I know I come to this discussion a little late but I wanted to suggest an approach to battery level monitoring that is extremely low component count and low cost as well. This idea uses the Maxim MAX6348 chip as a battery level monitor. The chip is a SOT-23 size package and the implementation only requires three additional resistors. Here is a link to the data sheet for the MAX6348.

Here is the schematic for the concept of using this chip as a battery level monitor.



Note that the MAX6348 is available in a number of various voltage detection levels and so after selection of the appropriate device the resistor divider values (430K and 220K shown) must be tuned to the proper setting to cause the MAX6348 to emit a low level on its output pin for the low battery level. The selection shown should be made so that the Vbat level that causes the trigger is the low battery voltage suitable to the chemistry of the 9V battery you are using. It is also advisable to select the two resistors such that they put a load of about 10 micro amperes on the battery. This is because the Maxim chip draws about 1.5 uA and the divider current should be larger enough to pretty much swamp out variations in the current draw of the MAX6348.

Michael Karas