Avalanche Energy, a fusion energy startup, has won a $5.2 million contract to build radioactive batteries that can power a laptop-class system for months. The contract is part of the Defense Advanced Research Projects Agency’s Rads to Watts Program, which aims to build next-generation, compact, resilient nuclear batteries with higher densities.

Radioactive batteries aren’t an entirely new concept. They have been around for years and have made their way even to Mars on NASA’s Perseverance and Curiosity rovers. Another type of radioactive battery is used in medical implant devices and sensors, but all these devices face the same issue: low energy density. The project aims to resolve that issue.