Update:
1) I've seen the controller hitting its max 15A into the batteries, at slightly over 200W power from the array.
2) Currently the ratio of power from the roof to power from the off-grid array is about 30:1 even though the array size is about 10:1. If my over-build experiment is to 'work' this ratio had better improve in favour of the off-grid system by mid-winter!
3) I've tracked down some of the supply impedance that I was detecting, ~0.1 ohm from battery to input to the RPi 12V-to-5V switch mode supply (Vbus). I'm taking measurements at the battery and load terminals of the controller, and Vbus, and an output snippet from my program with the system under some load is:
Verbose mode ($Id: powermng.cpp 15411 2016-09-11 08:40:19Z dhd $).
SS-MPPT-15L Vb_f slow battery voltage (mV): 12473
SS-MPPT-15L Adc_il_f load current (mA): 1472
Load power (mW): 18361
Input flag DUMPING is unset.
SS-MPPT-15L Adc_ic_f battery charge current (mA): 658
External flag file /var/log/SunnyBeam/LOW.flag is absent.
ADC command 6a 80
ADC (6a 0) response 1307 (5 1b 0)
Battery voltage 1 local (LA) 12262mV, 2 (Li) -1mV.
Supply drop B1 to BV 211mV; implied supply impedance 0.143342ohm.
Adc_vl_f load voltage (mV): 12397
Supply drop controller to BV 135mV; implied wiring impedance 0.091712ohm.
The upshot is that maybe 1/3rd of the voltage drop I see happens between the battery and load terminals of the controller, probably as part of its measuring and short-circuit protection. The rest is still higher than I'd like (and the lumpiness of the measurement for small voltage drops means that these absolute values are probably only roughly right), but at least its getting closer to my estimates of what it should be.
I took the opportunity to tidy up the wiring yesterday.
4) It was open day at my house yesterday for SuperHomes and again today and I get to show off my little off-grid system to the visitors!
http://www.superhomes.org.uk/superhomes/london-kingston-upon-thames-willingham-way/Rgds
Damon