If you want to know how to connect to multiple databases in Codeigniter, I’m going to show you.
Step 1:
Open file database.php in the application/config folder.
Step 2:
Find database settings and make a secondary database details in this file. For example:

$db['default']['hostname'] = "localhost";
$db['default']['username'] = "root";
$db['default']['password'] = "";
$db['default']['database'] = "blog";
$db['default']['dbdriver'] = "mysql";
$db['default']['dbprefix'] = "";
$db['default']['pconnect'] = TRUE;
$db['default']['db_debug'] = TRUE;
$db['default']['cache_on'] = FALSE;
$db['default']['cachedir'] = "";
$db['default']['char_set'] = "utf8";
$db['default']['dbcollat'] = "utf8_general_ci";

//And copy and paste listing above like this:

$db['sec_db']['hostname'] = "localhost";
$db['sec_db']['username'] = "root";
$db['sec_db']['password'] = "";
$db['sec_db']['database'] = "blog";
$db['sec_db']['dbdriver'] = "mysql";
$db['sec_db']['dbprefix'] = "";
$db['sec_db']['pconnect'] = TRUE;
$db['sec_db']['db_debug'] = TRUE;
$db['sec_db']['cache_on'] = FALSE;
$db['sec_db']['cachedir'] = "";
$db['sec_db']['char_set'] = "utf8";
$db['sec_db']['dbcollat'] = "utf8_general_ci";

As we can see that I have sec_db as secondary database. Don’t forget to change the Controller to load the model and of course the Model to connect to the sec_db.