You are combining the old and new api, just stick with the new one. In the new API, you don't need cr, uid & ids, you can achieve most things via self.env. So your send_to_order function could be greatly simplified to:

    @api.multi
    def send_to_order(self):
        for temp in self:
             sales_order = self.env['tbl.sales.order'].create({
                      'product_id': temp.product_id.id,
                      'batch_ids': [(6, None, self.batch_ids.ids)]
                    })

Things I have changed:

removed @api.one, it has been deprecated as it was confusing to use and was often used wrongly

creation of new recordsets can be done via the new self.env['model_name'] syntax, instead of the old pool.get('model_name')

And the part you were actually asking for, how to write the many2many field batch_ids. This is using Odoo's tuple syntax (the details can be found here: https://www.odoo.com/documentation/10.0/reference/orm.html#odoo.models.Model.write) . Basically, you have to somehow pass recordsets in the return, which is converted to json, so you can't pass records directly. Instead you give Odoo a command and a set of ids. In this case the 6 means: set this Many2many field to the records with the ids given in the last tuple element (the middle element is ignored in this case, but it must be present).