Question about having more panels than my controller can normally use?

IvarForkbeard · December 2014

Hi everyone,

Long (long!) time lurker, but first time poster. This forum is always the first place I head to or wind up at when I have questions, and the answers are always fantastic.

I have a smallish medium setup here. I currently have a single Tristar MPPT 60 controller hooked up to three 230w panels, each in parallel to help with the very tiny bit of shading one tree that the rest of the family likes too much to let me cut down. So I've got 24v going in, each panel producing around 7amps.

My battery bank and inverter are also 24v. Surrettes and a Magnum 4024.

In November and December here, the solar output is poor. I am thinking that as the budget permits, I would like to triple the panels I have up on the roof. Go to three strings of three panels each - voltage would still stay under the Tristar's 150v rating, even when it hits 40 below for that week in February.

The thing is, that many panels would produce (back of the envelope) 21 amps @ about 100v to the charge controller, and then 63 amps @ about 30v on the output to the batteries. I assume that the Tristar would just clip that down to 60 amps (its rating)...

I'm not worried about the batteries taking 60 amps of current when charging, but will it harm the Tristar to set things up this way? My solar guy is suggesting that I should have a second controller for this many panels. I don't mind if on the sunniest days the Tristar clips off some or all of the production of one of my panels, but I don't want to be doing something that may be prematurely wearing it out or stressing it.

Thanks for your thoughts.

solarix · December 2014

Re: Question about having more panels than my controller can normally use?

Solar charge controllers easily protect themselves by just misadjusting (or de-tuning) the array power point to off-maximum thus reducing the incoming power to what it can handle. Given your 9 panels are all 230W or 2,070w total and that is only under ideal conditions and with controller losses, you will rarely lose any of your new array's productivity.

BB. · December 2014

Re: Question about having more panels than my controller can normally use?

Welcome to the forum Ivar!

I do have a question about your panels... What is their Vmp and Imp ratings?

Most 200+ watt panels have Vmp in the range of 30 volts Vmp. That is not high enough to properly recharge a 24 volt battery bank. You would normally want a Vmp-array minimum of ~35 volts to properly recharge a 24 volt battery bank (which needs ~29 volts minimum+controller and wiring voltage drops, plus hot panels have Vmp suppression too).

Basically as the panels heat up, their Vmp voltage can fall as much as ~20% of hot/sunny summer days.

So--If nothing else, changing to a 4 panel array with 2 in series and 2 in parallel would be worth the effort.

Regarding trees and shading, can you move the array elsewhere? If you go to a higher voltage array, and you already have a MPPT charge controller, moving the array could be a big help in total power. Even partial shading can really drop the available energy from the solar panels by a large amount.

-Bill

mike95490 · December 2014

Re: Question about having more panels than my controller can normally use?

My thought, if the batteries are not extremely low, and your panels are mostly aimed south, you will find that 6am - 10am charging will bring the batteries up so much, that they may not accept the full array power that late in the AM. The saving grace is that a good MPPT controller will limit power to it's spec, and perform even more limiting if it's internal temps get too hot. The Tristar MPPT 60 should be able to deal with the extra panels, in my opinion.

IvarForkbeard · December 2014

Re: Question about having more panels than my controller can normally use?

BB>

That is a great question that I have considered before. Originally I was working with 12v batteries, so it wasn't an issue. I didn't think to note the Vmp or Imp on the back of the panels before they were mounted (on the roof, so it's not really an option to move them. The shading only occurs for about an hour a day, and only for a month or two of the year before the sun is much higher than the tree).
When in sun, the panels and controller are able to get the batteries to absorption and equalization voltages, so I haven't thought it to be an issue. Looking at the Tristar log for the past month, I can see that the panels Vmp has ranged from 31-39 volts, likely depending on the outside temperatures. Hopefully you feel that that will be okay?
Of course, if I double or triple my panels, I will series them to drive the voltage up to the 100-120v range and not have to worry about it even being close.
I'm pleased that most people agree that while I may have more panels than the controller will permit current to the batteries, it won't hurt the controller... And yeah, I also think that it would only be under the most ideal of conditions that I would ever achieve output that may be in need of clipping. I wonder that by the time the sun is high and direct enough to produce full output, wouldn't the controller ALREADY be clipping the amps because the batteries will be in or near absorption?
Thanks for the thoughtful and (reaffirming) responses so far. Of course, it's always easy to be thankful for positive news.

Photowhit · December 2014

Re: Question about having more panels than my controller can normally use?

Other things to consider when ramping up your system, is the size and rate of charging, I assume you have had outsde charging sources n the past and a pretty good sized battery bank, since you have a 4024 inverter...

...and you might check the actual panel rating, I'm surprised with that high of numbers from a 230 watt panel. They are invariably 60 cell panels, you might be able to count the cells from the ground 60 cell panels will have a hard time properly charging a 24 volt system, 72 should be fine, but even then MPPT charge controller like to have a bit more room above the charging (out put) current.

Coach Dad · December 2014

Re: Question about having more panels than my controller can normally use?

Morningstar states that you can exceed the controller wattage as long as you don't exceed the max input voltage of 150V...

Go to their site and read this publication.

http://support.morningstarcorp.com/wp-content/uploads/2014/07/MPPT-Technology-Primer.pdf

BB. · December 2014

Re: Question about having more panels than my controller can normally use?

As said above, you really need to know the specifications for your panels (or at least count cells per panel and take a guess).

You can usually put 3 panels in series with Vmp~30 volts... However, with 3x 72 Cell (~36 volt Vmp) in series, that is getting high enough to be problematic for folks in colder regions (Voltage open circuit cold can exceed 140-150 volts).

-Bill

Vic · December 2014

Re: Question about having more panels than my controller can normally use?

Hi Ivar,

As has already been mentioned, over PVing your MPPT CC will often mean that the system will reach Absorb and Float. typically, earlier in the day. This can mean that the CC will experience higher input voltages, when the CC is in a voltage-regulating stage. This higher input voltage will cause almost any MPPT CC to be less efficient (higher internal losses)

But, yes, do calculate the Voc for coldest weather conditions at your location. We do run strings of three 72-cell PVs here, but our mild climate allows us to run about 10 volts under the maximum permitted Vin on our oldest (MX-60) CCs with the coldest conditions.

Most 230 STC watt PVs are 60-cells, but there may be several, now, that are 72-cells.

If you know the model number of the PVs that you are using, you should be able to find the Data Sheets online, somewhere. Counting the number of cells from a distance, if necessary, IS a great idea. Good Luck with the system improvements. Vic

Question about having more panels than my controller can normally use?

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