Interesting. As is common for folks with Icom 7300's who also unplug them for an amount of time.
They use a Seiko backup battery soldered to the main board. As do the IC-R8600 and possible others.
The specs. for the battery state they can be discharged to (from memory) 10% repeatedly with no short term degradation.
But complete discharge/charge cycles will kill them quite fast. As is shown in the forums and user groups.
Everyone has a phone. With a reminder app.
If its your practice to divorce standby power for a week or so, set the reminder to alert you to plug it back in for at least 24 hours.