[wkat4242]

> One of the more difficult features was the rechargable battery.

Flat ones do exist. I have one eink TOTP card that's no thicker than a normal credit card.

And also an airtag style tracker that's about twice as thick. Maybe it uses the same kind of battery.

[bookofjoe]

O.4mm thick rechargeable battery:

https://www.lithium-polymer-battery.net/ultra-thin-lithium-p...

Standard credit card: 0.76mm thick

[uticus]

seems dangerous to have terminals of battery exposed, shorting (and with rechargeable, burning) hazard

[spacedcowboy]

Some conformal coating spray would probably be all that’s needed.