Another observation during all of this wiring work is that there was a drop in the voltage from the alternator terminal to the splitter input – up to 0.5V. A drop at the batteries is expected as the relationship between amps and volts changes during charging, but in the ~4 metres from the alternator to the splitter, there should be a negligible drop.
The old cable looks to be about 8 – 10 mm sq or so, the new one is 25 mm sq. The old cable also had a drilled out connector on the splitter end, so it’s possible that the lighter cable combined with the not-great amount of connector surface area both contributed to the drop.
With the cable changed out, I’m now seeing about 0.01V of drop over the 4 metres, and about another 0.01V drop to the batteries from the splitter terminals during the absorption stage. Pretty good really. When Blue Opal had a diode splitter, she’d only ever get about 13.2V into the batteries, so they never hit the absorption stage. She’d previously been kept on a marina with shore power going permanently, so the diode splitter wasn’t a problem.
Old cable lug vs new cable lug 
Old alternator cable vs new alternator cable 
Heat-shrink tubing to ensure no contact with the chassis of the alternator 
In place on the terminal 
How do you cut 25mm sq? With the cutters for cutting a mast free 
The Draper hammer crimper for doing battery lugs
While I had the galley drawer assembly out to run the alternator cable, I checked the voltage at the fridge compressor. It’s dropping about 0.1V on a 5A draw. Acceptable (way better than the 2V drop at 9A for the water pump).