I just got the guts to let it run 6.5 hours. Set HSM to away, was gone for about 3 hours.
YMMV based on apps, activities plugins etc. I have 128 devices
Impressive, but I don't completely understand the use-case here.
Is your interest in
- protecting the HE itself from corruption due to an unclean shutdown (ie., surviving a brief power outage (<1min - 15min))
- keeping the HE up during a long outage
- or something else?
I can't see any benefit to a backup battery other than scenario #1 -- which is a substantial convenience.
In any other case, almost all the devices would be unreachable (even the battery-powered devices would be off the zigbee/zwave network if the powered repeaters are down and the network meshes haven't repaired, right), the external alert system (ie., internet) would be down (unless you've got a battery-power cellular backup, and the local cell system isn't overloaded with people calling each other to ask if the power is out), the home LAN would probably be down (unless there's a UPS on critical switches & routers), etc.
However, it's still a cool project, and something I may well through together.