Original source: http://www.dba-oracle.com/t_v$_flash_recovery_area.htm

If you manually delete files under recovery area after you get an ORA-19815 in your alert log, run the following to refresh v$recovery_file_dest and v$flash_recovery_area_usage.

[oracle@rac1 ~]$ rman target /

Recovery Manager: Release 11.2.0.4.0 - Production on Fri Jun 6 13:24:23 2014

Copyright ? 1982, 2011, Oracle and/or its affiliates. All rights reserved.

connected to target database: ORCL (DBID=1377135549)

RMAN> 
RUN {
change archivelog all crosscheck;
report obsolete orphan;
report obsolete;
crosscheck backup;
crosscheck copy;
crosscheck backup of controlfile;

delete noprompt expired backup;
delete noprompt expired archivelog all;
delete noprompt expired backup of controlfile;
delete force noprompt expired copy;
delete force noprompt obsolete orphan;
delete force noprompt obsolete; 
}

You should avoid deleting files in recovery area with OS commands. They should be managed by RMAN.

Refresh recovery area usage data after manually deleting files under recovery area