There's a prototype betavoltaic battery which can sustain 50 microwatts on 20 Curies of tritium [1] -- an efficiency of about 2%. Scaling linearly by 10^4, current technology could get 500 milliwatts on 200 kCi, or 20 grams, enough to charge a 5 watt-hour phone battery in 10 hours. The tritium cost would be on the order of $600,000 today [2]. If you could do this, the battery charge time would be a few decades.
