Canon EOS 1D - Manual (Page 248)

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The date/time (back-up) battery’s service life is approx. 5 years. If the
date/time is reset when camera is turned on, replace the back-up
battery with a new CR2025 lithium battery as described below.
The date/time setting will also be reset, so be sure to set the
correct date/time (p.49).
1
Set the power switch to <2>
and remove the battery pack.
The back-up battery is on the ceiling
of the battery compartment.
2
Remove the back-up battery
cover.
Use a small screwdriver to loosen the
screw and remove the cover.
Be careful not to lose the cover and
screw.
3
Remove the battery.
4
Install a new back-up battery.
The plus side of the battery must face
up.
5
Attach the cover.
Replacing the Date/Time Battery
(+)
For the date/time battery, be sure to use a CR2025 lithium battery.
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