VTech IS7121_-2-22 Manual - Manual (Page 14)

Getting started
9
For longer battery life, lithium batteries (not provided) are recommended. Lithium
batteries provide approximately 30% additional battery life than alkaline batteries.
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Doorbell battery installation
You can power up the doorbell by connecting it to the domestic power supply or two
AA alkaline batteries (provided). Even if the doorbell is connected to the domestic
power supply, we recommend you also install two AA alkaline batteries. This
guarantees continual operation in case of a power outage.
Install two AA batteries into the battery compartment, matching the polarity markings
(+ and -) inside the battery compartment.