Single model per aplication .

Is there any drawbacks in using a single model extending basemodel per aplication ,?

Code:

class Basic_Model extends CI_MODEL {

private static $instance;

    function __construct() {
        // Call the Model constructor
        parent::__construct();
    }
    public static function getInstance()
    {
        if (!self::$instance instanceof self) {
            self::$instance = new self;
        }

        return self::$instance;
    }

}

PHP Code:

Class myModel{
var $model ; // base model
public function __construct() {

$this->model = getModelInstance() ; // basic_model singleton

}

function find($params = array())  {
$this->model->db->from(my_table());...
} 


You don't need the code you show. Any model loaded using $this->load->model(...) already creates a singleton.
You can use the Auto-loading Resources feature to make the model available all the time for the application.

BTW, you do not need the call to parent::__construct(); in Basic_Model because CI_Model does not have a constructor to call.