Fitness trackers are marketed on their battery life at launch — five days, a week, sometimes more — but almost nobody talks about what that number looks like two years in. Here’s what’s actually happening under the display.

Battery Health, Not the Screen, Is Usually the First Thing to Go

Lithium batteries in fitness trackers typically lose 15-20% of their original capacity after roughly 500 charge cycles, which for a daily-worn tracker charged every few days works out to somewhere around 18-24 months — the screen and sensors are usually still fine well past this point.

The Degradation Curve Isn’t Linear

Battery capacity loss is steepest in the first year and levels off somewhat afterward, meaning the drop from 5 days to 4 days often happens faster than the later drop from 3 days to 2 — which is why the decline feels sudden even though it’s a gradual, predictable curve.

Temperature Exposure Speeds This Up More Than Usage Does

Leaving a tracker in a hot car or in direct sun for extended periods accelerates battery degradation more than normal daily wear does — heat is one of the most consistently cited factors in lithium battery aging across all small devices, not just fitness trackers specifically.

What Actually Slows the Decline

Avoiding letting the battery fully drain to 0% repeatedly, and avoiding leaving it plugged in at 100% for extended periods, both help preserve capacity longer — keeping it generally between 20% and 90% when possible is the standard advice across lithium-battery devices.

When Replacement Makes More Sense Than Living With It

Once daily battery life drops below roughly half of the original spec, most people find themselves charging frequently enough that it interferes with actually wearing it (especially overnight for sleep tracking) — at that point, a lot of the tracker’s core value (continuous data) is already compromised.

TL;DR:

• Fitness tracker batteries typically lose 15-20% capacity around 500 charge cycles, often 18-24 months in
• The decline is steepest in year one and levels off somewhat after
• Heat exposure speeds up degradation more than normal daily use
• Keeping the charge between 20-90% when practical helps preserve capacity longer

A tracker draining faster than it used to isn’t malfunctioning — it’s just further along a curve every lithium battery eventually follows.