Current off the shelf betavoltaic batteries produce 50-350 nA @ 2.4v. I don't expect them to be producing much more than that.
These are the class of battery that would replace the CMOS battery in something that can never lose power to its RAM, rather than the much more powerful RTGs that space probes use.
[1] - http://www.citylabs.net/index.php?option=com_wrapper&view=wr...