Casio VL-80 Operation Manual page 8

5)After replacing battery, set thé mode selector to "CAL"
and press thé P switch on thé back of thé unit with a pointed
object.
* Before inserting new battery, be sure to thoroughly wipe it
off with a dry cloth to maintain good contacts.
* Do not leave dead battery in thé battery compartment as it
may cause malfunctions.
* Remove thé battery when not using for an extended period.
* It is recommended that battery be replaced every 2 years to
prevent thé chance of malfunctions due to battery leakage.
6