Efficiently sensing the voltage of a battery pack
Instead of leaving the resistors attached all the time, consider turning them on and off. The procedure would be:


Turn on the divider by asserting the POLL signal. Enable your ADC.
Read your ADC (average the number of readings you need)
Turn the divider back off. Disable your ADC.


Repeat the above only as often as you need to update your reading of the battery voltage, which is probably rather infrequent. Therefore, the resistors are on for a really low duty cycle.







simulate this circuit &ndash; Schematic created using CircuitLab

Notes:


Since they are rarely on, just select R1||R2 for a reasonable resistance for your ADC to read.
I assume you made an error and interchanged the values of the divider in your question. You need to get the maximum input voltage down to 3.3V or whatever your ADC voltage is.