I believe it can use the aux battery. Here is a doc that Surepower sent to
me and on the second paragraph it states that it will assist in starting by
comparing power in both batteries.
http://www.thacker.ca/auto/westfalia/surepower-1314-1315.pdf
Hope this helps clear this up.

./athacker
87westy

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@GERRY.VANAGON.COM] On Behalf Of
mark drillock
Sent: Monday, July 11, 2005 10:01 AM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: SurePower Battery Searator NOT Isolator!

No, it can't do this. Both models leave the starter always connected to the
main battery. Both models allow the aux battery to assist the main battery,
provided the installer hooked up the extra wire needed if this added
function was desired. The 1315 merely adds bidirectional charging capability
while the 1314 allows only for charging from the main battery side.

Mark

Frank Condelli wrote:

>Yes, but the more important feature of the 1315 is  that it ALWAYS 
>chooses the highest voltage battery to start the vehicle with  !  For a 
>couple bloody bucks more why dick around with the less featured  model 
>?
>
>