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Old 10-23-2017, 05:57 PM   #1
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Adding a 24v panel to a 12v solar system

For 6 years I have had a Unisolar 272 watt solar system on my roof and it has served me well. As I add more technology to my RV home I'm considering adding additional wattage to my system. I bought what was called a 12v system, but I wonder if the charge controller is what makes it 12v and the panels themselves are actually 24v.

Uni-Solar went out of business, but there are still "new from old stock" 24v panels for sale on eBay. I'm looking at a 126v panel. I have 4 x 68v panels stuck to my roof. The photo shows all that I can find to identify them.

My charge controller is a SunSaver 20L. The 4 panels currently on the roof are wired to a combiner box. Two cables then go to my battery box where the charge controller lives. There is a 30 amp inline fuse on the red wire just before the 2 Interstate golf cart batteries.

So can I add the 128 watt, 24 volt panel to my system? Do I need to step down the 24v to 12v using a DC to DC converter or give this new panel it’s own charge controller? Where should I connect the new panel? At the combiner box? At the shunt?

As you can probably tell I know just enough about solar to get me into trouble. I'm also an auditory learner, so figuring this out from a solar text is almost impossible without help. I do well with step by step directions, tho.

Also, since my roof is full and the 128v panel is 18’x15.5” I’m planning to mount it to something like roof flashing and using hinges to attach the flashing at the outer edge of the roof. That way I can tilt the new panel when parked and fold it against the side of the RV while driving. Any input here appreciated as well.

We will be spending the winter at La Posa South LTVA near Quartzsite beginning about November 20th.

Thanks! For any help.
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Old 10-23-2017, 06:34 PM   #2
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I don't think that is a good idea. I say forget about 24v Unisolar panels and find other mfg 12v panels that closely match your old ones. What are the specs on the existing panels?

Here is a decent article on the subject;
http://solarpanelsvenue.com/mixing-solar-panels/
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Old 10-23-2017, 07:24 PM   #3
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There's a chart on this page that compares all of the Uni-Solar panels.

http://store.affordable-solar.com/si...0510141659.pdf

I'm not too concerned about my current panels reducing the output of the new panel as the new panel is rated higher. Would it make a difference if I put the new 138 watt panel on it's own charge controller?

There are not any 68 watt panels available anywhere. Without starting over how do I increase my solar capacity?
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Old 10-23-2017, 07:53 PM   #4
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85-100w 12v panels should be close. Your 68w panels are at 16.5v and 4.13a, so as long as you get close to that you should not loose too much.

These 85w are close.
https://www.amazon.com/ALEKO%C2%AE-8.../dp/B007YT9WHC
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Old 10-23-2017, 09:05 PM   #5
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Your charge controller is a PWM and will not be good for mixing panels of different voltages. Also, it is limited to 20A input from your panels and adding another large panel will most likely exceed the amperage capacity of the controller. Based on the pdf above, it looks like your panels should produce about 4-5A each so you are already at or near the 20A total capacity of the controller. If you want to add the larger 24v panel you will need another charge controller, preferably an MPPT one take advantage of the additional voltage that the bigger panel will produce.

All that is about the PV system, but what is your battery bank? If you have been running low on battery power recently, maybe you need more/better battery bank?
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Old 10-24-2017, 08:56 AM   #6
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Since you are near maximum current on your existing charge controller, I would go with a new 150 to 165 watt panel with a new controller. The down side of that is that it will require some additional wiring.
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Old 10-24-2017, 10:14 AM   #7
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No cheap way around it, for sure! But what I don’t spend on camp grounds in a month or 2 will pay for it. W
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Old 10-25-2017, 01:24 AM   #8
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okmunky,

You mention that you have a shunt, what brand/type of battery meter do you have???
If you have a Trimetric battery meter, I would suggest similar to what John said. Add 12V solar panels via a separate SC2030 charge controller that is made by the same company as the Trimetric and is designed to work/communicate together.
Right now Renology 100W panels are costing just slightly above $1 per watt and give you the most output per square inch for your limited roof space.
You will need to make another run for the new panels. You can run them via the combiner box, just keep the wires run separate from the current one, as in "do not hook the existing wire run and the new one".
As for hooking up, the wires should come from the panels, via the combiner box, go through a fused disconnect (us a disconnect that can run two and from charge controller. This way both sides are protected, the run from the solar to the charge controller and the run from the charge controller to the battery.
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Old 10-25-2017, 06:17 AM   #9
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You have some room to oversubscribe the controller. With flat mounted panels you lose 15-50% of max rated capacity, depending on lattitude and time of year. Also check with Morningstar as to the controllers behavior if presented with over current - it may gracefully handle it and just dump it. The larger Morningstar controllers do this.
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Old 10-25-2017, 08:18 AM   #10
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My battery monitor is a Trmetric-2030. This just keeps getting more complicated.
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Old 10-25-2017, 01:18 PM   #11
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We have a 315W and a 100 W panel on roof of Roadtrek 19'. It is all the real estate available. Son designed and fabricated system but it requires a different controller for each panel to go to 12 V nominal LFP bank.
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Old 10-25-2017, 01:35 PM   #12
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I think I would just go with an auxiliary portable panel or two with their own charge controller rather than re-configure a already working system.
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Old 10-25-2017, 03:19 PM   #13
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Quote:
Originally Posted by Motor7 View Post
I think I would just go with an auxiliary portable panel or two with their own charge controller rather than re-configure a already working system.
That is actually a good option, but the controller should be mounted in the RV fairly close to the batteries. The panels that come with the charge controller mounted on the portable panel are not effective, as there is too much voltage loss in the long thin run to the batteries.
Get one or two 100W Renology panels and when the sun angle is low or it is cloudy/shady, you could tie the portable panels right into your charge controller. It's not hard to set up a pig tale adapter where you could plug the portable panel(s) into your existing charge controller.
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Old 10-26-2017, 09:59 PM   #14
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Quote:
Originally Posted by Motor7 View Post
I think I would just go with an auxiliary portable panel or two with their own charge controller rather than re-configure a already working system.
I'll look into this option, but would move the new panel's charge controller from the back of the panel to inside and shorten or replace the cable. I agree that there's too much lost by that long, thin connection between the panel and the battery. Thanks!
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