[Amstrad PC real-time clock battery]
 They didn't keep time properly if you used alkalines here - also, they
 would usually moan about the batteries being flat.  This is because the
 batteries were in fact providing too *high* a voltage!  Putting
 mostly-flat alkalines in worked okay. 
Right... Isn't the other trick to wire a diode or two in series with the
battery pack? I wonder if 3 cells + a 'dummy' (as you use to fill up
spaces in some battery chargers) would work
-tony