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AlanDavenport
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# Posted: 10 Aug 2015 11:29am
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Hello, four years ago I bought a small camp (cabin) in the Adirondacks in NY state. The cabin had a single panel on the roof and three large AT&T batteries which I suspect were salvaged from an old cell phone site. The charge controller was a brand that I have not been able to find anything about even using Google. I suspect that it is an off-brand MFM controller.
The previous owner had been ignoring the system in favor of running a generator 24/7. I bought the camp for peace and quiet so that the generator option was not an option for me.
I swapped out the unknown controller for a MPPT Genasun controller and added 12v lighting. I also added a car stereo hooked to some old stereo speakers. In addition, I am an amateur radio operator and run some two-way radios. All was well however I became concerned because that even on a good day, I was unable to completely top off the batteries. I had a slight negative power budget. This became worse during the fall when the sun dipped lower. It never illuminated the panel and I was just picking up around an amp of charge from the shadowed panel.
With this in mind I bought a second charge controller and a 100w panel, 50 feet of cable and built a portable wooden frame that allows me to move the panel to wherever in my yard the sun was shining. Even in late fall I can get this panel into direct sunlight. I now easily had a positive power budget again and easily topped off my batteries daily.
Pardon me taking so long to get to the point. Here is where it gets interesting. From what I have read the best solution would have been to hook the panels in series and let the MPPT controller do the rest however I could not do this since the resulting open circuit voltage would have exceeded my controller's rating. I opted for option number two which was to use a separate controller for each panel. The problem I had was that performance was less than I had hoped for. The problem with this solution was that as the battery voltage begins to rise one controller or the other will see this raised voltage as an indication that the battery has reached full charge and switch to float mode. The reality was the increased voltage was due to the charge coming from the other controller and not from a full battery. I was losing charge capacity.
I tried paralleling both panels into ONE controller and saw a very slight increased charge rate. Better but nowhere near as much of an increase as I should have seen.
The ultimate answer was so off-the-wall that I am still shaking my head that it worked. I paralleled EVERYTHING. I hooked both panels in parallel and took that and hooked that to BOTH charge controllers whose inputs I hooked in parallel. Wired like this each controller takes half the current. In this way I finally realized the charge rate I expected from the two panels. Here are my results below:
Unknown panel that came with the camp alone ~ 4 amps.
New 100w panel alone, best case ~ 5 amps.
Both panels, each with its own controller ~ 5.5 amps.
Both panels in parallel to ONE controller ~ 6 amps.
Everything in parallel best case ~ 9.75 amps.
I am still cringing at this solution but it is hard to argue with success. (I know, what I really need to do is get a charge controller that can handle to full open circuit voltage of both panels in series.  I will someday.)
Thanks for listening,
Al
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bobrok
Member
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# Posted: 11 Aug 2015 12:58pm
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Hey Al, I'm glad to see you on the forum.
I recently had a conversation with a solar installer person about this very setup, as I am thinking of doing something similar.
The answer I got was that the smaller, less effecient panel would 'dumb down' the system.
I'm going to share your results with her. Should be interesting.
There are some very knowledgeable solar power folks here. Let's see what they have to say.
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davestreck
Member
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# Posted: 11 Aug 2015 05:36pm
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I'm following this thread closely too. I have a similar setup: 30w panel feeding into a 10A Morningstar charge controller, 100w panel feeding into a 30A Blue Sky CC, both directly connected to the battery bank. I based my design on advice from the Solar Electricity Handbook - 2012 p.89-91, which states that multiple CCs can feed a battery bank, allowing you to mix and match panels of multiple amperages/voltages.
I hope this is the case. I have purchased a pair of 150w panels to add to the system this year. I was planning on eliminating the 30w panel, and putting the new panels on the 30A cc and the old 100w on the 10A cc. If this won't work I need to come up with a new plan ASAP. We leave for the cabin in 4 days!
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creeky
Member
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# Posted: 11 Aug 2015 06:18pm
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well. sometimes stuff works in weird ways and you can't always control or indeed affirm the variables. coughonepostercough.
btw. two controllers going into one battery bank are in parallel. but anyway.
your idea with the dual controllers is perfect. make sure that the bigger panel set on the blue sky has the correct finish voltage. that'll be the one to rely on.
and probably all you need.
man. movies. tunes. new solar panels. happy cabin days indeed.
say. if you have a 30 just taking up space 
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davestreck
Member
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# Posted: 12 Aug 2015 06:36pm - Edited by: davestreck
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Quoting: creeky man. movies. tunes. new solar panels. happy cabin days indeed.
Yup, things are coming together up there for sure. My wife insists that we can no longer call if "camping", because the cabin has gotten so comfortable.
Quoting: creeky say. if you have a 30 just taking up space
Don't worry, I still have a use for it. Its mounted up high up on the cabin, out of reach of the run-of-the-mill local thieves, and I'll use it to keep a trickle charge on the battery bank after the main panels have been hidden away for the winter:
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