Low current divider for battery voltage monitoring
Use a simple circuit like this. The P-Channel MOSFET can be some other low current device - The one shown is just the one that showed up in the schematic tool. 







simulate this circuit &ndash; Schematic created using CircuitLab

The idea is to connect the GP_OUT signal to your MCU and have the software drive that high at the time an A/D measurement is to be made. I show using 100K resistors for the divider but these may need to be scaled down depending on the input impedance of the A/D input pin. It's been my experience that 1Meg resistors used for this purpose lead to too much measurement error. 

Some delay from setting the GP_OUT high will be needed till the A/D conversion is commanded to allow for settling of the A/D input.

Note that when the NPN transistor is off the only load on the battery is leakage current.