Voltage divider for ATtiny85's ADC
The divider idea works for the A/D input but it does put a continuous load on the battery. Depending on the type of battery this could discharge the battery in time. You may want to consider optimizing the resistor values to be larger to put less load on the battery. Another alternative is to use a  small MOSFET to gate power to the voltage divider only at the times when you want the A/D reading to be taken.