc. and the larger capacity system for other needs. Tying the larger system into the trailer's existing power center when needed would be wonderful if I need to power onboard systems more frequently, such as when winter camping when the furnace fan is working a lot.

[Franswa]

can't think of any reason you'd want to have TWO systems...(???)


if it was me I'd install the two new panels (200W) on the roof and carry the 45W inside the trailer to deploy/set-up at will/need
(100W panels get rather large when you go looking for a place to store them conveniently)


My system is all 12V (3 panels wired in parallel, two on roof, one portable)



I have a BF with the same propane locker...I ran a wire for the portable panel from under the left dinette seat into the propane locker (drilled hole straight through)...I have about fifteen feet of wire stored there wound around the top of one propane tank.....makes for a quick/easy connect of the portable

[Tony O]

Originally Posted by Franswa

can't think of any reason you'd want to have TWO systems...(???)


if it was me I'd install the two new panels (200W) on the roof and carry the 45W inside the trailer to deploy/set-up at will/need
(100W panels get rather large when you go looking for a place to store them conveniently)


My system is all 12V (3 panels wired in parallel, two on roof, one portable)



I have a BF with the same propane locker...I ran a wire for the portable panel from under the left dinette seat into the propane locker (drilled hole straight through)...I have about fifteen feet of wire stored there wound around the top of one propane tank.....makes for a quick/easy connect of the portable

Yes, I gave a lot of thought to doing what you've done but decided to rig the 100 watt panels as portables for three reasons: 1.) The AGM batteries charge faster than some other types. 2.) Aiming the panels at the sun will maximize charge rate and shorten the time to full charge. 3.) I can park in the shade when possible and string the big panels out in the sun away from the trailer. Besides, the 45w panel is more or less permanently mounted to the roof and the connecting cable neatly secured and threaded into the trailer.

BTW, I really like your panel storage idea and noticed the cool removable shelf you affixed to the entry door grab handle. I may have to rig one up like that for myself. Well done!

[Raspy]

Jim,

In looking at your diagram, I'm wondering if any power is lost from the portable panel to the fixed panels if the fixed panels are in the shade while the portable one is in the sun?

I've wanted to hook up two arrays as shown, but didn't know if it was a good idea.

[Jim Bennett]

Originally Posted by Raspy

Jim,

In looking at your diagram, I'm wondering if any power is lost from the portable panel to the fixed panels if the fixed panels are in the shade while the portable one is in the sun?

I've wanted to hook up two arrays as shown, but didn't know if it was a good idea.

My understanding is that there would be no loss like that at all, as the amperage is additive in parallel, and it would put whatever it supplies right into the controller. In practise I know I see a huge jump in charge current when I plug it in. I have seen good 5A boost when plugging it in with good solar gain on it, though have never really done any proper testing.

All I know is that it sure is nice to use that electric toaster in the morning.

[CarlD]

Raspy,
If two panels are connected in series the current will be the lower of the two. If one goes dark you get no current. If they are connected in parallel AND the voltage curves are the same the currents will add. If one panel goes dark the current will be cut in half. That being said it is better to operate them in parallel unless you can keep them all in the sun. It is more efficient to operate them in series if they all have similar current characteristics, all receive the same amount of sun and you use a MPPT controller.

[Raspy]

Carl,

Yeah, I get that. But I thought one of the functions of the solar controller was to prevent reverse current flow at night to the collectors, and away to the night. Isn't that the function of the diode?

So, if two collector arrays are connected in parallel and one is in the shade and one in sun, and the line after the parallel connection has to overcome a diode to get to the controller, I was suspicious that there may be a loss from the producing collector to the shaded collector. Or that the shaded collector could become a slight load.

I don't know this, just asking the experts. If the collectors, at night, don't become a load, is there a need for the diode function other than just keeping voltage away from the collectors when they are not producing?

[Raspy]

I really enjoy having collectors hooked up while camping. Before I brought power in to my new house we had a trailer hooked up with a solar panel to keep the batteries up.

I tried using collectors on my boat but it was mostly a bust. Any lines or stays that shadowed it, even slightly, cut the power. I think it was an amorphous collector. Sail boats swing around on the hook, the mast, shrouds and halyards are very good at shading.

I now have a suitcase setup that I'm getting ready to use this season on the trailer.

[Franswa]

yeah, "it depends" as they say....


your situation might be different than mine....with me, I tend to move every day when I am out on a trip (don't stay in one place)...so for me having more rooftop than portable makes sense.....and yes, from my very "ballpark" observations a panel directly angled to to sun delivers TWICE, at least, what a panel of the same size would deliver on the flat (roof)....in my latitudes anyway


if you have the same hardware the little shelf is a very "good thing" and dead simple....keeps all small items "outside" in one place and under cover of the awning....."work with what you got" are words I live by

[CarlD]

John,
You are correct, in a parallel configuration, a dark panel will draw power from the battery and it will also draw power from a parallel panel that is in the sun. The model of a panel is a forward biased diode in parallel with a current source. A typical panel designed for 12 volt operation has a nominal voltage of around 18 volts. There are 36 cells connected in series each with a 0.5 volt drop. The VI curve is an exponential function typical for a diode. When a panel goes dark it acts kind of like a leaky voltage limiter set at 18 volts. This will limit the maximum voltage the illuminated panel can produce thereby reducing the output power. A series blocking diode will prevent current from flowing back into a dark panel, however you will incur a voltage drop and hence a power loss. A charge controller per panel is the best option for highest efficiency, but at a higher cost.

[Franswa]

most solar panels sold as "kits" (panel, basic charge controller and various connecting wires) like the ones I have been buying come with a diode built in....I have enough CCs to use one per panel as suggested...but I found that these basic controllers have a cut-off voltage that is too low...this suggest the battery bank would never get to absolute "full charge" (according to Handy Bob anyways)

[Jim Bennett]

If using an MPPT controller uses all energy possible from an array of panels by adjusting the voltage to what it needs to charge. In a nutshell, an MPPT controller helps to minimize output losses associated with partial shading and other panel output mismatches.

I have had my trailer in heavy shade, tied my portable into the array, which resulted in full charging from the portable.

[Raspy]

Thanks guys.

The conclusion sounds like: It's best to have a controller on each panel array if cost is no object, but with an MPPT controller it doesn't really matter, at least partly because that style adjusts the voltage to always get to full charge if there is enough capacity in at least one of the panel arrays, vs the loads. There can or will be some losses to the shaded, or darkened array, but it'is too small to worry about.

With all of that, a parallel connection upstream of a single MPPT controller is fine. This also simplifies the use of a portable system by allowing a simple plug somewhere to plug in the portable array directly from the terminals on the panels. Just just two conductors with no controller.

I hope that sums it up to your liking, and thanks.

[justlookin]

Originally Posted by Tony O

Being a devout boondocker that always prepares for worst case scenarios I still haven't accurately measured my typical power use to gauge whether I can "survive" without a generator. Having used one for years I'm finally at the point where I would like to leave that noisemaker at home.

TonyO, I have 300w flat on the roof charging two 6v GC batteries. I boondock exclusively and have not been without enough power ever. That was out west where sun is plentiful. Here in the northeast you are likely camping under the tree canopy and will not get a full charge everyday. When not hot I'll park in the sun till batteries are full then move to shade.

I just bought a Sportsman 800w generator from Home Depot for $149. It will run my 30 amp battery charger so I can park in the shade and put a bulk charge on the batteries. I also just tapped in with 20ft of cable for a 100w suitcase solar panel that I can put out in the sun and track throughout the day.

The little generator is super quiet, weighs only about 20lbs and sips gas. Sell that big noisy generator and look into one of these little 800w gennys.