My current bike has sram batteries. On a minimum of 2 occasions I have had my battery die on me due to my forgetfulness to keep them charged. So I purchased a "backup" battery to keep in my saddlebag for those moments. It also helps me to remember to charge my existing battery!
The Sram Red eTap batteries hold a charge for many rides (I think Sram notes that they should last 600 km and I think I've been able to exceed that) and they have survived for a couple of years of hot summer and cold winter riding, much of which was on chipseal roads. However, recently I've had to replace a couple of them as the tabs that fit into the slot on the derailleur have broken off, no doubt due to the extreme temperature changes and the vibration caused by the road surfaces. It is also important to remove these batteries when traveling long distances with the bike on a bike rack as the derailleurs recognize the motion and keep the units on which will discharge the batteries.
No matter what I did, the tabs on my eTap batteries would crack, break and would eventually fall. I went through 10 of these batteries on 2 different bikes.
Tried to use electrical tape where the plastic comes in contact with the metal part on the derailleur, tried glue as a preventative measure, tried to avoid any bike cleaning products, but I always ended up with using rubber bands to secure the batteries, because the tabs would break.
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.