To add to Francois' answer, it depends a lot on how 'old' your camera is. A new digital camera sans battery is basically a paperweight. In contrast, my old Minolta SRT-101 was all 'manual', except for the match-needle light meter. With a handheld meter, I could have used the camera w/o battery with no problem. Another example, the Hasselblad 501CM and 503CW are fully manual (except for the prism viewfinder, which has it's own battery), and so they don't need batteries at all.
I was thinking in terms of digital cameras
I forgot that not so long ago we could use cameras without any external power source.