Great, we do have a lot of land to fill. 5 acres.
nice , thanks for the lead on the panels! They have lots of selection.
we have this .... do we need another one for 10 more panels of 240w?
Rather than using more Midnight Classic charge controllers, to add panels to a Sunny Island system I suggest using Sunny Boy or other AC couple grid-tie PV inverters. If other brands than SMA, the inverters need to do "Frequency-Watts", which means they reduce power output as frequency increases.
What this will do is let Sunny Island control battery charge current to precisely what you have programmed. Your FLA batteries want a particular charge current, which helps by stirring the electrolyte. Look it up in their technical manual, but let's assume that is 0.12C:
You have two, 540 Ah batteries for 1080 Ah total. 0.12C = 130A. 130A x 50Vbat = 6500W
You'll need about 6500W just to charge batteries at this rate.
A good size system would have at least 10kW of PV. Up to 24 kW is suitable for 2x Si 6048US. For instance, three of the new model SB 7.7, or some quantity of older models. Set them for "off-grid" or "Island".
When additional power is available beyond the 6500W needed to charge at 0.12C (or when battery is near full and in absorption charge mode), Sunny Island will raise frequency above 60 Hz. Between 61 Hz and 62 Hz, Sunny Boy will linearly reduce power output from 100% to 0%. Frequency will hover between those points, so Sunny Boy will deliver exactly the power needed for battery charging plus your loads. The ramp up/down takes a few seconds when loads vary, and Sunny Island makes up the difference by momentarily sourcing/sinking power from battery.
With Midnight Classic also providing charge, Sunny Island needs to know how much it contributed, in order to manage battery SoC. I've been told these DC Solar trailers have a battery shunt. Connect that to master Sunny Island if not already connected (2-wire sense leads.) I think Midnight's negative battery cable need to go to Sunny Island side of shunt, not battery side. When Sunny Island charges battery, it also sees its own current measured by the shunt and knows not to double-count. When Midnight charges battery, Sunny Island sees measurement from shunt.
(Apparently, if a system also has data link from Midnight to Sunny Island, in that case Midnight must connect to battery side of shunt. Sunny Island learns of Midnight's current from data link, so shouldn't also see it measured by shunt.)
You can over-panel both Midnight and Sunny Boy to about 140% by having multiple PV strings, half oriented different from the other half. For instance, one string aimed at 10:00 AM sun and another string 4:00 PM sun. This gives more hours of charging with 70% as high a peak current as all oriented same way.
If you use a lithium battery, it will typically accept charge rate up to 0.5C, but only within a temperature range around 25 degrees C. Lithium battery will work best with a BMS that talks to Sunny Island such as REC. If your PV is capable of more than 0.5C, using AC coupling lets Sunny Island limit charge current, but makes the additional power available for AC loads.
If temperature gets cold, BMS should prevent charging below freezing. But even at temperatures like 5 or 10 degrees C, maximum allowed charge rate is reduced; 0.5C would cause damage. Programming a charge rate like 0.15C and setting BMS low temperature disconnect around 10 degrees C could be suitable to protect battery.
