It is an ordinary battery, I forget if it was a AA or AAA. An IT person had to open ours to reset it. In addition to RFID and unlocking, it needs to call home periodically for the room key numbers (aka Magic Band) presumably over WiFi and TCP/IP, although it could be any radio signal. I'm assuming that when the battery is low it can call home and say so, or just blink a light, but I don't know who changes it.
In our case, our Magic Bands were not unlocking the door, despite 2 days of cast members telling us it's fixed, try it again. The IT guy plugged his computer into it and just reloaded the firmware, and that did the trick. Pretty bad bug when you have to do that. The IT guy did not go into any detail whatsoever and barely spoke english, or maybe it was his 100th door that day.