SRAM Red eTap Battery Charger And Cord
SRAM says you'll get a 1,000km or 60 hours of constant use before you need to recharge.
eTap batteries are literally a snap to remove and charge.
Just flip the tool-free battery latch up and slide the battery up and out of the derailleur. Then snap the battery into the USB powered charger and connect the charger to any USB power source or adaptor