Canon eos 40d 1901b014 User Manual

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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 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 be sure to set the 
correct date/time.
1
Set the power switch to <
2>.
2
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.
 
Then load the battery and close the 
cover.
Replacing the Date/Time Battery
(+)
(–)
For the date/time battery, be sure to use a CR2016 lithium battery.