Intermec PB21 User Manual page 16

Chapter 1 — Using the PB21/PB31 Mobile Receipt Printer
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2 Insert the battery with the tab facing away from the front control
panel.
3 Insert the edge of the battery under the lip of the PB21/PB31 and
then push down until you hear the battery latch click.
PB21/PB31 Mobile Receipt Printer User's Guide