A miniature nuclear battery, Betacel, using the betavoltaic principle has been designed for use in heart pacemakers and telemetering instruments, or in any remote unattended location where small amounts of long-term reliable power are required. Eighty-eight prototype Betacel batteries were manufactured in 1970 and the performance characteristics of a large number of them have been monitored since. These batteries are compact, predictable in output, and rugged. Safety tests have been performed to ensure containment of the 147Pm radioisotope fuel under all normal use and credible accident conditions.