John_S:
Have a little crank on the outside of the time capsule that turns a generator and charges a battery. Put a sign that reads "Please turn crank".
FTFY: "Please DO NOT turn crank."
That should guarantee it gets turned once a week or so.
Seriously, though. It's going in an attic? Has anyone owned an attic for 100 years? Maybe I'm just American, but after 60 years or so, houses tend to get remodeled and/or demolished.