The OP said it was a Renogy Rover 100A if that helps.
That helps, thank you! I think I have a clue as to what is going on. The installer put them all in parallel, used cheap thin wire, and is running the cabling too far from the array to the controller. That's why the voltage would be ~12.
Now, PNM, exactly how are you reading the voltage? Are you using a voltmeter, or are you reading the voltage on the controller? If reading the controller, it will be displaying the voltage the battery is charging at, not the voltage of the solar panel itself. To do that, you need to put the probe terminals of a voltmeter directly on the PV in wires. The MPPT controller then transforms the higher raw solar voltage down to battery charging voltage.
If this was a system I was operating, I'd go out and buy one more additional panel, then change the wiring to 4S3P. That means three parallel strings of four panels in series. The voltage going into the controller would be ~72Vmp.
If you bought a 12th panel and tried wiring them 6S2P, the resulting voltage might get dangerously close to 150V in below-freezing weather. Assuming your panels have a Voc of ~21V, then six in series would be 126Voc. But, that is at room temperature. Right at freezing, the Voc will be ~141V, and ~158Voc at -40 degrees. How cold do your winter lows go.
If you can't buy a 12th panel, you could re-wire the panels to 5S2P, setting panel 11 aside.
Lastly, contact the battery manufacturer, an ask specifically what the optimal charging voltages should be set at. You then go into your controller setup menu, and change the charge settings to what the manufacturer wants.
When it comes time to actually test your system to see what the maximal output is, you need to run a load that is about as big as your whole array. Let's say you have 1200W total. Turning on a 1500W toaster oven would an excellent load to see what your maximal output is. If you don't have a toaster oven, then maybe just a toaster will do. So, plug in the oven at ~ noon and switch it on, while watching the info on the controller. After turning of the oven, you should see the watts coming on shoot up to >1000W. If it comes nowhere close to that, more troubleshooting needs to take place.
