These nuclear batteries make Mini Nuclear Power Plants look massive.
"University of Missouri engineers are building a nuclear battery the size of a penny. Their aim is to develop a long-lasting power source for tiny sensors, actuators, and labs-on-a-chip. While nuclear batteries sound, er, problematic, they're actually relatively common in larger form factors to power pacemakers and instruments aboard space vehicles."
Picture embedded below. (from http://news.cnet.com/8301-17938_105-10372233-1.html)