It has started discharging in 3-5 hours without any excercise tracking on regular usage. Needs replacing?
Is there a Hard Reset / reset option which may help ?
A Samsung Experience Store or Samsung Service Location could run a diagnostic on the device.
Daily Driver > Samsung Galaxy s²² Ultra 256Gb in Phantom Black.
The advice I offer is my own and does not represent Samsung’s position.
I'm here to help. " This is the way. "
Might be worth a quick check that all of the software (including the Gear Manager app) is up to date, along with turning Wi-Fi off if you're not using it. Other power-saving tips are: decreasing the screen brightness; deactivating the Watch Always On feature; and customising the Notification settings to limit them.
There's some guidance on backing up (here) and resetting (here) a Gear Fit2 on the US Samsung site if you need to follow through on the first step of BandOfBrothers' suggestion.