I spent an hour yesterday putting an RTC chip and backup battery on a Pi Top proto board: easy to do, and it works perfectly. If people are interested, I would be happy to draw up the layout and post it.
I wrote a page on my web site, then tried to post a link here, but either that's not allowed or it's just taking a long time in moderation. You can find the same page by googling "Spivey's Corner" then using typing "real time" into the wiki's search box.
Nice work, and important, if one uses the pi-top offline.
I was never able to enter any contributions with a link. What works for me is to enter the website address just like for example youtube.com. Users can then copy and paste the address into their browser. Sounds stupid. It also strips off quite often all the new lines in my contributions.
For those who have not found it, the web page is at spivey.oriel.ox.ac.uk/corner/Realtime -- Mike