My car is parked in an area with no electricity so no battery tender. When I store the car I remove negative lead. On March 15th I took the car out of storage after 4 months and a few drives later received the notice about an update and declined because I was coming back to the car in a few minutes. Next time I parked, I was asked to update and accepted. There was a few messages and, as I exited, I could see a progress bar that I watched. Returning to the car next day I checked the status and could see a new software version.
There are 2 steps to the update: 1: Is a download and as far as I can tell this happens when you are driving around and continues if you stop etc. After download 2: The software is residing in the vehicles memory (an EPROM?) and may not require wifi etc as its just updating software.
With no battery tender the car started perfectly.