I'm not sure beta voltaics will ever reach LiPo densities. All materials I know would be unwise to place in your wallet, or anywhere near your body.
If we are OK with a battery and a beta voltaic source, a tritium one is reasonably safe and can trickle charge the battery when the device is in deep low power mode. The battery can still be charged by the induction coil.
This post - the title made me remember ... ( as a credit card is about the same size as a business card )
A Linux Business Card CD is a miniature, credit-card-sized optical disc containing a stripped-down, bootable Linux operating system. They hold around 50MB to 100MB of data and were highly popular in the early-to-mid 2000s
If we are OK with a battery and a beta voltaic source, a tritium one is reasonably safe and can trickle charge the battery when the device is in deep low power mode. The battery can still be charged by the induction coil.
A Linux Business Card CD is a miniature, credit-card-sized optical disc containing a stripped-down, bootable Linux operating system. They hold around 50MB to 100MB of data and were highly popular in the early-to-mid 2000s
More info at https://en.wikipedia.org/wiki/Bootable_business_card