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Old 04-08-2011, 16:51   #1
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Solar Voodoo ? 24v Panels in a 12v System ?

I have a pretty good background in electronics, and understand how a charge controller could be made to power a 12v system with 24v panels...

Is this being commonly done now? I know that my panel has a nominal 17v open voltage (up to 21v IIRC)... but the voltage of 24v panels would seem to make them less then ideal in a 12v system.

Has something changed WRT controllers that makes the higher voltage preferable?
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Old 04-08-2011, 17:40   #2
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Re: Solar Voodoo? 24v Panels in a 12v System ?

I'm not sure what WRT means so I might be missing your question entirely, but MPPT seems to be the missing link, yes?
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Old 04-08-2011, 17:49   #3
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Re: Solar Voodoo? 24v Panels in a 12v System ?

MPPT controllers are most efficient taking a higher voltage and reducing it to a lower voltage. Don't know how much of that is less resistance from the wires because of the higher voltage or actually magic inside the MPPT controller. It's just supposed to work that way. To get 24v, you need to hook 2 12v nominal panels in series. You'll get a much higher than 24v at max production but the controller is designed to handle it.
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Old 04-08-2011, 18:46   #4
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Re: Solar Voodoo? 24v Panels in a 12v System ?

The benefit of wiring the panels in series to get the higher voltage is that you can use smaller gauge wire for a given loss (or less loss with the same wire gauge.) An MPPT controller is needed to convert this higher voltage to an appropriate battery charging voltage. The extra efficiency from the MPPT is because it is able to load the panels at their "Maximum Power Point" voltage and convert this voltage to the battery voltage with minimum loss. It's not voodoo, but the gain over a traditional "12V" panel is perhaps 10-20%, depending on the battery state.

One issue with panels in series is that they are more vulnerable to shading issues. With two panels in parallel, the shaded panel does not prevent power from being delivered from the un-shaded one. When the panels are in series the whole string is compromised when one is shaded. "Bypass" diodes can help when there are many series-connected panels (as in rooftop systems), but probably not so much in a 24V system.
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Old 04-08-2011, 19:16   #5
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Re: Solar Voodoo? 24v Panels in a 12v System ?

Here's mine.

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It all started with 2 x 120 watt 12vdc panels. What was shipped to me from america was 2 x 150 watt 24vdc panels. It wouldn't have been that bad but I already had a 20A 12vdc solar controller..........So what to do....after getting no response from the supplier to see if they would cut me a deal on a new controller and quite a bit of research I went with a MPPT contoller as has been previously mentioned.
The key point I believe, is the maximum allowable voltage that the controller can handle and the Vmp (maximum voltage output of the panels) My panels Vmp is rated at 27vdc, my controller is rated for 70vdc max. Where I'm located my panels will deliver on average 32vdc paralleled to the MPPT controller.
All in all I'm very pleased with the mistake. Now I'm considering a voltage sensing relay to switch the panels to series and get closer to max voltage of the controller during overcast days Just a thought.

Cheers
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Old 04-08-2011, 20:59   #6
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Re: Solar Voodoo? 24v Panels in a 12v System ?

Think of a MPPT controller as an automatic transmission, except Voltage= RPM and Amperage= torque.
24Volts @ 5Amps = 120watts, but after going thru a MPPT controller, it= 14Volts@ 8.6Amps.
(Assuming 100% efficiency).

I love my BlueSky MPPT controller !
(No financial interest)
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Old 04-08-2011, 21:10   #7
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Re: Solar Voodoo? 24v Panels in a 12v System ?

Yes, I have the MMPT theory of operation... it seemed to me that the 40v open input voltage was going to be used less efficiently then a voltage closer to the charge voltage of your bank.

Question for those of you who have placed 2 12v panels in series (that were in parallel)? How have your real world 'observed' rate of charge changed?

Thanks,
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Old 05-08-2011, 15:59   #8
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Re: Solar Voodoo? 24v Panels in a 12v System ?

Quote:
Originally Posted by senormechanico View Post
Think of a MPPT controller as an automatic transmission, except Voltage= RPM and Amperage= torque.
24Volts @ 5Amps = 120watts, but after going thru a MPPT controller, it= 14Volts@ 8.6Amps.
(Assuming 100% efficiency).

I love my BlueSky MPPT controller !
(No financial interest)
Steve,

Not asking for 'stated' efficiency, but observed. Have you seen these kind of numbers?

I appreciate the replies, just trying to clarify that it does indeed work out in practice.

Thanks,
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Old 05-08-2011, 16:09   #9
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Re: Solar Voodoo? 24v Panels in a 12v System ?

Regarding series or parallel, please take a look at my post on a previous thread:
Four Solar Panels - Parallel , Series or Series-Parallel

Since all modern solar panels include multiple bypass diodes, you will generally be better off wiring in series. I have a pair of panels in series and they do handle shade well, as a panel is only partly disabled by partial shading, vs parallel which would cause the entire panel to cease output.
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Old 05-08-2011, 16:17   #10
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Re: Solar Voodoo? 24v Panels in a 12v System ?

Quote:
Originally Posted by jaredko View Post
Regarding series or parallel, please take a look at my post on a previous thread:
Four Solar Panels - Parallel , Series or Series-Parallel

Since all modern solar panels include multiple bypass diodes, you will generally be better off wiring in series. I have a pair of panels in series and they do handle shade well, as a panel is only partly disabled by partial shading, vs parallel which would cause the entire panel to cease output.
I think you have that backwards. Parallel is better if you have shade problems.

Re the numbers I get, I used to have a couple of panels (which I still have) and on a perfect day at noon with partially discharged batteries, I could get 3 amps.

I then added the 120 watt 12 volt panel and the mppt controller.
The 120 watt panel has a short circuit rating of 7.45 amps.
Add that theoretical max to my 3 amps previously and you get 10.45 amps.
With the mppt controller when my batteries are down to 75% charge, I've seen over 12.5 amps in full sun.
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Old 05-08-2011, 16:28   #11
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Re: Solar Voodoo? 24v Panels in a 12v System ?

You can't mix different panels in series, they all need to be the same. The string will be limited by the cell with the lowest current level.

I don't have it backwards, series is definitely better in most circumstances, but the popular opinion is that parallel is better. Don't know where that came from, and I'm doing my best to change it :-)
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Old 06-08-2011, 18:14   #12
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Re: Solar Voodoo? 24v Panels in a 12v System ?

If you are using an MPPT charge controller and panels in series, a panel dropping out due to shading will not impact the other panels output as long as the voltage produced by the surviving panels is still within the limits of the MPPT controller's input.

The old-school rule that you lose the whole series string if one panel is shaded comes from the pre-MPPT days. In these olden times, the series string voltage was matched to the battery's voltage. Loss of a panel would cause the voltage to drop below the batter voltage thereby losing ALL output from the string.

In practice, MPPT controllers work really well. I've seen as much as 30% boost over conventional PWM chargers. The "magic" is that panels have a very narrow voltage/current operating point where they produce max power. The MPPT charger just has a DC/DC converter that allows the panels to operate at their max power point while converting that to the battery voltage.
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Old 06-08-2011, 20:56   #13
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Re: Solar Voodoo? 24v Panels in a 12v System ?

twistedtree,

That would be true as long as all panels have bypass diodes.

Some don't.
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Old 07-08-2011, 01:55   #14
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Re: Solar Voodoo? 24v Panels in a 12v System ?

It would be quite rare to find a modern panel without at least 1 bypass diode built in. Reason being, they are designed to be used in rooftop installations. most of these installations use many panels all in one series string, and any shading without bypass diodes will eliminate all output of the system. I certainly wouldn't by a panel for a rooftop system that didn't have bypass diodes.
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Old 07-08-2011, 01:57   #15
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Re: Solar Voodoo? 24v Panels in a 12v System ?

Quote:
Originally Posted by twistedtree View Post

In practice, MPPT controllers work really well. I've seen as much as 30% boost over conventional PWM chargers.
There is very little real world information on MPPT gains and losses on boats, so your results are interesting.
A couple of questions
How did you measure the difference?
What was the average gain?
What sort of conditions gave the most gain or loss?
Thanks
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