Something like the cct below (it's a bit of one of my circuits).                                 
It uses an NFET, switching the DC-, and the comparator is <1uA.                                  
To swap to switching Batt+  rather than Gnd (Batt-)you'd need a PFET and                         
to swap the opamp pins 3/4 inputs around.                                                        
Tweek R10/R11 to set the switching voltages, R11 to set hysteresis so it                         
doesn't chatter on /off at the limit.  Maybe you'd be better off swapping                        
R11 for a cap, to give a time delay on switchoff?