To charge Rival eTap AXS derailleur batteries, remove the battery from the derailleur to charge them on the eTap battery charger. These derailleur batteries are very small and easy to remove without any tools. You can even carry a spare or two for bike packing or long travel adventures where recharging isn’t an option.
See the SRAM RED eTap System Overview video for battery removal, installation, charging, and battery indicator LED information.