At the rear are two side by side rectangular compartments measuring 72mm wide and 29mm long. Using AA batteries the compartments are not long enough to place batteries in flat, facing front to back and placing them in flat left to right while they will take 5 batteries each as the dividers are not full height the batteries easily jump front one compartment to another unless the box is kept constantly flat.
As such batteries have to be stored standing upright and given the size of the compartment quite a lot of batteries have to be stored at once to prevent them from falling over and shifting to another compartment or worse having the terminals on one battery coming into contact with those on another.
Personal testing would indicate that the absolute minimum number of AA batteries that you would want to store in one of these compartments is 6 stood upright. This number prevents the batteries falling flat or from shifting to another compartment.
In total these compartments will hold 10 AA batteries upright and if the dividers had actually isolated each compartment it would have been possible to store 1 to 6 batteries on their side or 6 to 10 stood upright rather than only being suitable for storing a minimum of 6.