Canon EOS Rebel XTI - Manual (Page 155)

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The date/time (back-up) battery maintains the camera’s date and time.
Its service life is about 5 years. If the battery is replaced and the date/
time dialog appears, replace the lithium CR2016 battery with a new
one.
The date/time setting will also be reset, so be sure to set the
correct date/time.
1
Set the power switch to <2>.
2
Open the cover and remove the
battery.
3
Take off the battery holder.
4
Replace the battery.
Make sure the battery is in the proper
+ - orientation.
5
Insert the battery holder and
close the cover.
Replacing the Date/Time Battery
(–) (+)
For the date/time battery, be sure to use a CR2016 lithium battery.