Adding an auxiliary solar panel in series?

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Adding an auxiliary solar panel in series?

Postby lfhoward » Wed Sep 16, 2020 7:30 pm

Hi folks,

My trailer has a 200 AH battery bank and a 140 watt solar panel permanently mounted on the roof. It’s an older panel from Craigslist and so I don’t think I’ve ever gotten 140 watts out of it. We also tend to camp next to trees that provide at least partial shade. In a parking lot in full sun I’ve seen 7 amps of charge going from the controller to the battery, but that would only translate to 85 watts. In a typical camping spot, the most I’ve seen is 2.5 amps of charge or so, only during the hour a day where the panel gets the most sun.

We run a fantastic fan at night, which is about 2 amps. Later in the season we will have a Propex heater replacing the fan, but it’s still roughly 2 amps. We both have phones that charge at 1 amp a piece, and other electronics probably take another 1 amp. Assuming devices are on about 4 hours a day, that’s 5 amps X 4 hours or 20 amp hours used in a 24 hour period. (Very rough estimate I know.)

If my solar is providing 2 amps for 8 hours a day (doubtful), that’s 16 amp hours of recharge in a 24 hour period. I think in reality 10 amp hours is optimistic with the trees. So we are using about 2x the power we make in the shade. That’s why I have a large battery bank so we can camp for many days in a row. However, i just bought a small efficient fridge, which would add additional load. The Alpicool fridge (see Slim Potatohead on YouTube for an excellent review) is rated at 45 watts, which at 12 volts is 3.75 amps. However it would cycle on and off. Let’s say it’s only running the compressor for 8 hours during a 24 hour day, so that would be an additional 30 amp hours used. What have I gotten myself into? Lol.

Let’s sum up.

200 Amp hour battery bank, of which 100 amp hours are usable (50%)
Fan or Propex 2 amps x 4 hours = 8 AH
Phone charging 2 amps x 4 hours = 8 AH
Other electronics 1 amps x 4 hours = 4AH
LED lighting is negligible.
Total use per day adds to 20 AH
Solar charge (full sun) will bring this back to full (7 amps x 3 hours)
Solar charge (trees) would only replenish 10 AH per day
So a net of -10 AH a day means we could camp 10 days on our battery bank under trees and be fine.

Now add in the Alpicool fridge 30 AH for a total of 50 AH used per day.
That’s replenished in full sun (assuming 49 amp hours of charging), so we could be fine if we camped in the desert. But we are in Pennsylvania and there are trees.
If we only get back 10 AH per day under the trees, that’s a deficit of 40 AH so we can camp for 2.5 days until we run out of power.

Advantages to the fridge would be no more having to buy ice or have food spoil in melted ice water. We could save money on ice and food. I like the concept. Can we sustain it is the more important question, and one I should have thought about more before clicking “Buy” on Amazon! But I really want this to work.

So... this is turning into a manuscript... sorry. Here are some remedies I am thinking of.
1. Camp in sunnier spots to be able to stretch the battery bank across more days.
2. Get a portable solar panel to move into the sunny spots around us to supplement our power generation.

I really like #2. I can imagine splicing in a smaller portable panel that can be set up and angled directly at the sun in open areas. 20 feet of wire would probably be enough to stretch it out to where it would need to go. I think wiring it in series not parallel would be the way to go since it wouldn’t be identical to the existing panel. I have an MPPT controller.

Here is my question.
My controller isn’t rated for more than 140 watts of photovoltaic input. However, I would only use the additional panel when the 140 watt panel is in the shade. If my big panel is making only 2 amps, that’s only 24 watts of input, right? I am thinking a 50 or 60 watt auxiliary panel positioned in full sun could get us another 3-4 amps. It would never be utilized when the main panel has access to full sun, so as not to overload the controller. Is this doable? It should provide another 24 AH of charge at 3 amps x 8 hours, basically making our shady spot into the equivalent of full sun and allowing us to be out camping with the fridge for 5 days instead of 2.

Thanks if you read this far! Ideas? Assumptions? Warnings? Experience?
Last edited by lfhoward on Wed Sep 16, 2020 7:50 pm, edited 1 time in total.
My off-road camper build on an M116A3 military chassis:
http://www.tnttt.com/viewtopic.php?f=50&t=62581
Tow vehicle: 2008 Jeep Liberty with a 4 inch lift.
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Re: Adding an auxiliary solar panel in series?

Postby lfhoward » Wed Sep 16, 2020 7:40 pm

Hold on, I just realized a 2 amp iPhone USB car charger is only at 5 volts, not 12. That would make it less than 1 amp draw on the 12 volt system.

Going to recalculate above...

... Fixed.
My off-road camper build on an M116A3 military chassis:
http://www.tnttt.com/viewtopic.php?f=50&t=62581
Tow vehicle: 2008 Jeep Liberty with a 4 inch lift.
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Re: Adding an auxiliary solar panel in series?

Postby saltydawg » Wed Sep 16, 2020 7:54 pm

you dont want to wire panels in series as the one in the shade becomes a load and eats up power. You can do parallel as long as the voltage of both panels is the same. If the are "12" volt panels their open circuit voltage should be around 18 volts. So run them in parallel, easier to connect easier to deal with.

next go lithium for batteries. less worry about hurting them by discharging them too deep. And they can take more charge faster.

edit get a bigger charger rated for your full solar, all you need is for the panel on the roof to die tomorrow and if you replace it with the same size your now over the capacity of the charger.
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Re: Adding an auxiliary solar panel in series?

Postby rjgimp » Wed Sep 16, 2020 7:59 pm

I would also lean towards option #2. Having the rig parked in the shade will remain a positive particularly considering the addition of the fridge. Just as your ice in the cooler will melt faster if you leave it out in the full sun, the fridge will cycle on more often under full sun. Having a portable panel will get you the best of both worlds.
:thumbsup:
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just as soon as the steering committee gets around to scheduling one!
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Re: Adding an auxiliary solar panel in series?

Postby lfhoward » Wed Sep 16, 2020 8:19 pm

I just went back and re-watched Slim Potatohead’s review of the Alpicool fridge. You can find it here.

https://youtu.be/tFzy-6LCnTE

He did an amp hour analysis and even though the specs say 45 watts, he found a maximum of only 5.76 Amp hours used per day on the hottest days. Now that’s entirely doable! Maybe this wasn’t such a bad idea after all. :)

Here is a screenshot of Slim’s math.
Image

I love data.

So 6 amp hours a day instead of 30 for the fridge in my original estimate means we don’t have to change much. It’s only a deficit of 16 amp hours a day in the shade, which translates to 6 days off the grid. An auxiliary panel would still be pretty cool and could make us self sufficient. 8)

[edit: The 45 watt / 30+ AH per day spec for that unit is accurate if it is used as a freezer, and the compressor would run a lot more than if it is being a fridge.]
Last edited by lfhoward on Wed Sep 16, 2020 8:54 pm, edited 1 time in total.
My off-road camper build on an M116A3 military chassis:
http://www.tnttt.com/viewtopic.php?f=50&t=62581
Tow vehicle: 2008 Jeep Liberty with a 4 inch lift.
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Re: Adding an auxiliary solar panel in series?

Postby lfhoward » Wed Sep 16, 2020 8:44 pm

@ saltydawg, here are the specs of my permanently mounted panel:

Image

I’d have to find something with 44 volts of open circuit voltage to match, to be able to wire them up in parallel. I’m afraid that might make it too big to handle or to store in the trailer when not in use. I am all ears though if you have one in mind. I’ve seen the 100 watt panels that are fairly common but as you said their voltage is too low.
My off-road camper build on an M116A3 military chassis:
http://www.tnttt.com/viewtopic.php?f=50&t=62581
Tow vehicle: 2008 Jeep Liberty with a 4 inch lift.
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Re: Adding an auxiliary solar panel in series?

Postby saltydawg » Wed Sep 16, 2020 9:33 pm

Two chargers. simple answer
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Re: Adding an auxiliary solar panel in series?

Postby John61CT » Wed Sep 16, 2020 9:37 pm

Did not parse the details nor read responses.

My reco is if you want a good system, do not f*** around cobbling stuff together from bits and pieces.

Match a single good controller to a single panel that gets best value from it

or vice versa. Forget about cheap-chinese consumer grade silly low voltage panels, just because that is what you find at Home Depot or from mass marketers online.

Example: Victron 75/15, maybe $125

Will get you 15A peak, so 7-12A pretty often for 4-6 hours in good conditions depending on season latitude and whether you tilt / move the panel to get good angles.

To get best increased output from MPOT vs PWM, get a panel at 40V or so, or higher.

The 75V rating of the SC is important, but only as a maximum, keep the panel VOC spec below 70 to be safe.

The 15A is also a maximum, but for **output**, input can go quite a bit higher no problem (aka overpanelling), so for a 12V nominal system, shoot for a panel capable of producing 250+W (Pmax, aka Vmpp x Impp)

If you find a great bargain locally that is rated 300W or even a bit higher, go for it, even though your (rare) peak output will never go there, you're getting higher **average** output, far more important.

Of course if you can fit a bigger (single) panel than that, spend the extra $50 on a 100/20 SC then 300+W becomes your baseline and your stretch is ~ 400W. If you stumble across higher voltage panels from a big install, up to 90Voc would be safe.

And so on.

The key to a good value panel at under $2/watt delivered, is sourcing locally, and that may take a bit of legwork, looking out for contractors finishing big jobs with surplus panels left over.
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Re: Adding an auxiliary solar panel in series?

Postby lfhoward » Wed Sep 16, 2020 9:38 pm

saltydawg wrote:Two chargers. simple answer

Thank you. I didn’t realize it was that easy. Charge controllers aren’t that expensive either.
My off-road camper build on an M116A3 military chassis:
http://www.tnttt.com/viewtopic.php?f=50&t=62581
Tow vehicle: 2008 Jeep Liberty with a 4 inch lift.
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Re: Adding an auxiliary solar panel in series?

Postby John61CT » Wed Sep 16, 2020 9:41 pm

Sorry did not emphasize enough, 1:1 ratio is the way to go, multiple panels per SC is suboptimal, and really they should all be the same model if you go that way.

Ballpark daily production in a 12V system, is ~ 150Ah into the bank, for every 500W of panelage Pmpp, in good-to-ideal conditions.
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Re: Adding an auxiliary solar panel in series?

Postby lfhoward » Wed Sep 16, 2020 10:05 pm

John61CT wrote:Sorry did not emphasize enough, 1:1 ratio is the way to go, multiple panels per SC is suboptimal, and really they should all be the same model if you go that way.

Ballpark daily production in a 12V system, is ~ 150Ah into the bank, for every 500W of panelage Pmpp, in good-to-ideal conditions.

Sorry John, I think our posts went up at the same time and didn’t see yours first.
I like this idea too. It looks like good quality high voltage panels that are available in the same size as my 140 watt are now in the 200 watt range. (58x26” panel so I would not have to change my roof mounts). Technology has certainly improved since whenever mine was built (10 years ago probably).
My off-road camper build on an M116A3 military chassis:
http://www.tnttt.com/viewtopic.php?f=50&t=62581
Tow vehicle: 2008 Jeep Liberty with a 4 inch lift.
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Re: Adding an auxiliary solar panel in series?

Postby John61CT » Wed Sep 16, 2020 10:13 pm

Yes, best strategy is roof mounts adaptable to any size, and choose the panels not only highest energy density Watts/sq ft, but also just maximizing the wattage that fits, fills every available bit of roof.

Then go to folding portable ones, can get real creative that way, seen rigs running aircon in the desert.
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Re: Adding an auxiliary solar panel in series?

Postby MickinOz » Wed Sep 16, 2020 11:51 pm

lfhoward wrote:@ saltydawg, here are the specs of my permanently mounted panel:

Image

I’d have to find something with 44 volts of open circuit voltage to match, to be able to wire them up in parallel. I’m afraid that might make it too big to handle or to store in the trailer when not in use. I am all ears though if you have one in mind. I’ve seen the 100 watt panels that are fairly common but as you said their voltage is too low.


Something suss about your panel, I think. The first thing I would be doing is checking that.
It isn't hard to check Open Circuit Voltage and Short Circuit Current with a cheap multimeter.
A good MPPT will be 90% efficient or so, so you should be able to get 9 or 10 amps if the panel is good.
This will only happen if the battery and load are demanding that many amps, but the first step is to check that your panel is still capable of delivering it.
They do age.

As stated by others, by far the best thing to do is have one matched panel and charge controller.

But, if all you can run to is another Craigslist panel, all is not lost.
As mentioned by others here, some MPPT controllers are rated on the output, not the input.
Maybe, check the specs, your 140W MPPT controller is one of these.
The other limiter would be the maximum input voltage.
So if you get, say, a 100w panel that is close to the same voltage as your existing panel, you can actually parallel them.
Somewhat inefficiently I concede, but still.
What it would definitely take to make this work is isolation diodes.
You would need to put one in each positive line from each panel, before where they join in parallel.
Sorta like check valves where two pipelines meet.
The check valves stop the water running back up the other pipe, if you get my drift.
With an Andersen plug and socket and a couple of diodes wired right, you could easily have plug in supplementary power.
I'm at work in my lunch break, tonight I will try to sketch out what I mean.
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