read battery value

If you did want to measure the voltage, you'd need to divide it down with a potential divider (but then it seems for an accurate reading you also need a capacitor on the analog input).

So for example:

• A capacitor (0.1uF should do it) between GND and A0
  
• A 100k resistor between GND and A0
  
• A 100k resistor between Battery volts (or even the 5V/vBat/VCC pin) and A0
  
  

Then analogRead(A0)*3.3*2 is your voltage.

Obviously you want to use high-value resistors for the potential divider, as they'll be constantly drawing power.