How often is it used? Unlocking it manually does not drain battery, and sending zwave commands (check to see if it is locked) is low power.
There are also a few reports of batteries dropping off a cliff, where it was 100% then dives to like 40% or even stop working.
What device driver are you using? Is it the default or Enhanced Z-Wave lock? ((Enhanced) Z-Wave Schlage Touchscreen Lock)