Honeywell RET97C Series Owner's Manual page 18

Battery replacement
Batteries are optional (to provide
backup power) if your thermostat
was wired to run on AC power when
installed.
Install fresh batteries immediately when
the LO BATT warning begins flashing.
The warning flashes about 30 days
before batteries are depleted.
Even if the warning does not appear,
you should replace batteries once a
year, or before leaving home for more
than a month.
Grasp thermostat and pull to
remove from wallplate
Turn thermostat over, insert
fresh AA alkaline batteries,
then reinstall thermostat.
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