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