After extensive research on the Internet, I found a battery that will fit in the aux batt compartment under driver's seat, WITHOUT modifying the compartment. 

 

Like many owners, I was attracted to the Optima yellow top, but didn't want to modify either the battery or the compartment. This meant Optima batteries wouldn't be suitable. 

 

Fortunately I found the Hawker Odyssey PC 925 MJT battery. 

 

Its advantages: 
-- with dimensions of 7" x 6-3/8" x 5-3/4"h it fits the compartment easily in any orientation 
-- it is a sealed AGM battery that does not require venting.

-- its charging voltages similar to the starting battery, so it can share the same AC charger
-- it can be deep-cycled
-- it generates enough cranking amps to start the vehicle if the starter battery fails

I have now installed the Odyssey as the auxiliary battery in my '87 Westfalia and am running all the Fuse 3 loads (interior lights, radio memory, lighter outlets, clock, etc.) off the battery. I hooked up a charging system that I will describe in detail later. 

Details on the batteries:

http://odysseyfactory.com

East coast source:  http://www.batterymart.com/battery.mv?c=odyssey

West coast source:  http://odysseybatteries.com/auto.htm

 

If you are near a Batteries Plus store they may be able to order it for you:

http://batteriesplus.com/Product/slacidrechg.html

 

Location of the vendor is important: the batteries are heavy and shipping over distance can get expensive.

 

Chris Earl

'87 Westy