24V Panel Charging 12V System

[gavinseim]

OK new here. This might be a lame question, but I have been hunting threads here and elsewhere for over an hour and cannot find a straight answer. Maybe it's just something everyone else knows.

I have a 21ft travel trailer (photo below) with a standard 12V system using 2 group 24, 12V batteries.

I just bought one Uni Solar PVL-124. 124 watts at 24v that I plan to put it on the roof. I don't expect a ton, but we travel months out of a year and I just want a bit of power supplement so I can run the gen less when in the wild.

Initially I was looking at something like an HQRP Solar 10A Charge Power Controller / Regulator. But as far as I can tell, I have to somehow convert this 24v panel to a lower voltage to charge the 12v system. This and other controllers I can find seems to provide little info on this subject. They say they work with 24V, but not if the convert.

Bottom line. What do I need to to to start charging my RV with this system. Preferably something simple, small and not crazy expensive.

Gav

[bmet]

Welcome to the forum. The specs read that the Imp is about 4 Amps, a little low for the 'recommended' minimum 5% charge rate on a Flooded, SLA group 24 battery. You will need to invest into a MPPT controller to bump up the Current for 12V charging. I know you have your reasons for going with a flexible panel, but for about the same money you could have gotten a Kyocera 7.5 Amp, 135W panel at 12V (the Kyocera would essentially provide twice the Current while allowing for a less expensive controller).

About the HQRP charge controller you describe, I started out with that product, and it would not charge my 12V 75AH marine battery higher than 12.7 (80W panel). After replacing it with a MorningStar SunSaver 10A CC, my Group 24 battery is now maintained at 14.1 V

OK new here. This might be a lame question, but I have been hunting threads here and elsewhere for over an hour and cannot find a straight answer. Maybe it's just something everyone else knows.

I have a 21ft travel trailer (photo below) with a standard 12V system using 2 group 24, 12V batteries.

I just bought one Uni Solar PVL-124. 124 watts at 24v that I plan to put it on the roof. I don't expect a ton, but we travel months out of a year and I just want a bit of power supplement so I can run the gen less when in the wild.

Initially I was looking at something like an HQRP Solar 10A Charge Power Controller / Regulator. But as far as I can tell, I have to somehow convert this 24v panel to a lower voltage to charge the 12v system. This and other controllers I can find seems to provide little info on this subject. They say they work with 24V, but not if the convert.

Bottom line. What do I need to to to start charging my RV with this system. Preferably something simple, small and not crazy expensive.

Gav

[waynefromnscanada]

Morningstar MPPT Sunsaver controller will take that extra voltage, drop it down to what's required for your batteries and change the excess voltage into extra charging amps! It automatically finds the max power (wattage) point of your panel/s then dumps that wattage into your batteries. Example: For simplicity and ease of understanding, I'm ignoring normal efficiency losses: Panels send 96 watts (4 amps @ 24 volts) to the MPPT controller, which converts the voltage etc and sends 96 watts to charge the batteries (8 amps @ 12 volts). Of course in the real world there will be efficiency losses, and the voltages/amps will vary depending on many circumstances, but this is the general idea of how MPPT controllers work. Personally I highly recommend the Sunsaver MPPT controller for your situation. Others opinions may vary of course.
Best of luck Gav!

[gavinseim]

Thanks both of you. Bmet as you've said, I had my reasons for the flexible panel. Seemed light, thin, no glass, screws etc. Also I picked it up for about $250 on Eb which seemed fair so if I have to invest a little more it won't kill me. less is still better :)

So just to clarify (want to make sure I understand all this). I do NEED a controller that will reduce that 24 volts down so I don't screw up my batteries.

A regular (read inexpensive) controller like the HQRP, or these MornringStar units. Will not cut it? I specifically need this Morning Star 15L MPPT? Also based on the size I'm assuming this unit would leave me room for another panel should I want to expand?

So would you say the above MS MPPT the best value in a unit that will control the charge. Will any MPPT controller do that or just specific ones like this.

I did not initially realize in planning that I would be dealing with 24V and it seems a pain. I'm guessing it's done to prevent voltage drops over distance. But wondering if I would have been better off with a 12v panel. That said, I really did want to try these flexible panels.

G

[bmet]

Before purchasing a MPPT controller, I'd measure the Current coming from it (while pointing it straight at the sun) to see how close it comes to spec'd Isc (Short-Circuit Current) of 5.1 Amps. IF it does, then you would need a product like the Morning Star 15L MPPT. In the choice of MPPT charge controllers, there are many imported products with suspicious names. Most of them are PWM controllers with bad English translations. In contrast , the MS unit you've listed is an excellent choice.

My concern for your investment is the price you paid. Retail listings around the internet are much higher, I hope it isn't too good to be true. For this reason I'd want a short circuit Current check before going any further.

Thanks both of you. Bmet as you've said, I had my reasons for the flexible panel. Seemed light, thin, no glass, screws etc. Also I picked it up for about $250 on Eb which seemed fair so if I have to invest a little more it won't kill me. less is still better :)

So just to clarify (want to make sure I understand all this). I do NEED a controller that will reduce that 24 volts down so I don't screw up my batteries.

A regular (read inexpensive) controller like the HQRP, or these MornringStar units. Will not cut it? I specifically need this Morning Star 15L MPPT? Also based on the size I'm assuming this unit would leave me room for another panel should I want to expand?

So would you say the above MS MPPT the best value in a unit that will control the charge. Will any MPPT controller do that or just specific ones like this.

I did not initially realize in planning that I would be dealing with 24V and it seems a pain. I'm guessing it's done to prevent voltage drops over distance. But wondering if I would have been better off with a 12v panel. That said, I really did want to try these flexible panels.

G

[gavinseim]

This is a seller on Eb who seems to sell a lot of these. The price was low, but feedback was solid and on this item. Guessing he got a large batch of them somewhere.

By Short-Circuit Current do you just mean than checking the amp output. Checking to make sure it puts out what its supposed to?

If everything checks out I'll get one of these Morning Star 15L's. Unless someone has a recommendation for something better run the meantime. Would like something a bit less costly that would do this, but I don't want trash so if I have to pay I will.

Thanks for all the info. Helps a lot.

[bmet]

Yes. Solar panel's performance are conducted under a standardized illumination. In the real world that light level is rarely achieved. A panel's 'nameplate' rating is typically derated to about 80% of Isc. Your panel's Isc is listed as 5.1 Amps, 80% of that would be 4.01 Amps. Measuring the short-current Current of your flex-panel should be close to 4 Amps if you have good alignment to the sun on a clear day. Anything less than that might mean the panel has a bad cell.

This doesn't even address the actual Current you get under Load, or the Imp. That is listed as 4.1 Amps, 80% of that would be 3.28 A under full sun (still referencing 24 V, before MPPT controller would down-convert).

By Short-Circuit Current do you just mean than checking the amp output. Checking to make sure it puts out what its supposed to?

[mike95490]

Do you have a nameplate on the back of the panel - what are the listings for :

Voc (volts open circuit
Vmp (volts max power

If Vmp is over 18V, then you benifit from MPPT, if under 18v, not a lot of benifit, but just a little.

And measureing Isc (amps short circuit) you should just hook your ampmeter up to the + - PV terminals, and hope you get 80% of the Isc spec. (when you aim the panel at noon)

[bmet]

Voc = 42.0
Vmp = 30.0

16 ft. long? I am guessing that is made to tack to the top of your RV?

[mike95490]

Voc = 42.0
Vmp = 30.0

holy cow ! The only way you are going to get power, is to use MPPT controller. If you use a PWM, you will only get about 40% of your panel wattage.

Yes, the high voltage/low amps, lets you run long runs of thin wire, and when you place charge controller near battery, you only need a couple feet of the heavy gauge stuff.