On a UK model 2010 petrol i20, so maybe the 2009 as well, the battery has ridges at its base front and back that locate under clips on the car. To remove the battery:
Loosen or remove the rear clip. It is not obvious where it is. To find it, look down on the battery from above; immediately behind it you will see a shiny metal box containing electronics, on which is mounted the rear clip. Do not try to take the clip off the box. Instead, remove the box, complete with the clip;
Start by taking the electrical connectors off the battery. There are lots of generic guides on the web on how to do this safely, so if you're not sure about this bit, check it out before you start.
Then, start the physical removal of the battery by loosening the nut on the metal retaining arm that goes out from the box of electronics to the wing (on top of the suspension strut, just by the battery earth lead connector). If it does not want to move, smother it in penetrating oil and wait, and go gently - you do not want to snap it. You'll probably only need to loosen it a bit.
Then, look behind the box of electronics, and down to the level of the battery tray, and you will see two 12 mm bolt heads. They may look quite rusty. If you then look carefully, you will see that they are holding a mounting bracket for the shiny box of electronics.
Use a long extension and a 12 mm socket to remove these two bolts completely.
The shiny box, complete with battery retaining clip, is now free to move and there is nothing holding the battery in.