The battery compartment is on the back on the opposite side of the film door (shares the hinge.) Just pull gently on the door near the silver button. While the original battery was 4.5v, people have found that 3v is sufficient to drive the shutter. I use a CR123A lithium battery with a spring to make up the length difference. You can also find instructions on other custom battery adapters via google. You can verify proper shutter operation by dry firing while looking through the back.