AAA batteries have a magnetic signature that is strong enough to influence compass readings. In order to negate the batteries effect, it is important that the Kestrel 4500’s batteries remain in the same orientation as they were when the unit was calibrated. To accomplish this, the Kestrel 4500 has a flexible plastic shim that slides in between the two batteries to keep them in place. Insert the batteries in the orientation indicated on the battery door, then insert the shim between the new batteries with the clear tab out and folded toward the end of the battery with the positive pole (the “bump”)pointing up. Press the tab down over the battery with the “bump” through the hole (it will not remain completely flat). Close the door tightly. When changing the batteries, lift the clear tab folded over the positive pole of the battery, remove the shim, and then remove the batteries and replace them with new ones.