I am getting a "Battery Fault!!! Replace Battery" message on LabQuest.

This is an indication that your battery is failing and will need to be replaced soon. If the battery is generally holding a charge, there is possibility that the message is incorrect and that the battery is actually okay. To see if this is the case, swap batteries with a unit that is working as expected to see if the problem follows the battery or stays with the LabQuest.
TIL 2189: How do I remove or replace a LabQuest battery?

If the problem follows the battery, replace the battery.
LabQuest Replacement Battery, order code LQ-BAT, $19

If the problem goes away, note which unit has the suspect battery, an monitor that unit to see if the problem comes back. If it does, replace the battery.

If the problem stays with the LabQuest unit, try updating the LabQuest software. This may help even if you are currently running the latest version.
http://www.vernier.com/labquest/updates/

If you continue to have problems, contact Vernier technical support for additional assistance.

For additional battery troubleshooting tips, see
TIL 2077: Original LabQuest Battery Troubleshooting and FAQs