Building a additional solar panel

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Building a additional solar panel

Postby daveesl77 » Fri Mar 17, 2017 5:34 pm

Ok, before I get yelled at, I know I could have actually purchased one for about $30 more than what I spent for just the parts for this project. While the goal was to save some money (maybe), primarily I just wanted to build one. I'd never worked with solar cells before. I'm making a 50 watt to go along with my factory built 50 watt. This was more involved than I originally thought, but it has been fun to do. Got the cells on eBay - forty, 3x6, untabbed, plus all the tabbing/connector wiring strips and a tube of flux. Actually they sent like 45, as some were broken (I knew that in advance). Had a couple of yard sale picture frames with archival glass, and picked up a 39"x78" EVA film from eBay. I have a bunch of aluminum framing that I'll build the actual frames from and already had a 30 amp charge controller in addition to the 20 amp I already have in the camper. Total cost so far is $53.

I'm building the unit to consist of 2 panels, each having 18 cells, in series. The frames will be hinged so it closes together to be a little smaller, but opens up to full size. Finished panel 1 today and should have panel 2 done tomorrow. Did a power test after encapsulation in EVA and was happy with the results. 10.2v open, 3.1 amp under load (this is an 18 cell arrangement per panel, 36 cells total). So when the two are finished I should have an open voltage of about 20v and amperage capability of about 3 amp. That matches up very close to my factory built panel.

Neither of these are permanently mounted, they are portable with a 25' feed line to the trailer. Trying to decide whether I should link the two systems in parallel and run everything down a single feed line to 1 controller, or have each of the systems (factory built and my 2 panel build) feed independent controllers (don't have a MPPT) and then have those feed the battery in parallel. My concern is that my home-built (or perhaps the factory built) will underperform the other system and thus pull it down in efficiency. What do you folks think?

Oh, and Conch Fritter's new varnish should be done this weekend. Had to stop for a few days due to temps and wind.

dave
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Re: Building a additional solar panel

Postby bdosborn » Fri Mar 17, 2017 7:51 pm

Throw some pictures up already!

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Re: Building a additional solar panel

Postby Bearbait in NM » Wed Mar 22, 2017 11:50 am

Dave,

And I thought I was nutty when I built my 70ah portable generator box last year rather than buying a factory offering. You are actually building pv cells, you are more nutty than me :thumbsup: Seriously, ain't this solar stuff a lot of fun? I am having a blast.

In my portable box, I have a sunsaver 20 amp, and a sunsaver duo. I like redundancy in general, and as I run an Arb fridge when camping now, I really like redundancy in components. Hence, two controllers give me that. One upside to your two controller thoughts.

I am now the new owner of a 6x10 cargo trailer that I am in the process of making offroad camping worthy. So I am having to make some of the same decisions as you. From what I have read, parallel panels on a pwm controller might default to the lower performing panel. You might need to actually test under duty to see if you have a real mismatch. Apparently mppt can mitigate this. Sounds like you know all of this.

I have spent a boat load of hours over at Expedition Portal in their 12 volt area, where the good folks over there have helped me a lot. I do not recollect a lot of discussion about dual controller feeds to batteries. My newby thoughts are that the charge controller is really the heart of the system, if you want to maximize solar performance, and maximize battery life. By this I mean temperature control for charging, low voltage cutoff's, and efficient conversion. Expensive batteries really like good charging profiles. I am not sure how that would work with two controllers? I guess if they were identical?

I have a cheapy controller in my new trailer at the moment, using my existing 135 watt fixed panel, and can also use my portable Goal Zero's. Going to add a second 135 watt panel, and looking hard at the Sunsaver mppt's. I saw some nifty connectors in a video the other day, that let's one easily switch the panels between series or parallel, for testing. I plan on doing so when I get my second big panel, and look forward to thoughts here about duo controllers. had not pondered that. I could swing two cheap mppt's, but not two quality mppt's, and back to my thoughts on battery life, I think I will go one quality. And my other thought is that if I go one good quality mppt, if I shop correctly it should be able to handle the new generation of batteries. Not able to afford those right now, but I did learn from my solar suitcase build that having expansion built into buying choices is a good thing.

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Re: Building a additional solar panel

Postby daveesl77 » Fri Mar 24, 2017 5:44 pm

Just finished putting together the two home-built solar panels for our new 2nd array for the camper. Waiting until tomorrow for the silicon to fully cure, will then link them together and do a full system test starting on Sunday. Tested voltage and amperage today, late afternoon. 15-17+v unloaded (depending on cloud cover and time of day). The total amount of voltage is lower than usual simply due to the fact that I could only use 34 cells instead of the normal 36. Went with slightly lower voltage in order to maintain current of 2.8 amp+. After running through the charge controller this array should put out about 40 watts/hr on an average amount of sunlight. The two arrays together will do about 90 watts @ 12v loaded (8+ amp under load). This is more charging capability during the day than my actual 120v plug-in battery charger.

I doubt I would ever build a full size panel again, but it was a fun experiment. I do plan on using left over materials to make some smaller charging units, for like USB or cell phone. I'll post photos of the units tomorrow.

Oh, and I didn't use my scrap aluminum pieces for the frame, instead went to HD and picked up two, 8' sticks of aluminum angle for $20 to make the frames. Was much easier. Brazed the corners and end-join with alumiweld, worked great. So all total I'm pretty much at break even on the cost of building vs buying a prebuilt. However, it was fun and educational.

dave
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Re: Building a additional solar panel

Postby Trebor English » Sun Mar 26, 2017 12:37 pm

There are two panels that are similar but not matched in any way. They have different open circuit voltage and different short circuit current. A thought experiment: you connect panel A to a partially charged battery and it puts out 2.1 amps at 13.7 volts. Remove panel A, connect panel B and measure 2.4 amps at 13.9 volts. Connect panel A in parallel with panel B. You measure 14 volts and 4.5 amps. At any voltage both panels contribute the current they can make at that light level. The voltage will depend on the battery.

If one panel gets total shade the illuminated panel may be heating the dark panel. Combining outputs with diodes would stop all that. The forward drop through the diode would be bad. Diodes / no diodes is a separate argument but if both panels have light no diodes are needed.
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Re: Building a additional solar panel

Postby daveesl77 » Mon Mar 27, 2017 11:51 am

Both solar arrays operating. Together I'm getting over 80 watts at load. The homemade array only brings the factory array down by 0.5 volts (unloaded) to 20.3. The way I put in my diode, even when fully closed, the homemade array only drops the factory output voltage by .8 volt unloaded and the output amperage by 0.7 amp loaded. That means any shading on any of the panels doesn't kill the system as many multi panel setups will do. The two systems can be placed up to 25 feet apart and 25 feet from the camper, or next to each other.
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