Battery replacement for tomtom spark cardio music

I'll be waiting too for electroguy's reply regarding where to source the battery from. Also maybe this thread can turn into a resource for repairing TomTom watches? So here's my contribution. I'll talk about screwdriver needed and O'Ring (for water-resistance) specifications. (warning: lots of words and theory; lacks in practicality).

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Screwdriver

If you are very careful, all you need is a Torx T5 screwdriver (T5 is the size) to open a TomTom Runner, which is the first version that comes before TomTom Spark (aka TomTom Runner 2*). It's fairly easy to disassemble and reassemble the watch with the help of a small 2mm flathead screwdriver (to gently pry out the case and other electronics). Hopefully the other versions use the same Torx T5 screws (*other versions are: Runner | Spark aka Runner 2 | Spark 3 aka Runner 3).

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O-Ring

If you need new O'Ring for the water-resistance, I measured the thickness using a micrometer and the best bet* is a 0.85mm O'Ring (*see methodology below). This small rubber ring is important to keep water away from the delicate electronics inside the watch.

The O'Ring sits in a 0.96mm width groove (bottom case) and is pressed by a 0.81mm width "protrusion"* (upper case). That's how the watch is sealed. (*there might exist a better word for this).

As for the length, unfortunately I did not measure it but that should be easy to obtain by placing any thin wire (ideally a less than 1mm thickness wire) inside the entire groove and then simply measuring the resulting length. Because of the elasticity of the rubber O'Ring, there's more leeway here when compared to the thickness (which must be small enough to sit inside the aforementioned tiny groove; but not too small compromising the sealing effect and water-resistance).

Once you have your O'Ring specifications in hand, you can source it from whatever O'Ring manufacturer you find without worrying about the application (you eliminate the variable 'TomTom Runner GPS watch'). Just a small ring made of rubber* with the specified dimensions (*a popular one is NBR aka Buna-N; you don't have to worry about chemical and temperature compatibility charts with this application). Maybe not that easy because it's very tiny (less than a mm), but for sure much easier than searching for "TomTom Runner O'Ring" (maybe you can prove me wrong).

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Methodology (why 0.85mm)

The discrepancy between the O'Ring thickness (0.85mm) and the groove width (0.96mm) is explained by the fact that the O'Ring gets squeezed (flattened) by the "protrusion". I took two measurements of the thickness in orthogonal directions. The squeeze effect gives the thickness an oval profile (instead of the factory round profile) and the first (larger) measurement lies between 875 and 880 micrometers and the second (smaller) measurement lies between 790 and 795 micrometers. Hence the 0.85mm seems reasonable, theoretically (I did not test it, fortunately mine came out intact and I was able to reuse it).