This battery lasts the life of the router under the operating environmental conditions specified for the router, and is not field-replaceable.
But who determines its lifespan?
Knowing there is a battery set to fail and I can't simply replace it makes me physically uncomfortable. Enough so that I'd rather it not have RTC.
Thanks Cisco.
As far as what battery it is, measure it in metric. (mm) Search up a button battery chart. One of those should have voltage and measurements you can compare against.
To go further, all those 3v button batteries are essentially the same. You can buy any one of similar thickness to replace one like that, that you will have to re-solder in place. It's definitely a 20mm across one, so the first numbers will be 20. For thickness I'm guessing by the looks of it, it's either 3.5mm thick or 4.0mm thick, so last two numbers will be 35 or 40. So you want either a 2035 or a 2040 as a replacement.
They're all 3v. The only difference in button style batteries thickness is how much capacity they hold. If it physically fits in a device and you can create the connection for the top and bottom, it will work. I've used a little piece of metal or wire to bridge a gap to make a thinner battery replace what was supposed to be a thicker one if I didn't have the right size on hand. Just means it will die a bit sooner.