For lack of another thread to post this, thumbs up to Fitbit's customer support. Been using a Charge 2 since January and suddenly over the last 3 weeks the battery has started dying in 2 days instead of the 5 it's supposed to last. My partner's one was setup at the same time and is still fine and still lasts 5-7 days. Emailed Fitbit with a copy of the receipt asking for their advice and they replied quickly with instructions to perform a battery test. This was charging it fully and syncing it 5-10 times a day until it was flat and then they could see the battery level at each sync from their end. So did this and it again died in 2 days and they replied last Thursday that they would send me a replacement under warranty.

Their replacement policy states you will get either a refurbished or new model and the shipping confirmation email said I would be receiving the tracker only so I should keep my original charger, straps and any accessories, so expected a refurbished one. But it turned up on Wednesday and was a brand new one in the original sealed box. And I don't need to return the old one either, so pretty good stuff.

I used a Garmin Vivofit for a couple of years before getting this and that also ended up having to be replaced under warranty but with them I had to send it to away to be assessed first and got refurbished one in return.