Just two resistors are needed.
Create a voltage divider and choose resistors that will give you close to but not over 5V output when the battery is full. In the diagram, Vin is from the battery and Vout goes to an analog pin.
If the math is confusing or you're uncertain of your result, post the details of your battery (type/peak voltage) and what you think the resistors should be. Take into account that the total value of both resistors should be ~10K ohms to prevent excessive power waste.