I’m a watch junkie.
This is my first smartwatch and the battery has finally given up the ghost. I don’t know if I’ll be able to find a replacement that fits inside the case but I’m going to try. As old as it is, it’s probably a NiCad. At least it’s not glued down and the solder tabs are easy to get to.
Edit: It looks like the battery is lithium-ion.
Take the battery out and see if there is ID on the other side. Remove the inventory tag and see if underneath it. Use Google to look at images of batteries to find a match. There are web sites with pictures of similar batteries that you can match to and order.
Adding to the problem of finding a modern replacement is the fact that it is a 6V battery.
Edit:
Now I’m reading that it is a 3.7V cell which actually makes more sense and is easier to find.
Makes more sense!
So, I quit reading Wikipedia and measured the battery voltage while it’s on the charger and it’s 3.9V so definitely a 3.7V rechargeable.
The build quality on this watch is very good which I guess is to be expected for Fossil and the fact that it cost $250 in 2003 ($417 in 2023). I have another one still in the sealed box and after seeing the inside of this one I still have hope it is not a corroded mess from a leaking battery.
$417 today will get you a nice smartwatch so I guess $250 was not that exorbitant a price considering that the Fossil Wrist PDA runs the Palm operating system and does basically everything a Palm Pilot will do.
I’m wondering now if it’s feasible to replace the battery with a Farad level supercap since the original battery was only 180mAh. Assuming it will fit in the case, of course.
Okay, not much mystery after all. The battery is a PD3032, easily found since they are currently used in Garmin products.
Duh, and now I see the part # is visible on the original battery.
I guess you didn't order ONE with a solder tab!
The options with the solder tabs had the wrong orientation. If this won't take solder, I'll have to rig one with tabs, I guess.
I wasn't aware that solder tabs had that problem.
While you are at it why not research changing the usb port…..
Is it possible???
The USB port is currently mini-usb. What would you suggest changing it to?
In my opinion, mini usb is not used very often.
Maybe USB-C or Micro USB?
I don't know if it's possible.....
I imagine getting the connector to fit the PCB and/or the case opening would be a problem, not to mention finding one that would mate up to the PCB traces well enough to be reliable. I'm not concerned with having a more modern connector since I have a cable that fits already.
I was successful changing the battery and it seems to be taking a charge. I'm a little concerned that the battery I got may have been discharged below a safe level. Time will tell.
I had hoped to be wearing it today but one of the springbars escaped when I was re-installing the strap and I'll have to get a replacement.
I've got heaps of mini USB cables....I guess @anon92864395 is just from a different generation or doesn't buy this kind of stuff. 
Yeah. I see the appeal of a USB-C connector for compatibility with modern devices but like you, I have mini USB cables and I only need one for this device.
Plus, as an example of an early smartwatch, I'd like to keep it original.
The next issue is to find a version of the software used with it that runs on some computer/operating system that I still have. Linux may be the answer.
