I have built a battery level indicator and have tested with a 12V battery Would it harm my arduino board if I connect a 48V battery?
You have a voltage divider there, which would give (if the pre-set pot was set to the maximum):

Vout = (Vin * R2) / (R1 + R2)
Vout = (12 * 10000) / (100000 + 10000)
Vout = 1.09


Thus a 12V battery would present 1.09V at the analog input. 

This same wiring would seem to be in range for a 48V battery:

Vout = (48 * 10000) / (100000 + 10000)
Vout = 4.36


Since the analog input can handle up to 5V, then that should be OK. However I would test the voltage divider output first with a multimeter to make sure it doesn't exceed 5V - before connecting it to the Arduino.



You would probably need to change the code to test for different ranges of input voltages, other than what is used for the 12V battery.