Sizing panel arrays for FlexMax 80 Charge Controllers

richardimorse · November 2018

HI after analysing the logs for an existing system being upgraded, here the max supported for a 48V battery is stated at 4000Wp but in reality the actual Wp from the two combined arrays on 2 separate controllers is as follows:

October = 4000+2 % = 4080 Wp for 3 days and 4000+7% = 4280Wp for 3 days, the rest is below 4000
September = 3,500 Wp
August = 3,800 Wp
July = 3,600 Wp with 3 days at 4,400 Wp = 4000+10% and 3 days at 4080 Wp = 4000+2%

The arrays have 4725 Wp in total, so my thinking was to remove one string and have it at 4200 Wp STC to prevent the high actuals
which should leave it at a max of around +3% on a few days a year, but then I saw the manual entry which talks about giving 15% design headroom which is 3400 Wp

I understand that Outback want to sell more charge controllers, but is this really necessary when the seasonality of azimuth and zenith will reduce the figure even further from 3,400 Wp to 3,000 Wp which brings me onto my question.

Q) If the current limit function in the FlexMax 80 works why is it necessary to reduce the capacity of the controller like this.

if i size at 4,200, The Wp figure of 4,000 will exceeded for 1 hour a day 20 times a year by max 2% and then the input and output circuit breakers as the next line of defence as well as the 80Amp current limit SETTING = 3,840 WATT to prevent it from being overloaded

Using the log figures recorded above for October to July reducing from 27 to 24 panels would reduce Wp by a multiple of 0.8 recurring
this would bring all the figures to below 4,000 am I missing something here - am I going to blow up my FlexMax 80 charge controller or drastically reduce its life expectancy with 4,200 Wp on a 48 Volt system and a single controller when a tracker is not being used.

Estragon · November 2018

IMHO, running a controller at max output for long periods is likely to make it run hotter than it otherwise would, and shorten its life expectancy. How much shorter depends on ambient temp, dust, altitude, etc., but my thinking is better to have two controllers run at 1/2 capacity than one maxed out. This also has the advantage of a degree of redundancy, leaving a working controller in place so a failed one can be repaired or replaced at a convenient pace.

AFAIK the current limit works, so you shouldn't blow it up with the small excess you have.

BB. · November 2018

What is the temperature in the room where the controllers are installed?

Thermal cycling (running from 0C to 40C) is a killer (can cause components to pop off the board, early life failure for fans, etc.).

And for every 10C over ~25C, life is cut by 1/2 (i.e., 35C, the controller will age 2x faster).

Try to keep the room temperature low and avoid thermal extremes (as best you can).

If the room is consistently >35C, then you probably would want to run the controllers as less than 100% rated current (run the systems cooler) for longer life.

-Bill

richardimorse · November 2018

Thanks, I am attempting to get them to buy into a third controller and array as part of a 20' container shipment done once, instead of having to upgrade at a later date, on the old array sizing for this system, I think the old owners made a sizing error when they decommissioned two large single axis trackers due to wind and put the panels on the ground, trackers give you a 25% boost

If you decommission you need 25% more panels

Hindsight is a great thing, how many people would focus on making sure that wind doesn't break the panels and forget to upgrade the number of panels due to the site visit access costs and difficulty of getting to the remote location, they didn't even harvest the trackers, left them on the ground for 8 years to rust and rot away, they are still on the ground like many wind turbine relics that were also decommissioned due to excessive wind.

Photowhit · November 2018

I take it this is for a grid tied system... Off grid systems So rarely reach/use the max in well designed systems, I'd gladly over panel them 30+ percent for cloudy days if inexpensive panels are available. This is in the beginners corner, perhaps a bit more explanation of the system and moving it to the advanced sections.

mcgivor · November 2018

Running controllers at, or near maximize capacity, can result in unwanted temperatures which may prompt the controller to reduce it's output capacity, a means of self preservation, the Schneider MPPT 60-150 begins this at 40°C, others may do the same, read the specifications, for your particular controller.

Vic · November 2018

Hi Richard..,

There are several aspects to "over-PVing" MPPT Charge Controllers (CCs).

One of these has to do with the CC's ability to quickly Limit the output current. Some have said that the OB legacy CCs (like the FMs), are indeed a bit slow to limit current. Total PV String Isc for these OB CCs is a factor in the safety margin, as is the String Vmp -- higher values of these, yield less margin.

Another factor, is that, when an MPPT CC Limits its output current, the PV string voltage rises, due to less loading on the PVs. This usually results in a reduction (relatively small) in CC efficiency, which results in additional CC heating.

The FM CCs have a maximum rated total string Isc of 80% of maximum rated output (48 A total EDIT-64 A total for FM80s). On a 48 V system, It is very difficult to exceed this current rating, unless the CC is seriously over PVed, but good to keep an eye on this, if the String Sizer is not checking this.

As has been stated, in general is it not a good idea to run Power Electronics at or near rated power levels. CCs (as you well know) often run for a number of hours at high power levels, which can result in a shorter life, especially when over-PVed (IMO).

FWIW, Vic

Sizing panel arrays for FlexMax 80 Charge Controllers

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