Nikon EL2 - Manual (Page 11)

inserting and properly seating the battery, snap the
chamber lid shut, unlock the mirror and remount the
lens preparatory to shooting.
Caution: Should the battery be installed incorrectly (i.e., in the
reverse direction) in the chamber, its energy will be depleted in
a matter of minutes; thus, take extra care to insert the battery
properly. Also, be careful not to touch the shutter curtain and
mirror surface while installing the battery.
Note; Remove the battery when the camera is not to be used
for a long period; this will prevent leakage within the camera.
Also, keep the camera as warm as possible when operating
under cold-weather conditions; otherwise, the battery may fail
to function. (See “Tips on Camera Care” on page 40 for
additional information.)