24V panel and 12V battery?

[timzog]

Hi,
I'm planning on adding solar to my 1980 20' Caravelle and have decided that 150 to 200 watts will be plenty for my needs. My question is the following. Could a 200+ watt 24V panel be used efficiently with a mppt controller like the Morningstar to charge a 12V battery system? I've seen several 24V panels that would fit perfectly on my roof that are ~240 watts with a peak of about 30V for less than $350. To me it looks like that would be cheaper with a $200 mppt controller than 2 120 watt 12v panels (at least $250 x 2) with a $50-$100 controller and would provide the alleged benefits of mppt for battery maintenance. Also, somewhat smaller gauge wire and connections could be used from the panel to the controller at the higher voltage. If the controller were placed close to the batteries than you would only need a short stretch of heavier gauge wire for that connection. One panel vs two also means less mounting hardware. Ay thoughts from experienced solar folks would be appreciated.
Thanks,
Tim

[RickDavis]

While this is speculation on my part since i have not used 24 volt panels I would not be inclined to do this myself. Panel wattage seems to be determined based on the max voltage the panel puts out.
For example a panel putting out 30 volts at 4 amps would be 120 watts. By the time you go through your regulator you will be down to aprox 15 volts, still at 4 amps so the practical results are 60 watts of charging power. In other words half is wasted by the regulator switching on and off and a good bit will disapate as heat.

The same logic applies to the 12 volt, really about 18 volt panels normally used.. except not as much is lost due to the lower difference between panel voltage and the voltage the charge is regulated at.

In reality the 250 watts of panels I am using as I write this will deliver a maximum of 14 amps (rarely, 10 is more common unless the air is really clear)

14 amps x 14.7 volts, where my regulator is set is slightly under 180 watts in the real world. As is typical of any product they do what the can to bump the numbers up.

At any rate, I would not go with 24 volt panels if i was buying them. If free that would be another story.

Please note I am not a solar engineer, didn't even stay at a holiday inn last night.

We are serious boondockers and have used solar for years.

Solar powered in southern FL. at the moment

[arodriguez60]

If you use a good quality MPPT controller you should do good. I have a morningstar MPPT controller and a Rogue MPPT controller. I have experimented with 12,27,and 36 volts to the rogue. It has been very efficient in charging my Batteries. What you should be careful about is not geing to high of a voltage to the controller . In the winter when the panels are cold they can produce a lot of power and overload the controller. The rogue is made for 12 - 24 volt systems . A 24 volt system is about 36 volt panels. Mppt controllers are not on off switched like your pwm controllers. They are very efficient.

[timzog]

Thanks so much for the quick responses. The Morningstar Sunsaver MPPT claims to be able to handle up to 15 amps and up to 75V max PV open circuit voltage. They claim the efficiency to be within 91 to 97% depending on output power. I believe that the mppt controllers convert the higher voltage to 12V so with an mppt controller and a 240 watt panel at 24V (10A), wouldn't it put out 20A at 12V (minus 2-10% efficiency losses)? I thought that Watts was the unit that would stay constant. This may be too much for the Sunsaver to handle actually, but there may be other controllers that could.

http://www.morningstarcorp.com/en/support/library/SSMPPT_ENG10_111.pdf

[barts]

What the MPPT controllers do is to place the optimum load on the panels for the amount of light presently striking them; this generates the most power possible from the panels under varying conditions. It is then converted (I'm not an electrical engineer but it looks like a switching power supply) to the right voltage and current for your batteries to accept the most charge possible considering their current charge level.

Whether or not the particular panels you're considering are a good match for the controller is a good question - it sounds like you're inside the specified limits - but make sure to buy sturdy panels that will handle the RV environment. The panels on my Airstream certainly lead a more interesting life than the one's on my neighbors roof with vibration, shock and routine 60 mph winds.

- Bart

[RickDavis]

Originally Posted by timzog

Thanks so much for the quick responses. The Morningstar Sunsaver MPPT claims to be able to handle up to 15 amps and up to 75V max PV open circuit voltage. They claim the efficiency to be within 91 to 97% depending on output power. I believe that the mppt controllers convert the higher voltage to 12V so with an mppt controller and a 240 watt panel at 24V (10A), wouldn't it put out 20A at 12V (minus 2-10% efficiency losses)? I thought that Watts was the unit that would stay constant. This may be too much for the Sunsaver to handle actually, but there may be other controllers that could.

http://www.morningstarcorp.com/en/support/library/SSMPPT_ENG10_111.pdf

The 24 volt panel will put out ten amps. This will not increase as you regulate the voltage downward so it affectively so the 240 watt panel affectively becomes a 120 watt panel as your battery sees it.

You are correct that 20 amps at 12 volts would be 240 watts but this is a series circuit so the current thru the rregulator will be the same as from the panel.

I am not up to date on the newer controllers so something may be different than what I am familiar with

[arodriguez60]

Here is an excerpt from the Arizona Wind And Solar site.
Using an MPPT controller with high voltage panels

The only way to get full power out of high voltage grid tie solar panels is to use an MPPT controller. See the link above for detailed into on MPPT charge controls. Since most MPPT controls can take up to 150 volts DC on the solar panel input side, you can often series two to four of the high voltage panels to reduce wire losses, or to use smaller wire. For example, with the 175 watt panel mentioned above, 2 of them in series would give you 66 volts at 7.6 amps into the MPPT controller, but the controller would convert that down to about 29 amps at 12 volts. Charge Controllers for Solar Electric Systems

[RickDavis]

Originally Posted by arodriguez60

Here is an excerpt from the Arizona Wind And Solar site.
Using an MPPT controller with high voltage panels

The only way to get full power out of high voltage grid tie solar panels is to use an MPPT controller. See the link above for detailed into on MPPT charge controls. Since most MPPT controls can take up to 150 volts DC on the solar panel input side, you can often series two to four of the high voltage panels to reduce wire losses, or to use smaller wire. For example, with the 175 watt panel mentioned above, 2 of them in series would give you 66 volts at 7.6 amps into the MPPT controller, but the controller would convert that down to about 29 amps at 12 volts. Charge Controllers for Solar Electric Systems

An interesting read.. Apparently the addition of the dc to dc converter changes the ball game and my previous statements would only be correct for the controllers previous to the MPPT. My apologies for not being up to date.

When i put the system on this trailer i purposely looked for a controller with out any of the fancy stuff and settled on a specialty concepts model that basically runs up to a certain voltage and shuts off until the voltage falls.
Nothing against newer technology, however the previously used PWM controller did generate RF noise I could hear in my Ham radio equipment. The same could be true of the MPPT. Wonder if anyone knows.

At any rate the simple controller I use must be ok even without all the fancy stuff as my exide golf cart batteries will be 9 years old in June and are still doing fine. I do monitor the system with a trimetric 20 20 meter.
Anyway, sorry for the partial misinformation

[idroba]

I also think that the MPPT charge controllers have a minimum voltage that they need, and so be sure you check that issue. One larger panel may not have a high enough voltage to make the MPPT controller work properly. Usually they use several panels in series and even the single higher voltage panel you are considering might not be high enough to make the conversion system work right. Strange, huh?

Ah, the wonderful world of electronics in 2012.... everything you thought you knew may be wrong. And I include myself in that category.

[arodriguez60]

You must be doing something right. 9 years is a long time. The goal with everything we do on our electrical systems is to get a good system.

[timzog]

Based on what I read it sounds like in principle the higher voltage panel with mppt could work. I think the savings from only one panel, less roof real estate, only 4 mounting feet, and smaller cables could easily offset the additional controller cost. Thanks for all your comments.
I'll try to put together a list of all the components each approach would require.
Tim