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Years back, there was a TV ad for a battery which had a special
reserve (almost like a second battery within a battery) capability.
The scene I recall was a guy's wife going into labor - they rushed
outside to get in the car and go to the hospital, only to find that
the battery had gone dead.  However, with this special feature,
they merely had to flip a switch of some sort, which called upon
the other side of the battery, and the car would start in
an emergency.

Does anyone remember who made this battery?  Or if it is still
being manufactured?

   I wouldn't be in a hurry to trust anything advertised on TV...
   Do Vanagons have the second battery bay? Why not install a
   second battery to use in a pinch?