Honeywell T4 Pro User Manual page 10

Batteries are optional (to provide backup power) if your thermostat was wired to run on AC power
when installed. If your thermostat was not wired to run on AC power, then batteries are required.
Install fresh batteries immediately when the low battery icon appears. The icon appears about two
months before the batteries are depleted. Even if the low battery icon does not appear, you should
replace batteries once a year, or before leaving home for more than a month. If batteries are
inserted within two minutes, the time and day will not have to be reset. All other settings are
permanently stored in memory and do not require battery power.
NOTE: When replacing batteries, alkaline batteries are recommended.