To get unlimited electric batarangs, simply press L1, R1, L2, R2 at the main menu.
All wepons and ammo
There is also a cheat for all wepons and unlimited ammo, but it's slightly harder to obtain. When in the Plant Electrocution 2 level, you start by sliding down towards some holes. Drop down into the second one and then glide downwards to the lit area. Pick up the key you find here and exit. The key will open the door right next to the vines behind the glass. Go inside this room and you'll discover an envrlope with the letter 'C' stamped on it. Now you must go on to compleate the game and the cheat will be selectable.