I have a very small off-grid system (with a decent store) used for keeping my Raspberry Pi server running the whole time. It generally only uses a couple of watts, so I could run it over wet string.
However, I can shift about another 12W/1A of load to it from the grid when the battery is full enough. This seems to cause ~0.25V drop, ie implying impedance from battery, via charger, to load (which is very close to the measurement point) of 1/4 ohm.
Cabling run from battery to controller to load is mainly 4mm^2, about 4m, so 8m there and back, and for that I estimate about a quarter of the voltage drop that I actually measure. You can see the sharp movements in the purple line where the extra load is switched on and off in the graph below:
a) Given the small load, is my wiring resistance reasonable?
b) What might be the rest of the drop? My couple of fuses either side of the controller (~20A MAXI automotive IIRC)? The SS-MPPT-15L controller? Bad connections (yes, I periodically check and tighten). Pixies?