Ask HN: Could a phone be powered in part by betavoltaics

I've had this question lingering in my mind for some time. In some cases (like camping or traveling) you need to be able to call for help. The problem is you need to charge your emergency device. '

Would it be possible to build either a portable charger or a phone/cell phone powered by a betavoltaic cell? If it is possible then would it be possible to, hypothetically, replace the battery in a smart phone with a capacitor and a betavoltaic cell?

And finally, if all of this is possible, is there any reason why it has not been done yet? Are there laws against building such devices on a commercial or personal level?