A 1500 watt inverter changes everything.
No battery that can fit under the driver's seat will power this up for long.
You are looking at a possible 150 A/H plus load here. You are in 2 or 1
gauge wire territory here. Now if you plan to use this while driving, you
have to consider that as the alternator will not keep up with the demand,
using a relay the chassis battery will start to supply this load also. So
any common path from the chassis battery to the aux battery  also has to
support at least part of this load. The existing wiring from the alternator
the chassis battery will not. I would limit inverter selection to the
600-800 watt range even though the larger inverters seem like more of a
bargain. The 2,000 watt inverter in my motor home is supported by 4 golf
cart batteries and voltage drops fast when using the microwave. A battery
under the front seat doesn't stand a chance.

Dennis

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
Tom Torrella
Sent: Sunday, March 04, 2012 1:50 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: Second battery in Westy

Dennis,

Two questions: Can an 8 gauge wire recharge the auxiliary battery of the
alternator (with relay) under the driver's seat?  Is that space vented
outside of the van?

Plan to permanently install a battery, relay and 1500 w inverter for light
electrical uses & refrigerator.  Our travel trailer has similar set up
that's charged off the trailer lights plug going down the road or with
charger when plugged into electriciy.