Two arrays, one charge controller. Bad Idea?

[shaniac]

I have inherited two PV arrays and one charge controller. I am hoping to describe to you the system, and would love to hear your comments about if it can be set up in a better way to make the most of the power it generates.

The system is 24V. There are two arrays (racks) One rack has a number of 12 volt panels that run in parallel to make each pair of two 24V. Each set of two panels runs in to one BlueSky Fuse box that leads to the ONE charge controller. This allows each 24V panel to be isolated from teh others.

The second rack has a series of 24V panels of different wattages running down in to another BlueSky junction box with fuses that also leads to the ONE charge controller.

Yes, I know this is not a proper set-up, but it is what i have to start with.

The charge controller is a RVProducts Solar Boost 50 with MPPT. (up to 20 amps.)

Array one has two 12V 64W panels, two 12v 130W panels and four 53W panels.

Array two has four 24V 160W panels and two 24V 150W panels.

As stated above each array is merged in a fuse box and sent to the charge controller where the two separate copper lines are connected (to each other) at the box.

My first question is simple: Does having the panels set up this way make my wattage decrease down to the lowest common panel in the system?  AKA: Am I throwing away power?

Next if so, is there a better way to set this system up? For example, do I get a second charge controller and isolate the two arrays each in to their own charge controller? Or is there a better way to maximize power?

Thanks for your guidance on this folks.

[richhagen]

Realize that you have the equivelant of four strings of two 12V panels, and six 24V panels all connected in parallel.  

Basically, all of your 12V panels are connected two in series of matching current ratings, and all of your 24V panels are connected in parallel.  All of the parallel strings from both arrays will be pulled down to the same voltage where they are tied together at the input of the charge controller.  The charge controller should adjust for the maximum power from this set of panels as a group.  

If the controller will take a large range of input voltage, you could configure the system with a higher voltage input, potentially reducing wiring resistance losses,  but with your hodge podge of mixed panels, matching panels by current for 48V in series would be tough, and would waste power because of the current mismatch.  

You could add a second charge controller and split the panels into two arrays with the higher open circuit voltage 24V panels or strings on one, and the lowest open circuit voltage 24V strings or panels on the other and it would likely marginally improve your performance, however if you haven't reached the current/power ratings of you controller, using the money to add another 24V panel would likely result in more energy output.    Rich

[shaniac]

Thanks so much for your response, this is very helpful and I hope someday will help someone else aswell.

So if I understand you correctly...

If my 12V array is: (Three sets of series in parallel.)

2. x Solarex64s 21.3 OCV (in series)
   
2. x KC130s 21.9 OCV (in series)
   
2. x SiemensM55s 21.7 OCV (in series) then all connecting to one fuse box makes it in parallel.
   
   

And my 24V array is (all in parallel)

4. x BP160s 44.2 OCV
   
2. x BP150s 43.5 OCV
   
   

Then in effect my 12 volt array is only putting out the lowest voltage x 2. Which would be 42.6.

And since the 12V panels and the 24V are wired together before the MPPT charge controller, then the whole system is being dragged down to the minimum 42.6V.

Yes I understand this is all in theory and if my oldest panels are actually producing less than 42.6V it looks even worse.

So I get that, now my next question is, (presuming I did understand you correctly) does the wattage get dragged down as well? AKA Are my 160W panels only producing as much as my 55s? Or is wattage not related in that way?

Thanks again for all you help. I believe a direction for a solution is starting to take shape.

[commanda]

My quick math of your total panel power is 1540 watts. At 24 volts that's 64 amps. How many amps did you say the charge controller was?

Amanda

[electronbaby]

Yes, the wattage is getting dragged down.(P=ExI)The controller will only sweep the max power point to the power point indicated by your lowest voltage module. If 2 kc130's in series give you 24v nominal, you have to go by the MPP of those mods. Most PV mods have the max power point published. It is commonly 70%-80% of the OCV. It is either on the cut sheet, or on the back of the modules (sticker). If you have series mods, double the max power voltage (that gives you the series MPP.

Compare that with your 24v mods. Whichever is lower, thats where your charge controller will settle. There is definately going to be a slight mismatch. You have to weigh it with the cost of an additional controller. You said you "inherited" them. It might not be worth the extra cost. Only you will know that.

Check all mods for the correct OCV. You might find the solarex to be the weakest. They are most likely the oldest. Depends on how much sun they have seen. It might be fine though.

either way, your power into the batteries will still be better than if you were to use a PWM controller.

[commanda]

Solar panels are essentially current generating devices. The voltage is more or less incidental. Groups of panels wired in parallel, the currents from each string will add together. So the 55 watt panels will not suddenly cause your 160 watt panels to only make 55 watts. So long as each pair of 12 volt panels in series are the same wattage.

Putting different panels in series, the current will not exceed that of the lower wattage panel. So don't do this.

Voc is one thing. What you really should be more interested in is Vmp. At what terminal voltage will the panel generate maximum power? In your case, you will find that the MPPT will find the best compromise to extract maximum power out of your array as a whole. And since solar panels are more akin to current sources, the power will not vary a great deal by forcing all the panels to one particular Vmp.

Basically, I wouldn't lose any sleep over it as is. Except for the current rating of that mppt controller.

Most solar panel manufacturers provide a chart of output voltage vs current. A quick google search and study any chart should make it a bit clearer (the general relationship between volts and amps, that is).

Amanda

[shaniac]

Yikes, I did say it was 20 AMPs but I must have been having a brain fart as the RV solar Boost 50 is 50A. Here are the specs from the manual.

SPECIFICATIONS

Output current rating.................... 50A

System voltage ............................ 12/24V nominal

Max. PV Open circuit voltage ...... 57V

Max. battery voltage..................... 57V

Output current limit ...................... 50±2A

Volt meter full scale range........... 60.0V

Volt meter accuracy..................... ±0.3% full-scale

Current meter full scale range..... ±60A

Current meter accuracy............... ±0.5% full-scale

Acceptance voltage ..................... 13-16/26-32V typical

Float voltage................................. 0-2/0-4V <Accept.

Equalize voltage........................... 1V/2V >Accept.

Power conversion efficiency........ 97% typical @ 40A

But you are right, it seems my charge controller is a bit underpowered for my arrays. Anyone have a charge controller they don't need anymore? Looks like I may be in the market.

[Volvo farmer]

Yikes! Your charge controller is not underpowered, it is overpowered. I cannot believe that a charge controller rated for 50A could consistently be powered at over 60A without the magic smoke being released.

You have quite the mixmash of panels there. As much as off-griders try and match stuff, it seems like things get harder to match as years go by.

You got money to spend on this? $500? Maybe get an MX60 for the higher powered 24V panels wired at 48V and let the Solaboost do it's questionable thing at 24V for the rest. MPPT technology undoubtedly works much better, and is more economical, when the panel voltage is higher than the battery voltage.

I've got two charge controllers on my system. A MX60 on the serious 170W commercial panels 24V, eight of them, tracking  at 48v into 24V batteries. Also have a C40 on a 24V array of fixed homebrewed solar panels that saw a whole 250W today. 10A @ 25V  max.

I've learned a little about letting charge controllers play nicely together. You need a master and a slave. If batteries are low, both controllers put maximum amps into batteries. If batteries are near absorb/float stage though, you've got to let one controller be smart and the other be dumb. My 250W array on the dumb controller is the slave to the MX60 in my case, it drops out after a shorter period of time (by default) and I've set the voltage slightly lower as well.

So my concerns with you system are: 24v nominal panel voltage @ 24V batteries, not much MPPT advantage going on there. 60+ amps rated panel amperage into a 50 amp controller? What's up with that? You got a trimetric? or similar amp/hour meter to tell you what's going on there?

[electronbaby]

70% of 1540 watts is only 45 amps. I doubt you are seeing 1000w/m^2 for long durations, but i might be wrong. not sure what latitude your in. It would probably work fine as long as alot of snow does not pile up in front of the array (reflection).  It would be wise to invest in an MX. To meet NEC requirements and all local codes (for the US) (also for the longevity of your system) get a controller that can handle more current safely.

very rare PV modules put out their nameplate wattages, but it does happen.

[richhagen]

Commanda, of course you are right about the Voltage at maximum power being more important, but I figured it would most likely be a relatively similar percentage of the open circuit voltage for panels of similar design type, and that the open circuit voltage could be more easily determined by a simple measurement with a meter.  Rich

[commanda]

In practice, I would expect that line resistance would tend to over-ride and even out any small differences between the panels anyway.

Amanda

[ghurd]

I don't see any real problem with it.  It works.

RVProducts have not been made in a couple years, so it must be up to the task or it would have smoked by now!

I would use a MX60 if the panels matched at all, but I doubt it will have a whole lot of gain over what it's doing now.

Volvo's idea sounds pretty good, but I'm not sure about the cost:gain.

I wouldn't worry about the OCV.

The Imp and Vmp are more important for MPPT.

It sounds like whoever put it together did the best they could do with what they had to work with.

It could be a LOT worse!

G-

[erne]

If you haven't mounted them in the air your best bet would be as listed below

2. -12v 64  watt
   
2. -12v 130 watt  
   
4. -12v 53  watt                  
   
   

               place in series 1-64 watt &1-130 watt & 2 53 watt = 48 volts @ 300 watts, parallel for 2 strings = 6oo watts total

2. -24v 160 watt
   
2. 24v 150 watt                                                    
   
   

       1-64 watt&1 150 watt= 48volts @ 310 watts, parallel for 2 strings @ 610watts

you can add each of these together and run @ 96 volts in a MX 60 1 - 310 watt & 1 - 300 watt = 610 watts divided by 24 volts = 25.4 amps times 2. Amperage out is the determining factor in mppt's. you could drop the 4-53 watt panels and be in range. It is best to stay in the 70% range of the controllers for maximum return out of the controllers.

erne

[wooferhound]

your math isn't adding up right

if you put a high watt panel in series with a low watt panel

you will be limited by the power of the smaller panel

a 130 watt panel in series with a 50 watt panels is the same thing as

putting two 50 watt panels in series

[shaniac]

Thanks so much for the advice. Here are the VMP readings for all the panels.

My 12V array is: (Three sets of series in parallel.)

   2. x Solarex64s (21.3 OCV) 17.5V VMP (in series)

   2. x KC130s (21.9 OCV) 17.6V VMP (in series)

   2. x SiemensM55s (21.7 OCV) 17.4V VMP (in series) then all connecting to one fuse box makes it in parallel.

And my 24V array is (all in parallel)

   4. x BP160s (44.2 OCV) 35.1V VMP

   2. x BP150s (43.5 OCV) 34.5V VMP

So it looks like I may fall just under 50A (I still need to look up how to do the math.) But it seems if I need to add another panel, I will more than likely let the magic smoke go on an extended vacation from Charge Controller land.

So it seems the preferred Charge Controller is the MX60 in this group. Thanks. Do I have the $500 plus for one right now? No, but it is better than being without power.

Theses arrays power my home. It is off grid, but it is too risky to screw up with overloading my MPPT box.

To answer the location questions, It is Northern California, snow comes once every 4 years if ever.

So if the numbers pan out, I need a bigger charge controller or I need a second used RV solar Boost 50 and play the slave and master game.

Looking up how to turn Volts to Amps now. Thanks again folks, you guys are invaluable.

[shaniac]

Sorry Volvo Farmer, when I said the charger was underpowered, I meant to imply it can not handle enough power for the task at hand. Thanks for clearing that up.

[shaniac]

Whoops, I just realized I messed up on the VMP for the 12V array, since the panels are in series, I need to double the numbers, so the correct numbers we would be looking at are:

12V array is: (Three sets of series in parallel.)

   2. x Solarex64s 35 VMP (in series)

   2. x KC130s 35.2 VMP (in series)

   2. x SiemensM55s 34.8 VMP (in series)

So since the 24V BP 150s are only at 34.5 VMP, it looks like my MPPT will settle at that. Is that correct?

Thanks again.

[shaniac]

VF asked: Yikes! Your charge controller is not underpowered, it is overpowered. I cannot believe that a charge controller rated for 50A could consistently be powered at over 60A without the magic smoke being released.

Some questions just stick in your mind and do not let you sleep until they have been batten down. Here is why I still have my magic smoke. In short, the unit is rated at 125 percent of it capacity. 62.5 Amps.

(snip from my Solar Boost Manual)

To be compliant with National Electrical Code (NEC), solar module controllers should be sized to provide 125%

more rated capacity than the 25°C short circuit current rating of the PV modules, and then all components be sized

another 125% greater. However, Solar Boost 50 can regularly produce more than 40 amps of output current from 40

amps of input current. The NEC does not cover MPPT type controllers with electronic current limit, but bases the rules

on older less sophisticated controllers which are unable to manage conditions of higher than expected current. Our

interpretation of the spirit of the NEC is that Solar Boost 50 connected to 40 amps of short circuit PV current, and

branch circuit protection of 60 amps is compliant with the spirit of NEC requirements. This is because the system is

rated for and electronically limited to 50 amps of output current. With wiring and branch circuit protection then rated at

60 amps, the 125% margin is always maintained. The final judgement of acceptability is to be made by the local

authority with jurisdiction.

(usnnip)

To sum up, I was lucky! Safeguards placed there to protect morons, protected this moron. I need a bigger charge controller. Period. You guys may have just prevented a fire, have certainly saved me form a BIG problem, and have really pissed off a bit of magic smoke that was looking forward to a holiday. : )

[erne]

Woof

OOH my gosh. I checked with S.E.I. and you are right. I sure didn't know that thanks for the info.... guss there is always somthing new to learn here.....erne