When Fitbit Zip need a new battery

Today, I replace the battery of Fitbit Zip. It is CR2025.

cr2025

I check the voltage of new battery and the exhausted one.

New one is 3.21V.

new-battery

 

The old one is 2.83V.

old-battery

Let me record it and see how long I need a new battery for Fitbit Zip.