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Old 02-05-2017, 22:13   #1
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Connect solar panels of different ratings

Hi Cruisers!

A friend recently gifted me a pair of 12V 50w solar panels after I complained that my 12V 140W solar panel just wasn't giving me enough power to cover my needs. With his addition I would expect to up the wattage to 12V 240W and hopefully get closer to 20 amps instead of 10 amps going to the battery.

In real world conditions I get about 6-8 amps from the 140W panel so maybe the extra panels could push that up to 12 to 15 amps?

The panels are all flexible (Monocrystalline silicone)but have different ratings. I currently have an mppt charge controller, but it is rated for 10A, So I know I'll have to change this.

My question is: can I just buy an mppt charge controller which is rated at 20A or even 30A (in case I want to add a fourth panel), AND Is it a simply just a matter of wiring all panels in parallel to a branch connector?
(i.e. 3 in 1 out on both the + and -)?

This is the spec from the 140Watt panel:
Rated Power (+/- 5%) - Pmax 140 W
Open Circuit Voltage - Voc 29.5 V
short circuit current - Isc 5.8 A
Maximum Power Voltage (Vpm) 24.9 V
Maximum Power Current (Imp) 5.5 A

These are the specs from the 2 50W panels
Rated Power Output: 50W
Open Circuit Voltage [Voc]: 21.2V
Short Circuit Current [Isc]: 3.05A
Optimum Operating Voltage [Vmp]: 17.6V
Optimum Operating Current [Imp]: 2.84A

This is the 30A Mppt charge controller I would like to get:from Amazon

Any solar gurus who can give me some advice?

Thanks!
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Old 03-05-2017, 01:54   #2
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Re: Connect solar panels of different ratings

Alternatively have 2 x MPPT controllers, especially as your 140w solar panel has quite a high voltage for a 12v panel, so the MPPT will be working well converting it down.

also I doubt adding 100w of solar to your 140w will give you 12-15 amps, more like 10 amps in the real world, so you could just use the existing MPPT and accept that there might be a slight loss in ideal conditions if the panels exceeded 10 amps slightly.

Pete
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Old 03-05-2017, 02:42   #3
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Re: Connect solar panels of different ratings

Given the different in Vmp, I'd second Pete7's suggestion of a second controller for the 50W panels. I also gives you redundancy.
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Old 03-05-2017, 05:45   #4
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Re: Connect solar panels of different ratings

There is a lot to be said for redundancy particularly if you run an automatic bilge pump from the battery and the boat is unattended for any length of time.
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Old 03-05-2017, 07:25   #5
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Re: Connect solar panels of different ratings

Thanks for the responses!

The trouble with adding a second mppt controller is running a second set of cables. That's another 10m of cabling, a through hull gasket, fuses, and mounting location for the controller.

I get the redundancy philosophy and it is absolutely valid, however, I would like to know if the voltage or amperage rating of different panels would "interfere" with each other. Would a lower rated voltage pull the whole system voltage down? Could the higher rated amperage of one panel cause overheating of the other? Are there any dangers to consider?

Thanks!
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Old 03-05-2017, 11:30   #6
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pirate Re: Connect solar panels of different ratings

Hello everyone, I have been told by the Solbian folks that hooking up two solar panels to one controller is not a good idea because shadowing of one panel disrupts the output of both panels in parallel. Again, they told me that and I repeat it for whatever its worth. I would welcome some criticism from the more savy out there.
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Old 03-05-2017, 11:50   #7
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Re: Connect solar panels of different ratings

Quote:
Originally Posted by sailon46 View Post
Hello everyone, I have been told by the Solbian folks that hooking up two solar panels to one controller is not a good idea because shadowing of one panel disrupts the output of both panels in parallel. Again, they told me that and I repeat it for whatever its worth. I would welcome some criticism from the more savy out there.
Ernie on the Mary Jane
Well, IMHO that is wrong. With parallel panels if one is shaded then the other will provide full charge. If the panels are mounted in series then shading on one panel will affect the output of both panels. However, in series with twice the voltage then the MPPT controller has the ability to maximise that for multi stage charging of the batteries. So there is no clear right or wrong way to wire two panels together.

Pete
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Old 03-05-2017, 11:51   #8
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Re: Connect solar panels of different ratings

Quote:
Originally Posted by sailon46 View Post
Hello everyone, I have been told by the Solbian folks that hooking up two solar panels to one controller is not a good idea because shadowing of one panel disrupts the output of both panels in parallel. Again, they told me that and I repeat it for whatever its worth. I would welcome some criticism from the more savy out there.
Ernie on the Mary Jane
Hum, Panels in serial would see a drop in both panels, if the first panel does not have bypass diode's. With parallel you loose the first panel but the second panel if unshaded will still give it's output.

Just a note, that the 10, 20, 30 amp controllers are generally nominal. That is the Mosfets generally used inside are all rated for 20-40 amps (or higher). The issue is how much heat sink does the mosfet have. With a 10 amp controller, not much. I will also note that I ran a 130 and 100 watt panels with a 10 amp controller for years. In real world your only going to get 11 ish amps out anyway on a good day, which is generally well within a 10 amp controller working range.
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Old 04-05-2017, 09:31   #9
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Re: Connect solar panels of different ratings

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Originally Posted by mangojam42 View Post
Thanks for the responses!

The trouble with adding a second mppt controller is running a second set of cables. That's another 10m of cabling, a through hull gasket, fuses, and mounting location for the controller.

I get the redundancy philosophy and it is absolutely valid, however, I would like to know if the voltage or amperage rating of different panels would "interfere" with each other. Would a lower rated voltage pull the whole system voltage down? Could the higher rated amperage of one panel cause overheating of the other? Are there any dangers to consider?

Thanks!

It is not unsafe as long as the controller and wires can handle total current. But you will have reduced charging then having 2 controllers. Each letting their panels run at its best power voltage. Rather then then both trying to find equal ground to run at.

We run one panel per controller. Even with same panels
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Old 04-05-2017, 09:54   #10
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Re: Connect solar panels of different ratings

With your (almost) 24V, 140W panel and 12V 50W panels wired in parallel, the 24V panel will effectively reverse-bias the 12V panels. The 12V panels will deliver no power at all. I don't know if the high reverse voltage on the 12V panels will cause damage, but it's possible.

The MPPT controller will not load down the 24V panel to the point where the 12V panels are putting out any power because the the maximum power point of the panel combination is well above that voltage.

You may be able to make this work by wiring the two 12V panels in series, and then wiring that series combo in parallel with the larger 24V panel. It looks like your proposed MPPT controller will handle the voltage. This will not be as efficient as using two MPPT controllers, and as there may be two MPPT points separated by a local minima I can't say how well the controller will actually function.
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