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.
There doesn't need to be a separate access panel. It can just be in a normal battery holder like so:
Replaceable CR1225 in Cisco 1802
Considering it's something that generally runs 24/7 for years, it may still be a good idea to clean out dust from the device when possible. That's also an opportunity to replace the RTC battery, assuming it's replaceable.
Oh, hey, it seems Cisco even used to provide some Li batteries until 2017.
Also, not all businesses need the networks up 24/7. There may be plenty of time for down time for maintenance.