We've intentionally avoided concepts that include a battery since a key promise SeedSigner makes is that nothing survives in memory after you unplug it. But if the power source is hidden within the enclosure, this is less trivially verifiable.

A built in battery doesn't necessarily break the security model but it muddies the water. When you explain how a SeedSigner works and what guarantees it makes, you just don't want a bunch of asterisks and caveats. Security isn't simple, but where the story can be simple and somewhat(?) foolproof, it should remain so.