[C320-list] Various Battery Cable Functions

[Dave and Ev]

I have a 1999 model #628. I just installed two brand new SRM-4D, which 
were replacing ones that were at least four seasons old. The old ones 
were functional but one battery had a bad cell. I have original factory 
wiring. There are three negative cables and two positive cables to each 
battery. What is the purpose of the each of these cables? I believe one 
of the two larger black cables connects the two battery negatives 
together and the other black goes to the ground bus, but I don't know 
the function of the smallest cables. While I had no problems at all for 
the last three seasons other than the bad cell, after the forward 
battery cabling was completely connected, I noticed that when I connect 
the thinner black cable to the battery post on the aft battery it 
appeared that current was flowing (saw an small arc.) The main battery 
1-2-both switch was in the full off position. While I had no issues with 
the batteries prior, I never saw this when I connected these cables at 
the start of the season. What is the smaller cables function and should 
there be anything happening when that smaller cable is connected to the 
negative battery terminal as it did. I looked a the Catalina manual and 
electrical diagram and never found a good detail of the original factory 
wiring for the two batteries. I hesitated to leave that thinner negative 
cable connected to the negative terminal until I have figured this out. 
I am on a mooring with no shore power connected. I charge with the 
original alternator on the Yanmar 3GM30F which puts out 13.9 volts at 
the batteries while charging. No new battery charging equipment added by 
either me or prior owner. All factory original equipment.  Any comments 
or thoughts?
Dave Cushman
It TIps #628