There is a CR2032 battery in at least one type of controller, one with a DSQC639 main CPU, not certain about newer Main CPUs, or for different main CPU architectures, e.g. Compact, OmniCore, etc.
I believe its primary purpose is to support CMOS.
Here's the error that will be seen if the voltage drops too low:
37053, Low CMOS battery voltage level
The CMOS battery on the computer board is empty.
On restart, the system will use an erroneous setup or no restart will be
possible. (All that has been experienced here is a loss of date & time)
1. Replace the CMOS battery