Etap is a dream when it works. Hard to describe that sinking feeling you get when you press that lever and nothing happens. I hope you don't ever experience it. Good luck.
I experienced it once in 11 months; I'd simply neglected to charge the batteries recently. Was on a group ride. I signaled to stop, pulled over to the side of the road, swapped the front and rear derailleur batteries, and we were back on the road. Elapsed time, ~15 seconds. No sinking feeling involved. Half the folks in teh group didn't even know there was a problem, they thought I was just investigating a "false mechanical"
It's two small batteries; one for front and one for rear derailleur.
In fairness, it's four: There's a small CR3025 (?) coin-type battery in each of the levers.