24v. solar panel for RV.

System · October 2007

Hi All,
I have access to a 24v. 200 watt solar panel which I would like to use on my RV which is a 12v. system. I know there are some expensive charge controllers out there $400-600 price range that will allow me to do this but i'm looking for something more economical, any suggestions??? Another question, when converting 24v to 12v will the amperage double or stay the same as when it was 24v.

I currently have 2 panels one puts out around 130w and the smaller one about 20w both are 12v. panels these were mounted on my old RV and I have a charge controller for them. In your opinion would it make sense you try and convert the 24v panel to 12v or just use the 2- 12v panels instead.

Let me know what you guys think, Thanks.

crewzer · October 2007

Re: 24v. solar panel for RV.

You might keep an eye out for the new MorningStar SunSaver MPPT controller; it’s due out imminently. Configured for a 12 V (all voltages nominal) system, it’ll handle an array rated at up to ~200 W, convert 24 V to 12 V, and I expect the street price to be ~$250 to $275. And, yes, when converting from 24 V to 12 V, the output current from this controller will be double the input current, plus whatever MPPT be benefit might be available (0% to 30%), less any conversion loss (probaly less than 2%).

Converting the 200 W 24 V module to 12 V would allow you to use all three modules (200 W, 130 W, and 20 W) on your RV.

HTH,
Jim / crewzer

mike95490 · October 2007

Re: 24v. solar panel for RV.

also:
Blue Sky SB2000E MPPT charge controller claims MPPT & PWM
http://store.solar-electric.com/sb2000.html Sale price: $212.40

crewzer · October 2007

Re: 24v. solar panel for RV.

Caution: The Blue Sky 2000 can't do 24 V to 12 V down conversion.

HTH,
Jim / crewzer

mike95490 · October 2007

Re: 24v. solar panel for RV.

crewzer wrote: »

Caution: The Blue Sky 2000 can't do 24 V to 12 V down conversion.
HTH,
Jim / crewzer

What's HTH ??

The data sheet @ http://www.blueskyenergyinc.com/pdf/SB2000Edatasheet.pdf claims a 30V max DC input, and 13 - 16V output.

[edit] OK, on paper, it looked good.

crewzer · October 2007

Re: 24v. solar panel for RV.

The data sheet @ http://www.blueskyenergyinc.com/pdf/...Edatasheet.pdf claims a 30V max DC input, and 13 - 16V output.

Mike,

No argument there. But, considering that a "12 V" module's STC Vmp and Voc are typically ~17 V and ~22 V respectively, that 2000's "30 V max DC input" spec just means that it can safely handle a "12 V" module or array.

Blue Sky's application overview file clearly indicates that the 2000E/2512i/2512iX controllers are intended for "12 V / 12 V" applications. See Note (1) in particular.

Typical STC Vmp and Voc specs for a "24 V" module" are in the ~35 V and ~44 V ranges respectively. A "24 V" module/array will kill an SB 2xxx MPPT controller.

HTH = Hope This Helps

Regards,
Jim / crewzer

System2 · October 2007

Re: 24v. solar panel for RV.

Hey guys thanks for the input and advice.

It appears my situation just got a little more complicated. When I got the specs for the panel they are as follows:
SunPower SPR-200-BLK-U
Peak Power 200W
Rated Voltage 40V
Rated Current 5A
Open Circuit Voltage 47.8V
Short Circuit Current 5.4A
Fuse Rating 15A
PTC Rating 180W
Size 61.39 x 31.42 x 1.81

The panel is a used panel that is not working at full capacity, so when tested in full sunlight it is putting out around 27v and not sure about the amps yet. I am not sure at this point if it is worth while to try and use the panel or not, any suggestions

BB. · October 2007

Re: 24v. solar panel for RV.

Technically, there may be another issue with using SunPower panels on your RV...

SunPower panels are rated for Positive Ground use and will lose upwards of 30% of their power if run with a negative ground installation (which I assume your RV has)...

I posted some technical details/links from several white papers (see this PDF file for clearest presentation)--and it may be that running at "low voltages" (one or two panels in series), that negative grounding may may reduce output a maximum of 5% or so.

It is reversible (just run the panels as positive ground)... But not really workable for an RV (don't know if the MX-60 or other brands of DC charge controllers can positive ground panels and negative ground the battery bank without letting the "magic smoke" escape).

Regarding the testing--full sunlight with an amp meter (10 amp scale) shorted across the output will give you a good idea of the health of the panel (should read around the Isc of 5.4 amps--you won't hurt the panel doing this test).

-Bill

crewzer · October 2007

Re: 24v. solar panel for RV.

Here's OutBack's application note for operating the MX60 with a positive ground:

Positive grounded systems

Telcom applications frequently require a positive grounded system. The MX60 switches the POSITIVE PV and battery leads. Keep these separate. If code allows, ground ONLY the battery positive lead in this case. Do not connect the MX60’s battery plus to the PV plus input while the MX60 is running. The OutBack HUB cannot be used in a positive grounded system.

SunPower SPR-200-BLK-U... Peak Power 200W... Rated Voltage 40V... Open Circuit Voltage 47.8V...The panel is a used panel that is not working at full capacity, so when tested in full sunlight it is putting out around 27v and not sure about the amps yet

27 V doesn't sound very encouraging... can you tell us more about your test procedure?

any suggestions

One of things I enjoyed most about our old camper was its solar energy system. It allowed us to quietly camp in quiet locations and still enjoy a few modern conveniences, such as having the Sun keep my adult beverages properly chilled. 8)

I'd be concerned about relying on a module that appears to be operating at 27 V / 40 V = ~67% of spec. If it and your other modules will meet your energy generation needs, then it may be worth a shot. But, it's a large module, and, whether it works well or not, it'll need an DC-to-DC MPPT controller to be at all useful for your 12 V system.

Since you have a couple of 12 V modules already, it might be worth considering one or two big new 12 V modules, wiring it (them) in parallel with your existing modules, and then controlling the entire array via a relatively inexpensive PWM controller from Morningstar or Xantrex.

HTH,
Jim / crewzer

24v. solar panel for RV.

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