You can load a database with:
$this -> load -> database();
Whenever you want to load a specific database, you give the name of the database to the load function:
$this -> load -> database('DBname');
Where you specify your database propeties in config\database.php
.
Note that you define your database name/group with the first key:
$db['This is your database group']['hostname'] = '127.0.0.1';
see here.
Hope this was helpful.