The internal CR2032 watch batteries in the MGL Avionics EFIS can be found at any electronics store or similar. They are used to store date and time settings, as well as Hobbs time, maintenance timers and map declutter settings. These batteries are not required to save critical engine and flight settings in the EFIS.
The first sign you have a flat battery is normally Hobbs time reverting to 0000 hours, or, map mode switching to Raster mode with no raster maps viewable. (Pressing SHIFT > MAP  > Change Map Mode to Vector will fix this problem, but setting will revert to Raster at next restart (Enigma/Voyager/Odyssey)).
IMPORTANT: When replacing the CR2032 battery, be sure to first note the Hobbs time and Maintenance Timers (if used) for re-entering if necessary. Also, copy your flightlog to your SD card. If you would like to continue with the flight log (rather than start a new one), copy the flightlog back to your RAM drive.
Enigma, Voyager, Odyssey:
The CR2032 Watch Battery in the back of the Enigma, Voyager and Odyssey EFIS units needs to be replaced every 2 years. It can be accessed through the little plastic case in the rear of the EFIS.
In the XTreme, the battery is located inside the case. Replace CR2032 every 2 years.
To open the XTreme:
1) Remove the 5 screws holding the back and front covers together.
2) Remove the 2 x DB15 securing nuts
3) Holding the unit, gently squeeze the barrel and press the back board (with the 2 brass connectors) through it. Make sure you reassemble it with no kinks in the tubing.
The iEFIS system (through the iBox) has a Keep Alive wire that feeds aircraft battery power directly into the iEFIS at all times, which saves the CR2032's life considerably. The iBOX distributes this power via the LAN cable to the iEFIS panels so they don't use their batteries either.
The CR2032 batterirs in the iBox and iEFIS displays should still be replaced every 5 years.
Note: On iEFIS systems the flight log is stored on the internal disk and does not have to saved.