GTA02 RTC backup battery
RTC keeps the current time while Neo FreeRunner is off or suspended. Rechargable lithium backup battery should keep clocks running for hours when main battery is flat or removed, but it has limited lifetime.
Replace by capacitor
RTC backup battery can be replaced by electrochemical double layer capacitor (EDLC), like XH414H-II06E or EECEN0F204. These supercapacitors have much longer lifetime and they are less sensitive to heat from solder iron than lithium baterries.