Texas Instruments TI-83 PLUS - Manual (Page 739)

TI-83 Plus General Information 736
After Message A is first displayed, you can expect the batteries to
function for about one or two weeks, depending on usage. (This one-
week to two-week period is based on tests with alkaline batteries; the
performance of other kinds of batteries may vary.)
The low-battery message continues to be displayed each time you turn
on the unit until you replace the batteries. If you do not replace the
batteries within about two weeks, the calculator may turn off by itself or
fail to turn on until you install new batteries.
If Message B is displayed, you must to replace the batteries immediately
to successfully download an application.
Replace the lithium battery every three or four years.
Effects of Replacing the Batteries
Do not remove both types of batteries (AAA and lithium auxiliary) at the
same time. Do not allow the batteries to lose power completely. If you
follow these guidelines and the steps for replacing batteries, you can
replace either type of battery without losing any information in memory.