Last couple days I have been running my solar shed system as two independent systems. This still seems to be working good.
To recap: I have a small 6 panel system with 3 east facing panels and 3 west facing panels. I am using 2 Midnite solar "Kid" charge controllers. I have a variety of battery options, but I am running Tesla Model S battery modules (24volts) while the weather is still warm enough.
I originally setup my system to according to the instruction manual. They recommend setting up two Kids to behave as a single controller via "twin" mode. To do this, you need to combine all of your strings of panels into a combiner box which will parallel the two strings. If you only have two strings, you may be actually be able to do this without a combiner box. But if you were to have more than two strings, you absolutely need the combiner box to put circuit breakers between the strings, otherwise it would be in danger of fire should one of the panels have a short. Anyway, once combined you have a common GFP (ground fault protection) breaker, one large breaker to accommodate the whole array, then two smaller 30amp breakers going to each Kid charge controller (they are only rated 30amps each).
So I started with it that way. I ran into a few problems with this.
1) I connected the Kid controllers to twin them using a phone cable as described in the instructions. After running the "seach/sync" mode to find kids connected to each other, I am not sure if they are actually running in twin mode as there seems to be no visual confirmation of such mode. They don't seem to communicate settings from one to the other. I had to setup each with the correct battery and voltage settings independently.
2) When the sun starts shining, the first Kid to turn becomes the only kid to turn on. Once one of the kids pulls the array voltage down, it makes the other kid think that the array voltage is too low to bother turning on and make any power.
3) With only one kid operating, the two collective arrays are too powerful for just one kid. The kids seem to be poor at measuring actual current flow (separate issue) such that the Kid does not "clip" the input from the solar arrays as expected or turn on the second kid to share the load. So the one kid gets overloaded and trips the breaker, stopping all solar charging.
4) Although the panels still produce a lot of power when paralleled together, I don't think they operate very efficiently when they are paralleled in the east-west arrangement. It would be much better if they were facing the identical direction.
So with these problems I decided to change up the configuration, despite the warnings in the instruction manual not to. Right now I am running the two arrays independently. I have one kid connected to the east and one connected to the west. Both kids are connected to the same battery negative and same battery positive (with separate 30amp beakers going to each kid). I have the east and west PV- connections on a common bus bar. The PV+ from the two arrays are isolated. One goes to one kid, the other goes to the other kid. I have the surge suppressor and GFP breaker going to one kid and intent to connect the other kid with it's own GFP breaker and surge suppressor. I don't have a surge suppressor or GFP breaker on one of the kids at the moment while I wait for parts. One of the arrays just goes for it's array PV+ breaker directly to the kid. All devices are on a common earth ground.
I have been running this configuration for a couple days now. The kids seem much happier in that they are not being overloaded. One array is properly sized for one kid. Also they kid can now independently control array voltage. This seems to help in the early morning/late afternoon. In late afternoon the west array is getting direct sunlight and is producing much more current and the optimum voltage seems to be at a high voltage (~90 to 100 volts). While the shaded array is still producing power but can be allowed to operate at a much lower voltage ~40volts. This seems to be more optimal. I think I am getting much more total power from the system when I operate it this way. Just yesterday it was a dark cloudy day, but I still got over 2+ kwhs of juice into the battery (almost fully charged a dead tesla module).
There are a few downsides to this configuration unfortunately. For one, I need twice the number of some misc components. Such as the GFP and surge suppressor. There is more wiring as well. However, the biggest danger to this setup as far as I can tell is that I have poor control over charging the batteries themselves. For instance: on a lead-acid battery setup, there are bulk charge, absorb, and float charge modes. When one controller is trying to go into absorb, the other might still be in bulk or float. Thus not getting the proper absorption time. There are also other possible issues relating to float charge effectively boiling the batteries dry. Another problem with in-accurate cut-off amps. So there are real reasons why this might be bad. I think if I can at least get the two kids to communicate with each other, those issues would be mitigated, regardless if they are connected as one array or two independent arrays.
The other thing to note, is some of these issues might not be a problem for the lithium ion setup (Tesla modules). I don't need to go from bulk charge to absorption time (rest period). I just need to go from constant current (i.e. bulk charge) to constant voltage (i.e. float charge). I am not fully charging the modules, so I don't think I need to worry about minimum amp cut-off. However, If I was fully charging the modules to near 100% SOC, I would definitely want a minimum amp cut-off.
Anyone with some more thoughts on this? Anyone think I am heading to disaster with a flaming solar shed in my future?
