Canon EOS 5D - Manual (Page 41)

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The date/time (back-up) battery maintains the camera’s date and time.
The battery’s service life is about 5 years. If the date/time is reset when
the battery is replaced, replace the back-up battery with a new CR2016
lithium battery as described below.
The date/time setting will also be reset, so you must set the
correct date/time.
1
Turn the <4> switch to <2>.
2
Unscrew the battery holder
screw.
Use a small Philips screwdriver.
Be careful not to lose the screw.
3
Take off the battery holder.
Push out the battery in direction 2.
4
Replace the battery in the battery
holder.
Make sure the battery is in the proper
+ – orientation.
5
Tighten the battery holder screw.
Replacing the Date/Time Battery
For the date/time battery, be sure to use a CR2016 lithium battery.