Various ways of generating electricity from small radioactive power sources have been long known.
The Voyager space probes launched in the 1970s use thermoelectric generation from plutonium sources to generate ~100W of power to keep the probes running when they go very far from the sun. Some small devices have used other sources and other conversion mechanisms to prove power for very low power applications like pacemakers where long lasting power that doesn't need to be replaced is an advantage.
But recent claims suggest a breakthrough where devices powered by carbon-14 could replace the batteries in mobile phones (Nano diamond Batteries web page highlights stories claiming their produces could "revolutionise mobile phone batteries").
These claims seem like uncritical hype but should be easy to check.
Ignoring engineering issues (like conversion efficiency) how does the energy density of a power source based on C-14 compare to a mobile phone battery?
(I'm guessing a simple calculation based on the radioactive properties of C-14 could give a decent answer and that any battery experts can tell us what a Li-ion battery achieves so we can compare them. Also, nuclear power sources are not "batteries" but they are compact power sources so please don't be distracted by the terminology).