@bruttium cool, with a couple of them you should get a head start on the new dead one while the old one still functions. They're very rewarding to work on and pretty simple to service. The silicone clip will keep it secure without glue, and when you rebuild that one, remove all residual glue carefully with an Exacto knife or similar before regluing so everything fits perfectly. I swear, removing glue takes more time than any other part of the process, LOL removing the case from an intact one can be done carefully with a razor blade. I'd highly recommend inserting it gently from the side opposite of the charging terminals as they're too easy to slice through if unfamiliar with the required pressure. If you need some assistance opening it, you can warm them with a heat gun briefly to loosen the glue, though be careful with this step to not damage the circuitry with excess heat, even a few seconds is enough, then you can insert the blade (or another super-thin pry tool) and work your way around to lift it open. If these steps aren't helpful, let me know what you're encountering, I've opened and reassembled dozens of these successfully. I've found excellent batteries on eBay, right now I see 45 results for 'Fitbit One battery', the top result being a vendor I've purchased from before, it's a 60mAh and works wonderfully. TL/DR: Slice around the near-invisible seam to reveal 2 parts; top and bottom, briefly heat existing solder to remove old wires then directly replace with new ones, hit "on" to test before gluing as most have a little juice in them 😉 LMK any questions and welcome to the new years! 
Best Answer@AlbertSnodgrass I've not seen a good exploding battery report in like 15 years, would you mind sharing the links with us, please? I've got some direct vendor relationships and also purchased good replacement batteries on Amazon recently, and I'm in the states. As I'm unsure where you're located that you're prohibited from purchasing batteries, you'll maybe you'll have better luck on eBay or even AliExpress? Unsure what you're finding that's dodgy. Several sources for good One batteries. Perhaps stick with reputable vendors with solid feedback and several previous sales.
I'm installing a new battery right now, the original battery is the one I'm scared of exploding, it's expanded greatly, much larger than the tracker's chassis was designed to accommodate due to how LiPo batteries degrade over time. Inspecting everything carefully, cleaning where needed, and successfully installed the replacement 🙂
Here's a good original working perfectly Fitbit One battery, vs. one that's greatly expanded ...YIKES!
Top one = good, bottom one = YIKES!
LOL, the only problem with this particular one pictured was something that happened at the factory level, the battery wasn't properly soldered in, I found a perfectly brand new battery inside what was considered to be a dead tracker - WOOT!
TL:DR: eBay One Batteries
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