Canon BR-E1 - Replacing the Battery; Use one lithium button cell battery CR2032.; Make sure to insert the battery with the positive (+) side face up.

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If the indicator lamp does not light up when you press a button on the
remote controller, replace the battery. Use one lithium button cell battery
CR2032.
Replacing the Battery
CR2032
Make sure to insert the battery with
the positive (+) side face up, as
indicated in the figure above.