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If the cards are always on a table / special playing surface, they could be wirelessly charged during play.

Some variants use NFC power to drive the display update, so no batteries!

I didn't realize NFC could deliver enough power for that. I'll look into it.

The demos I saw of this were slow, on the order of 10 seconds to get enough power through.

aha! So there is a catch. Still worth checking out.

If you could store that power in a capacitor it might be fine.

Basically, only one change every ten seconds is probably workable, as long as you don't have a ten second latency when asking for a change.