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?
Pretty much.
On of the biggest catch 22's is the battery internal resistance changes as it charges in a non linear and non predictable way.
The wire resistance will stay linear but as the battery draws more current during the charging phase the voltage at the battery drops. That drop will limit the maximum voltage the battery will see which in turn limits the maximum charging current.
Toss on top of that is the fact that the solar panels also have internal resistance which is not 100% linear, that internal resistance is what limits the peak current the panel can deliver.
Resistance is the enemy, some resistance in the circuit can't be removed and what you can control is the distance and wire ga you use.
Basically, in a nutshell the line resistance does have a limiting effect so a 100W panel on the end of a long run of wire will deliver substantially less than 100W.. In reality even if one were able to harvest all 100W that is roughly 7A of charging for at best 10hrs per day. That nets you a absolute max of 70Ahr at best, but with the OPs setup, might get 30Ahr due to the controller being remote.. If that is acceptable, then roll with it.
Personally out of curiosity, adding in a ammeter between the controller and the battery would give the OP a much better read on the situation.. Voltage alone doesn't give the full story of what really happens.
