Make a voltage divider from two 100K resistors, connect it between +9V and ground, and connect the midpoint to an analog input pin of the Arduino. Also connect a 10nF or 100nF capacitor between the midpoint and ground. When you do an analogRead from that pin, 7.8V will correspond to (7.8/2) * (1024/5) = 799. If you get a reading less than that, drive an output pin high to light the LED. How you display the battery state on the LCD is up to you - I normally display the battery voltage to 1 decimal place if there is enough space.