Why does drupal_add_css not work on Drupal7 ?

Hi all,
I’ve a site on Drupal7 with a custom module (mymodule).
In this custom module I have a template that should upload some css files from the directory “mymodule/css/font-awesome.min.css”.

Now If I write in the template:
drupal_add_css(base_path().drupal_get_path('module', 'dashCust').'/font-awesome.min.css');

the file is not uploaded.
But if I upload it as an external file, it works:

  global $base_url;  
  global $base_path; 
  $base_address = $base_url . $base_path;
 drupal_add_css($base_address.drupal_get_path('module', 'mymodule').'/css/font-awesome.min.css', 'external');

Why does the first method is not working?
Thank you


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