At the start of the game, one is given 3 seconds of immunity to attacks before we can get killed. Enemies may be all around you in a 12 player deathmatch, so they must get the most protection possible. With Shield Battery, one starts off with 20% shields, making yourself much more secure until you get the Shield. With this, someone may immediately attack you at the start of the game, but this will make sure you are covered.