Emergency battery power for primary EFIS is the belt-and-suspender backup for total electrical system failure, but not for some Garmin G5 models that sit for long periods of time. That’s why Garmin has issued a mandatory Service Bulletin (22013 Rev. A) to prevent the problem of total backup battery discharge if the aircraft electrical power is not applied to the G5 for multiple weeks. That’s not uncommon, especially during winter months when aircraft are parked for longer periods of time.
The bulletin doesn’t apply to all G5 models, and for units with part number 011-03809-00, you can search serial number applicability at tinyurl.com/2p877yh2. Even if your serial number isn’t on the list, it’s worth checking back because Garmin said it will be revising the list periodically.