I have an 85 westy with an optima yellow top house battery connected
using the normal relay setup (e.g. a separate relay for the house
battery, triggered by the blue alternator wire).  The starting
battery is a no-name FLAPS battery, relatively new.   The van sits
for days, sometimes weeks at times w/o being driven.   Recently,
twice, I would drive the van with the headlights on after not being
run for days.   Drive about 20-30 minutes, then turn the headlights
off and go into a store.  Come back and it won't start.   Removing
the battery cover and doing a self-jump (from the house battery to
the starting battery) and it'd start right up.   So I was wondering
if there was some weird discharge thing going on, such that the
optima was stealing charge from the starting battery.

Just yesterday, it happened again (this time the headlights were not
on), and I repeated the jumper cable trick, but this time it wouldn't
start!   Checked both the battery voltages, and they were low but at
least 50% charged (12.4 volts ish).   Scratched my head a bit.
Tried starting again a few times with the jumper cables attached --
it really sounds like a dead battery (e.g. when you turn the key to
the "crank" position, the seatbelt buzzer noise drops as if the
voltage is dropping).    Took off the jumper cables, wiggled the
battery connections, and suddenly it would start with just the
starting battery.

It looked like the smaller red wire was loose at the battery terminal.

My theory is that this may have been the problem all along -- that my
"self jumpstart" was a red herring -- the act of connecting the
jumper cables just happened to wiggle the battery connectors enough
to make it have a better contact.   Since it is an intermittent
problem, I can't be sure it's fixed, but it seems like a decent
theory.   Does this make sense?  On the hot side of the battery, what
is the function of the smaller red wire (it's perhaps 2 gauge)?