HP -28S Manual page 288

Installing Batteries
If you have just purchased the HP-28S and are installing the batteries
for the first time, you can take as long as you'd like to complete these
procedures.
However, if you are replacing batteries, you should keep in mind that
there is a time limit for completing these procedures if you want to
preserve the information you have stored inside the calculator (Con-
tinuous Memory). Once the battery compartment is open, you must
replace the batteries and close the compartment within one minute to
prevent loss of Continuous Memory. Therefore, you should have the
new batteries readily at hand before opening the battery compart-
ment. Also, you must make sure the calculator is off during the entire
process of changing batteries.
To install batteries:
1. Have three fresh N-cell batteries readily at hand.
2. Open the calculator to expose the keyboard and display. If you
are replacing batteries, make sure the calculator is off. Do
not
press
I
ON
I
until the entire procedure for changing batteries is com-
pleted. Changing batteries with the calculator on could erase the
contents of Continuous Memory.
3.
Hold the calculator with the battery compartment door facing
up. To remove the battery compartment door, slide it towards
the back of the calculator (away from the product label).
A: Assistance, Batteries, and Service 287