Honeywell RTH2300 - Manual (Page 17)

Programmable Thermostat RTH2300
15
Install two fresh AAA batteries when Lo Batt flashes on
the screen. Lo Batt appears for 60 days before the
batteries are depleted.
Replace batteries once a year, or when you will be away
for more than a month.
Most settings are stored in non-volatile (permanent)
memory. Only the clock and the day must be set after you
replace the batteries.
Battery replacement
Set the System switch to Off before removing the batteries. Otherwise, the
heating/cooling system could remain activated even when batteries are
removed.