Arduino Pro Micro: how to access A1
There may be other problems, but according to the Arduino analogRead documentation 


  Syntax
  
  analogRead(pin)
  
  Parameters
  
  pin: the number of the analog input pin to read from (0 to 5 on most
  boards, 0 to 7 on the Mini and Nano, 0 to 15 on the Mega)


So as A1 is defined as 19U, you should change:

const int BatteryPin = A1;


To the following:

const int BatteryPin = 1;


Personally I would also take a look at using preprocessor #define statements in those cases rather than constants, which may consume additional memory depending on compiler optimizations, so you could use the following:

#define BatteryPin 1


Although by convention for preprocessor defines it would normally be:

#define BATTERY_PIN 1