The SRAM Red eTap system is terrific and the battery can power things for quite a while between charges - if you run down in the rear, it just takes moments to swap with the front to finish your ride BUT the plastic tab on the battery is fragile and can chip or break off so be careful when inserting the battery into the bike or charger - a small 'price to pay' for an otherwise wonderful shifting system.
These batteries and the efficiency of the SRAM Electronic shifting system is impressive. I use mine on a new AXS system. I keep a spare with me on all of my longer rides but Ive never had to use it. Im pushing 50 hours of riding on the last full charge and all lights are still green! No complaints!
I've never had a battery on eTap run out of juice on the road, but with some longer multi-day tours coming up (with no easy access to recharging), a spare seemed a good idea. I also carry an extra pair of CR3032 button cells just in case a shift lever battery goes out.