SRAM thought of that and they know some people will forget to charge it. That's why there are hidden ports for their spare "ready cable". Just keep the small cable in your saddle bag or wherever and when the battery dies, you attach it to the derailleur and shifter and throw some temporary zip ties on to hold it in place. The cable has tiny batteries built into it that'll double your battery length to get you through the ride and back home with no issues.
They also have a mini stick on power cell you can connect to harness the power of the sun and it'll charge while you ride, thus you'll never need to charge them. You just connect the small wire to the hidden ports on either the derailleur or shifter that is connected to the solar cell sticker, then you simply wrap the sticker around your handle bars or chain stays and BAM! Done. Never worry about it again.