If your BT521 measures voltage but will not measure battery resistance, a likely reason is that the fuse in the BT521 is open. The battery compartment can be accessed by removing the screw that holds the battery cover in place on the back of the BT521. You can remove the fuse and check it for continuity. If the fuse is open, please replace it. The Fluke part number for this fuse is 943121.