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

Re: 24V Panel Charging 12V System

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

gavinseim wrote: »

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

Re: 24V Panel Charging 12V System

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.:p
Best of luck Gav!

gavinseim

Re: 24V Panel Charging 12V System

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

Re: 24V Panel Charging 12V System

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.

gavinseim wrote: »

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

Re: 24V Panel Charging 12V System

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

Re: 24V Panel Charging 12V System

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).

gavinseim wrote: »

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

Re: 24V Panel Charging 12V System

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

Re: 24V Panel Charging 12V System

Voc = 42.0
Vmp = 30.0

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

mike95490

Re: 24V Panel Charging 12V System

bmet wrote: »

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.

gavinseim

Re: 24V Panel Charging 12V System

Thanks everyone. Great info. Ya it should fit nicely right on the roof. Excited to try it out.

niel

Re: 24V Panel Charging 12V System

another cc option is this,
http://www.roguepowertech.com/products/mpt3024.htm
but it's doubtful you'll need to go to 30a out. you will find that in downconverting the voltage with an mppt controller that the current will go up. you won't quite double your current, but after efficiency factors you could get to to about 1.5x the current if at full sun (1000w/m^2).

gavinseim

Re: 24V Panel Charging 12V System

Ya that looks pretty sweet, Though probably overkill in this small camper. Space is also an issue.

waynefromnscanada

Re: 24V Panel Charging 12V System

niel wrote: »

another cc option is this,
http://www.roguepowertech.com/products/mpt3024.htm
but it's doubtful you'll need to go to 30a out. .

But definitely worth considering if there is the slightest possibility that you may, like most of us have, get addicted to solar and later add more PV, perhaps ground mounted.
There now, aren't you sorry you found this form? :p

niel

Re: 24V Panel Charging 12V System

gavinseim wrote: »

Ya that looks pretty sweet, Though probably overkill in this small camper. Space is also an issue.

no problem as i'm only giving you another option.

be careful that the premium space that you refer too doesn't interfere with the controller's ability to dissipate heat.

gavinseim

Re: 24V Panel Charging 12V System

Hey all. Another question. I received the panel and it looks good. Voltage in sun comes in at about 39v which is just about right.

What I'm a bit confused on is the the short circuit test. If I put an ohmmeter directly to the pos and neg leads, I don't get a proper max amp reading because there's not enough current draw. Are you saying to actually run a separate wire between the panel outputs (truly shorting the connections) and then placing the meter leads inline with that short, so as to measure the amps being drawn? Won't that damage the panel?

What am I missing here.

niel

Re: 24V Panel Charging 12V System

oh oh. not ohm meter. ammeter. it must read amps rated to at least the isc current. you could damage your meter trying to read amps in the ohms setting. see if the meter is working and if not check to see if it has a fuse internally protecting it.

when you use the current meter (properly known as an ammeter) it is acting as a short circuit to the pv and will pass lots of current, but it must be in full sun to give a good current output. you aren't hurting the pv at all.

gavinseim

Re: 24V Panel Charging 12V System

Excuse me not an ohmmeter (that would be resistance I believe). I did mean an ammeter. I was in amps mode on the meter. But in all previous experiences and research with how to use an ammeter, it was necessary to have a load (ie a light bulb of resistor) with the meter in between. Not just use the leads. If I just place leads on +/- I'm only getting about 1.2 amps. But there' no load, so I don't know how that can be accurate.

Cariboocoot

Re: 24V Panel Charging 12V System

gavinseim wrote: »

Excuse me not an ohmmeter (that would be resistance I believe). I did mean an ammeter. I was in amps mode on the meter. But in all previous experiences and research with how to use an ammeter, it was necessary to have a load (ie a light bulb of resistor) with the meter in between. Not just use the leads. If I just place leads on +/- I'm only getting about 1.2 amps. But there' no load, so I don't know how that can be accurate.

Funny things, photovoltaic panels.
Current source, y'know.
Do as Niel says: set the meter to 10 Amps, hook one lead to the positive of the panel and the other to the negative. No loads; you're measuring short circuit current and the short is provided by the meter.

Put the panel in the sun. See the reading? Change the panel angle; watch the reading change! Outside in midday full sun you can get anywhere from a few mA to near full Isc rating according to how "square on" to the sun the panel is.

When you use an Ammeter with a load, you're measuring the current draw of that load. This is normal procedure for a Voltage-based system such as a battery. Never try to read short circuit current of a battery! It's instant fireworks to say the least.

waynefromnscanada

Re: 24V Panel Charging 12V System

gavinseim wrote: »

Hey all. Another question. I received the panel and it looks good. Voltage in sun comes in at about 39v which is just about right.

What I'm a bit confused on is the the short circuit test. If I put an ohmmeter directly to the pos and neg leads, I don't get a proper max amp reading because there's not enough current draw. Are you saying to actually run a separate wire between the panel outputs (truly shorting the connections) and then placing the meter leads inline with that short, so as to measure the amps being drawn? Won't that damage the panel?

What am I missing here.

Hi Gav, others have answered your questions perfectly, but I'll try to answer using somewhat different wording that may help you to more easily grasp the ideas.
Re the short circuit test. I assume what you actually have is a "multimeter", capable of measuring resistance (ohms); voltage, and current (amps).
A couple of cautions:
NEVER try to measure the resistance in any circuit which is energized (is connected to a power source) otherwise your multimeter will smoke and if you're lucky, the ohms function will be the only section of the meter that will be destroyed.
NEVER try to measure the current (amps) of any source capable of providing more current than your meter is designed to handling, otherwise your meter will smoke, or if you're lucky, your meter will have an internal fuse, which will blow and hopefully quick enough to save your meter. Your battery pack would be an example of a current source which would blow your meter, and for these purposes, you can consider your battery pack as a source of unlimited amperage, and putting a short circuit on that battery pack, would result in sever damage to something, or someone,, including your battery pack.
Solar panels on the other hand, can only supply so much current and doing so will NOT damage them. Therefore you can set your multi-meter to DC AMPS, connect it's leads to the + and - of the solar panel, and directly read the amperage with no damage to your panels whatever. The current will flow from one of the panels terminals, through the meter, then back to the other terminal, showing up on the meter face as amperage, all the while causing no harm whatever to your panels. And you can think of that amperage, not as being "drawn" as from a battery, but "produced" by the solar panel.
Hopefully this is of some help.

BB.

Re: 24V Panel Charging 12V System

Sears sells a decent/cheap DC Current Clamp Meter for ~$60.

It is about the cheapest/workable DC current clamp meter I have seen... Plus it has the usual volts/ohms and some other functions too (including AC current clamp).

Note that DC clamp meters are not "stable" and will drift over a few minutes and need re-zeroing--especially if measuring lower currents (uses an H-Field transistor to measure the DC current field strength).

But, DC clamp meters are just about impossible to use incorrectly. Instead of setting the meter to DC scale, breaking the circuit to put meter leads into the current path--and possible short circuits... The DC clamp just goes over the wire you want to measure the current in. Safe, no exposed connections, etc.

-Bill

soleil

Re: 24V Panel Charging 12V System

Did you take that picture of your trailer? That's a great photo.