Save the game during Act 1. Go to the Mission Briefing screen and select the saved game from Act 1. Press Square to control the Mk II while on the plane. Move the Mk II upstairs and search for an extra battery. Collecting it will double the operating life of all battery-powered devices in the game.

Note: If you complete the game then go to the mission briefings and work your way back from Act 4 to 2 you can get three extra batteries instead of the allotted one.