Accu-Chek COMPACT PLUS User Manual page 146

Insert two new batteries (type AAA, LR 03, AM 4 or micro).
The minus (-) ends of the batteries must match up with the minus
marks in the battery compartment.
Re-attach the battery compartment cover.
The three hooks on the underside of the cover must engage with the
slots along the edge of the battery compartment. The cover is then just
loosely held in position.
Slide the battery compartment cover back into place.
It locks with an audible CLICK.
146
Changing the batteries