MNRon wrote:
Gdetrailer et al - check my math here, good catch on return path:
Simple answer, solar panel placement affects charging more than the difference between 40ft and 70ft of 10ga.
More complex answer:
Based on OP's first post the controller puts out 14v and he measured 13.8v at battery and 13.5v at battery on 40/70ft runs. This would imply with 40f he is getting 2.5A charging current (200mV drop across 80ft (round trip) at 1mOhm/ft); and at 70ft he's getting 3.6A charging (500mV across 140ft...). The reality is that his measurements etc aren't that accurate and I'm sure that he's getting roughly the same current charging with either run, the limiting factor in his charging is *not* the IR drop in the wire, but the internal IR drop inside the battery (if you will, actually more complicated). The truth is that with ~13.5v across the terminals the battery when nearly fully charged is the current limiter and isn't accepting much current.
If he did the same experiment with 50% discharged batteries (12.1v) he will be putting a larger voltage differential onto the batteries and will drive more current. In this case the wire IR resistance might come in to play but I'm guessing second order compared with sun exposure on solar panels. Assuming full 7A charging from his panels he'd be putting ~13v across the battery with 70ft (including round trip) and 13.4v with 40ft; I suspect a 50% depleted battery will draw 7A regardless of 13v or 13.4v across it, and as it charges up the battery internal resistance will be a larger current limiter than the IR drop (as evidenced by his measurements in the first place).
Thoughts?
Yup, Once the OP corrects the poor (wrong) placement of the charge controller and gets it close to the batteries, He will find, like I have found, that long runs may loose a fraction of an amp in peak charging current with a PWM controller, but gain with best sun placement. Even a slight cloud cover or shade will have a much bigger detrimental effect on charge current than the long run. And once he does that he will find that peak charging current with a reasonably discharged battery will be close to Imax for the panel. The power loss in the cable is a linear function of current, while the the panel conversion efficiency is a highly nonlinear function of sunlight, slight drop in sun or panel shade= large drop in output.
And he won't notice the power loss in the cable unless the voltage drop is enough to drop the voltage at the controller down to around 15 volts. Given that loaded output voltage of most panels is in the 18+ volts you need either REALLY skinny wires or really LONG run to get that much drop with the panel size he has.
Recently we have had days with high thin clouds. My roof solar panel will go between 2A and 9.5A charging current with the sun goin "behind" the thin clouds, a difference in shading that one hardly notices when standing outside. Same with shading part of the panel.
