Ark,
Luckily you had the fuse there so as to not melt a bunch of wiring. Yes at
engine start both batteries will try to support the 200-300 amp starter
load. You have a have to have a method of isolation and if the starter can
draw off both batteries then all the wiring and devices has to be up to the
task. If using a relay it has to be disabled during cranking.

Dennis

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
Arkady Mirvis
Sent: Wednesday, February 10, 2010 2:24 AM
To: vanagon@GERRY.VANAGON.COM
Subject: CONFUSION

I confronted a situation I need help to understand.

The main DIEHARD INTERNATIONAL battery was purchased April 2007.
 In autumn of 2009 I placed under the Westy's rear seat DIEHARD battery,
bought April 2002 and previously used in diesel 1991 Jetta with no problems
to be an aux. battery. AWG 10 copper wire connected the aux. battery to the
alternator via 30 amp. fuse. I didn't install battery separator.
Routine checking a month ago discovered the fuse being blown. I replaced the
fuse. A week ago I found the fuse being blown again.
The only heavy wire is the one connecting starter to the main battery. In my
set up the aux. battery is connected to main battery's positive post via
alternator terminal and the wire from the terminal to the positive post.
I suspect that the fuse blows because at engine start the starter pulls
heavy current from the aux. battery. I have no means to measure the current
( amps ) flow at engine start.
Any thoughts why the 30 amps fuse blows?  Any suggestions on the best aux.
battery set up.  Thanks in advance.   Ark