Hello,

1) If you are only interested in 12 volts for the refrigerator,
   perhaps the refrigerator can be modified for 6 volts.  If it is
   the usual absorption cycle type, the power is only used for a
   heating element and perhaps a light.  It might be possible to
   modify the existing element, or just to make a new element to do
   the job.  This should be low tech stuff.

   If it is the electronic, Peltier (sp?) type, a bit of rewiring
   should do the trick ... two 6 volt strings of diodes rather than
   one 12 volt string.  This is a bit higher tech.

   However, if the refrigerator is of the motor/compressor kind, the
   motor is probably sealed and you are out of luck.

2) If the refrigerator can't be modified, or you need other 12 volt
   capability, the charge one, then the other trick should be quite
   workable.  Some set of high current relays would be needed, but the
   idea is to charge on battery in parallel with the existing bus
   battery with the other 6 volt battery in series with this, adding
   to 12 volts for the load.  While driving, it would be good to
   automatically switch the batteries positions every so often (5
   minutes?) to keep the charge and discharge cycles shallow to
   preserve battery life.    When the engine is off, there is no need
   to swap the batteries periodically, but they do need to stay
   connected in series to provide power to the 12 volt load.  The dumb
   refrigerator won't notice the switch at all, but something
   electronic, like a radio will experience a moment of discomfort.

In general, I have a lot of difficulty deciding if an how to make posts 
here.  There seems to be a lot of variation in knowledge among the folks on 
this list.  So, please forgive me if I have bored you or insulted you 
intelligence with my post.  Perhaps if anyone wants to discuss this further 
we should use email directly.

Mike

Michael A. Radtke  -  Z74               Voice: 602-862-4897
Bull                                      FAX:         4853
13430 N. Black Canyon Hwy.
Phoenix, AZ  85029                      Email: m.radtke@bull.com