The Fluke-805 and Fluke-805 FC use 2 AA Lithium-Ion non-rechargeable batteries and also an internal coin cell battery for real time clock backup.
If the unit shows a coin cell error, and/or the date and time setting must be reset often, it indicates that the internal coin cell battery needs to be replaced.
As this is not a user-replaceable part, we recommend you contact the Service Center.
Request a quote for a Performance Test, and inform the Service Center that the internal coin cell battery needs to be replaced.
The Service Center will quote pricing and lead time for the RMA.
Here is a photo of the coin cell battery.