Constand output voltage from batteries / rechargable batteries
If you want a consistent output voltage over the life of the battery, you will have to use a voltage regulator.  In the case of the "normal" or alkaline battery, each cell will have a cutoff voltage of around 1V when finished.  So, over the life of the battery, you need to take this 6-9V input and convert it to a constant 7.2V.  This will require a buck-boost switching regulator to accomplish.