This information comes from the manufacturer's website. It was posted in February of 2014.
An error message may appear on a unit when an internal battery is low or not working. This message may appear
frequently, but can just be dismissed to continue operation.
Swapping main battery packs, particularly in the second (top) battery slot on the TK6000 or Mesa, with a known good
fully charged battery pack may eliminate this error message.
Otherwise, a lithium CMOS backup battery is provided in all of our rugged handheld computer products (with the exception
of the Allegro CE and Archer 2 models). This battery maintains the clock and sometimes firmware (such as BIOS) settings
while having the main battery pack(s) removed for more than 5 minutes (a super-capacitor maintains all power while
swapping main battery pack(s) within 5 minutes, with the exception of the Archer 2).
This CMOS backup battery is not critical to unit operation as long as the main battery back(s) is/are kept charged every
night and weekend as is recommended.
If replacement of this CMOS battery is desired (to prevent the error message from appearing), the unit can be submitted
for repair.
http://www.junipersys.com/index.php/Juniper-Systems/support/Repairs