Batch Requests/sec reported by DMV millions of times larger than Activity Monitor
From the documentation:


  Note: For per-second counters, this value is cumulative. The rate
  value must be calculated by sampling the value at discrete time
  intervals. The difference between any two successive sample values is
  equal to the rate for the time interval used.


If you'd like something that already does interval sampling, sp_BlitzFirst is pretty neat, and free, and full disclosure: I am a contributor, etc.