"Spehro Pefhany" Isn't that going to unbalance the cells?
^^^^^^^^^^^^^^^^^
I assume the current draw is going to be small, so the battery probably
won't even notice. If you are really worried about it, you could switch to
the other half of the battery every month or so. If you are REALLY
obscessive about it, you could install a flip-flop type relay to do the
switching for you every time you start the car. That would probably cost
more than some of the more sophisticated electronic methods that have been
described, but it is more easily understood by an old fart like me.